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Re: [Electronics_101] A question about MOV's

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  • John Johnson
    Ken, The free-wheeling diode should be across the HV coil, to absorb the high voltage returned when the magnetic field collapses. This HV can make its way back
    Message 1 of 12 , Sep 1, 2007
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      Ken,

      The free-wheeling diode should be across the HV coil, to absorb the high
      voltage returned when the magnetic field collapses. This HV can make its way
      back through your circuit on other paths besides the Vcc and ground rails.
      For instance, in a simple NPN transistor with a relay connected to its
      collector, then to V+, the inductive spike could go from the collector to
      the base, to whatever is driving it. Polarity of the "diode" formed by the
      base-collector junction won't really matter, as the spike will exceed the
      breakdown voltage of the junction.

      Regards,
      JJ

      On 9/1/07, Ken Stuempges <ozarkshermit@...> wrote:
      >
      > I am considering adding a small MOV to a circuit board using an IGBT
      > driven by a 555 astable multivibrator (approx 200 Hz). A 6 Volt Lead
      > Acid battery is used for power. The IGBT drives a small High Voltage
      > induction coil, and I have had a few problems with the 555 or a
      > transistor on the board getting fried for some unknown reason. I have
      > a free-wheeling diode across Vcc and Ground directly on the board,
      > but still get an occasional failure. The coil draws significant
      > current when the oscillator is enabled, which is not constant however,
      > several "bursts" of 200 Hz per second. I have not seen any large
      > spikes on the O'scope, but that is what I suspect.
      >
      > My question regarding MOV's is:
      >
      > What is the difference between working voltage, breakdown voltage, and
      > clamping voltage. I get confused trying to figure out the specs. I'm
      > sure that "working voltage" is the DC steady state voltage, but what
      > is the difference between Breakdown Voltage and Clamping Voltage?
      >
      > The MOV I am considering is from DigiKey, part # 478-2489-ND. I think
      > it should protect the 555, the transistor, and a 741 op amp on the
      > board. I would install it directly across Vcc and ground on the board.
      >
      > Hopefully someone can provide some advice
      >
      > Thanks
      >
      > Ken
      >
      >
      >


      [Non-text portions of this message have been removed]
    • John Popelish
      Ken Stuempges wrote: (snip) ... MOVs are nonlinear resistors that exhibit a very high resistance when low voltage is applied across them, but their resistance
      Message 2 of 12 , Sep 1, 2007
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        Ken Stuempges wrote:
        (snip)
        > My question regarding MOV's is:
        >
        > What is the difference between working voltage, breakdown voltage, and
        > clamping voltage. I get confused trying to figure out the specs. I'm
        > sure that "working voltage" is the DC steady state voltage, but what
        > is the difference between Breakdown Voltage and Clamping Voltage?

        MOVs are nonlinear resistors that exhibit a very high
        resistance when low voltage is applied across them, but
        their resistance falls dramatically above some particular
        voltage.

        Working voltage is safely below the transition voltage,
        where the resistance is very high. They will pass an
        insignificant resistive current, as long as they are exposed
        to working voltage or less.

        Breakdown voltage is approximately the transition voltage
        between high resistance and low resistance operation, where
        a trickle of leakage current starts to occur, and where they
        will start to self heat themselves with I^2*R losses. So it
        is not safe to apply this voltage, continuously (it is above
        safe working voltage), but is a good voltage to test them
        with, to find out where the resistance transition takes place.

        Clamping voltage is the voltage that drives them deep into
        low resistance operation, where their heat generation is
        very large, the current passed, in series with other circuit
        impedances tends to keep the MOV voltage from exceeding this
        operating point (voltage is clamped) and internal damage
        will begin to accumulate. They are good for only so many
        pulses of a given current at clamping voltage, before their
        breakdown voltage will drift downward. When the breakdown
        voltage (where a specified leakage current occurs) reaches
        the specified working voltage, the MOV is worn out, and may
        self destruct by self heating, if working voltage continues
        to be applied.

        Al these specs are intended to describe MOV operation as a
        voltage surge suppressor.

        > The MOV I am considering is from DigiKey, part # 478-2489-ND. I think
        > it should protect the 555, the transistor, and a 741 op amp on the
        > board. I would install it directly across Vcc and ground on the board.

        If I read the data sheet correctly, that is a
        VA100014A300DL, with a working voltage of 14 (where it
        passes almost no current) and a clamping voltage of 32
        volts, where it passes a peak current of 40 amperes.

        Any current less than 40 amperes (but greater than 100 ma)
        will produce a voltage drop somewhere between the clamping
        voltage and the breakdown voltage of 18.5 +-15%.

        Whether or not this device protects your other components
        and also survives for an adequate lifetime depends on the
        peak current it is passing, how many times that occurs, and
        lots of details about how the other components are
        interconnected. I think you need to understand better how
        (or even if) the coil current is causing your problem before
        you try to fix it this way.

        So, please, back up and post a schematic of your circuit to
        the files or photo section for this group, so we can figure
        out, more precisely, what is going on.
      • alan00463
        ... Ken, what is the polarity of your free-wheeling diode ? Where does its anode connect? its cathode? What is its part no? Can you post a
        Message 3 of 12 , Sep 1, 2007
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          --- In Electronics_101@yahoogroups.com, "Ken Stuempges"
          <ozarkshermit@...> wrote:
          >
          > I am considering adding a small MOV to a circuit board using an IGBT
          > driven by a 555 astable multivibrator (approx 200 Hz). A 6 Volt Lead
          > Acid battery is used for power. The IGBT drives a small High Voltage
          > induction coil, and I have had a few problems with the 555 or a
          > transistor on the board getting fried for some unknown reason. I have
          > a free-wheeling diode across Vcc and Ground directly on the board,
          > but still get an occasional failure. The coil draws significant
          > current when the oscillator is enabled, which is not constant however,
          > several "bursts" of 200 Hz per second. I have not seen any large
          > spikes on the O'scope, but that is what I suspect.
          >
          > My question regarding MOV's is:
          >
          > What is the difference between working voltage, breakdown voltage, and
          > clamping voltage. I get confused trying to figure out the specs. I'm
          > sure that "working voltage" is the DC steady state voltage, but what
          > is the difference between Breakdown Voltage and Clamping Voltage?
          >
          > The MOV I am considering is from DigiKey, part # 478-2489-ND. I think
          > it should protect the 555, the transistor, and a 741 op amp on the
          > board. I would install it directly across Vcc and ground on the board.
          >
          > Hopefully someone can provide some advice
          >
          > Thanks
          >
          > Ken

          Ken, what is the polarity of your "free-wheeling diode" ? Where does
          its anode connect? its cathode? What is its part no? Can you
          post a schematic?

          Alan
        • P.Arunkumar
          Dear Ken The definition of the break down voltage is that from that voltage the TVS/MOV will come it to picture and start conducting the current to voltage
          Message 4 of 12 , Sep 1, 2007
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            Dear Ken

            The definition of the break down voltage is that from that voltage the
            TVS/MOV will come it to picture and start conducting the current to voltage
            curve will you the current ( leakage current ) taken by the Protection
            circuit. The clamping voltage is the voltage above which the voltage across
            the TVS will not increase . There will be trade off between the three
            voltage when we select a TVS. There are couple of application notes on how
            to select TVS and how it works in the following link .


            http://www.microsemi.com/support/micnotes.asp?MN=316

            Rgds
            Arunkumar





            On 9/1/07, Ken Stuempges <ozarkshermit@...> wrote:
            >
            > I am considering adding a small MOV to a circuit board using an IGBT
            > driven by a 555 astable multivibrator (approx 200 Hz). A 6 Volt Lead
            > Acid battery is used for power. The IGBT drives a small High Voltage
            > induction coil, and I have had a few problems with the 555 or a
            > transistor on the board getting fried for some unknown reason. I have
            > a free-wheeling diode across Vcc and Ground directly on the board,
            > but still get an occasional failure. The coil draws significant
            > current when the oscillator is enabled, which is not constant however,
            > several "bursts" of 200 Hz per second. I have not seen any large
            > spikes on the O'scope, but that is what I suspect.
            >
            > My question regarding MOV's is:
            >
            > What is the difference between working voltage, breakdown voltage, and
            > clamping voltage. I get confused trying to figure out the specs. I'm
            > sure that "working voltage" is the DC steady state voltage, but what
            > is the difference between Breakdown Voltage and Clamping Voltage?
            >
            > The MOV I am considering is from DigiKey, part # 478-2489-ND. I think
            > it should protect the 555, the transistor, and a 741 op amp on the
            > board. I would install it directly across Vcc and ground on the board.
            >
            > Hopefully someone can provide some advice
            >
            > Thanks
            >
            > Ken
            >
            >
            >



            --
            success is a point where hardwork and luck converges


            Regards
            Arunkumar


            [Non-text portions of this message have been removed]
          • Ken Stuempges
            Thanks for all the responses. I will try and figure out how to either scan a hand-drawn schematic, or find some circuit-drawing software, and post it.
            Message 5 of 12 , Sep 1, 2007
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              Thanks for all the responses. I will try and figure out how to either
              scan a hand-drawn schematic, or find some circuit-drawing software,
              and post it.

              Meantime, I will try to describe what I am doing.

              I am using a 555, configured in astable mode, to oscillate at around
              200 Hz. The output (pin 3) connects to the base of an NPN transistor
              (2N4401, or a 2N2222, depending what I have), through a 1K resistor.
              The emitter is tied to ground, the collector is pulled up to Vcc
              through a 2.2K resistor.

              At the 2.2K resistor-collector junction, I connect to the gate of the
              IGBT through a 470 ohm resistor. The emitter of the IGBT ties to
              ground, the collector to one side of the coil primary. The other side
              of the coil primary connects directly to Vcc. I have a small heat sink
              on the IGBT, but it does not even get warm, so that seems OK.

              The IGBT I am using is an International Rectifier IRGB14C40L. It
              was designed to be used with ignition circuits, and has on-chip active
              voltage clamps between the Gate-Emitter and Gate-Collector, which
              provide over voltage protection and self-clamped Inductive switching.

              When the 555 is oscillating (in short bursts), the coil will generate
              substantial hi voltage sparks, at the 200 Hz rate. The coil I am
              using is a Tecumseh coil, used on many small engines, and I have added
              a laminated core to the coil. I am not sure how much current the coil
              draws when energized, an amp or two I am guessing.

              Adding a free wheeling diode across the coil (cathode to the plus
              side) prevents it from working at all - can't figure that out -
              After I had a few failures I placed the diode across Vcc and ground on
              the circuit board. The diode is an ultra-fast diode , 1A, 800V,
              Digikey # MUR180EGOS-ND. In retrospect, I think adding the diode to
              the circuit board was a waste of time, but I was hoping . . .

              Power for all this is a 6V , 1100 MAH sealed lead acid battery.
              I have seen circuits of this type , but perhaps my choice of driving
              transistor or resistor value selection is what is causing problems.
              The failures are infrequent, but something is wrong if it fails at all.

              Usually the transistor fails, but the 555 has failed as well.

              Thanks again for the responses. Can anyone suggest some
              circuit-drawing freeware?
            • orvillefpike
              I am making a suggestion in case nobody has thaught about it, without having done any research on this IGBT and probably without knowing what I m talking about
              Message 6 of 12 , Sep 1, 2007
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                I am making a suggestion in case nobody has thaught about it, without
                having done any research on this IGBT and probably without knowing
                what I'm talking about but isn't the gate resistor of 470 Ohms a bit
                large? Doesn't it make the switching loss large enough to destroy the
                IGBT? Maybe somebody who's familiar with these devices could answer
                that.
                If your IBGT has a free wheeling diode, isn't useless to add another
                one? When your circuit failed to work with the added free wheel
                diode, is it possible that you had the diode backward?


                --- In Electronics_101@yahoogroups.com, "Ken Stuempges"
                <ozarkshermit@...> wrote:
                >
                > Thanks for all the responses. I will try and figure out how to
                either
                > scan a hand-drawn schematic, or find some circuit-drawing software,
                > and post it.
                >
                > Meantime, I will try to describe what I am doing.
                >
                > I am using a 555, configured in astable mode, to oscillate at around
                > 200 Hz. The output (pin 3) connects to the base of an NPN
                transistor
                > (2N4401, or a 2N2222, depending what I have), through a 1K
                resistor.
                > The emitter is tied to ground, the collector is pulled up to Vcc
                > through a 2.2K resistor.
                >
                > At the 2.2K resistor-collector junction, I connect to the gate of
                the
                > IGBT through a 470 ohm resistor. The emitter of the IGBT ties to
                > ground, the collector to one side of the coil primary. The other
                side
                > of the coil primary connects directly to Vcc. I have a small heat
                sink
                > on the IGBT, but it does not even get warm, so that seems OK.
                >
                > The IGBT I am using is an International Rectifier IRGB14C40L. It
                > was designed to be used with ignition circuits, and has on-chip
                active
                > voltage clamps between the Gate-Emitter and Gate-Collector, which
                > provide over voltage protection and self-clamped Inductive
                switching.
                >
                > When the 555 is oscillating (in short bursts), the coil will
                generate
                > substantial hi voltage sparks, at the 200 Hz rate. The coil I am
                > using is a Tecumseh coil, used on many small engines, and I have
                added
                > a laminated core to the coil. I am not sure how much current the
                coil
                > draws when energized, an amp or two I am guessing.
                >
                > Adding a free wheeling diode across the coil (cathode to the plus
                > side) prevents it from working at all - can't figure that out -
                > After I had a few failures I placed the diode across Vcc and ground
                on
                > the circuit board. The diode is an ultra-fast diode , 1A, 800V,
                > Digikey # MUR180EGOS-ND. In retrospect, I think adding the diode to
                > the circuit board was a waste of time, but I was hoping . . .
                >
                > Power for all this is a 6V , 1100 MAH sealed lead acid battery.
                > I have seen circuits of this type , but perhaps my choice of driving
                > transistor or resistor value selection is what is causing problems.
                > The failures are infrequent, but something is wrong if it fails at
                all.
                >
                > Usually the transistor fails, but the 555 has failed as well.
                >
                > Thanks again for the responses. Can anyone suggest some
                > circuit-drawing freeware?
                >
              • John Popelish
                ... wrote: (snip) ... A diode across the coil prevents it from producing high voltage kick, when the current through it rapidly quenched as
                Message 7 of 12 , Sep 1, 2007
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                  --- In Electronics_101@yahoogroups.com, "Ken Stuempges"
                  <ozarkshermit@...> wrote:
                  (snip)
                  > I am using a 555, configured in astable mode, to oscillate at around
                  > 200 Hz. The output (pin 3) connects to the base of an NPN transistor
                  > (2N4401, or a 2N2222, depending what I have), through a 1K resistor.
                  > The emitter is tied to ground, the collector is pulled up to Vcc
                  > through a 2.2K resistor.
                  >
                  > At the 2.2K resistor-collector junction, I connect to the gate of the
                  > IGBT through a 470 ohm resistor. The emitter of the IGBT ties to
                  > ground, the collector to one side of the coil primary. The other side
                  > of the coil primary connects directly to Vcc. I have a small heat sink
                  > on the IGBT, but it does not even get warm, so that seems OK.
                  >
                  > The IGBT I am using is an International Rectifier IRGB14C40L. It
                  > was designed to be used with ignition circuits, and has on-chip active
                  > voltage clamps between the Gate-Emitter and Gate-Collector, which
                  > provide over voltage protection and self-clamped Inductive switching.
                  >
                  > When the 555 is oscillating (in short bursts), the coil will generate
                  > substantial hi voltage sparks, at the 200 Hz rate. The coil I am
                  > using is a Tecumseh coil, used on many small engines, and I have added
                  > a laminated core to the coil. I am not sure how much current the coil
                  > draws when energized, an amp or two I am guessing.
                  >
                  > Adding a free wheeling diode across the coil (cathode to the plus
                  > side) prevents it from working at all - can't figure that out -

                  A diode across the coil prevents it from producing high voltage kick,
                  when the current through it rapidly quenched as the IGBT turns off, so
                  you can't do this, if you want the coil to produce high voltage.

                  > After I had a few failures I placed the diode across Vcc and ground on
                  > the circuit board. The diode is an ultra-fast diode , 1A, 800V,
                  > Digikey # MUR180EGOS-ND. In retrospect, I think adding the diode to
                  > the circuit board was a waste of time, but I was hoping . . .
                  (snip)

                  I suspect the rapid change in current being drawn from the battery as
                  the IGBT is turned off is generating voltage spikes on the wiring
                  inductance. This is what is blasting all the othe components
                  connected across that supply. I would try adding a low inductance
                  capacitor, like a stacked film or ceramic type of a microfarad or so,
                  in parallel with an electrolytic cap of several hundred microfarads,
                  across the 6 volt supply, directly between the coil supply connection
                  and the emitter of the IGBT, to supply the edges of that high current
                  pulse as local and out of the supply wiring as possible. Of course
                  the IGBT collector and all secondary wiring should be kept as far from
                  the low power stuff as you can, also, to reduce direct capacitive
                  coupling of the high voltage swings from the low voltage components.

                  Have you got a digital camera to photograph this construction?
                • Ken Stuempges
                  Thanks John I guess I might be on kind of the right track. I noticed that there is some voltage overshoot on the positive side of the pulse triggering the
                  Message 8 of 12 , Sep 1, 2007
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                    Thanks John
                    I guess I might be on kind of the right track. I noticed that there
                    is some voltage "overshoot" on the positive side of the pulse
                    triggering the coil. I tried adding some capacitance (electrolytic)
                    across the board supply terminals, and it pretty much eliminated the
                    overshoot, depending on the size capacitor. I tried this before your
                    last response, so will add to what I do next, based on your reply.
                    The supply voltage definitely drops each time the coil is activated.
                    I was surprised at that since I am using a battery, but it definitely
                    drops. The scope shows the battery voltage dropping at the frequency
                    of the 555 oscillator. I'll measure the total drop, as I remember is
                    is a volt or two. When I added the capacitor, it definitely smoothed
                    out the voltage drop on the battery, and pretty much eliminated the
                    spikes. Unfortunately I do not have any more caps of various sizes to
                    try, will have to wait till Tuesday for that.

                    I will take a digital photo tomorrow and post it - the circuit itself
                    is on a small board, about 1 1/2 by 1 inch, - BUT - I have
                    connecting jumpers all over the place, since the rest of the
                    arrangement is not final. I have long jumper leads going to and from
                    the coil, to and from the IGBT and battery, etc. This might be
                    contributing to my problem. I kind of wondered about that, but did
                    not think it would cause major problems.

                    Based on your reply, I will clean up the wiring, close to what the
                    final arrangement will be, and life test it some more and check the
                    waveforms. Also, I will size the inter-connect wires to a larger
                    gage, etc. Unfortunately, when the circuit might fail is totally
                    unpredictable at this point. I'll check what the supply voltage looks
                    like with some of these changes also.

                    Thanks again

                    Ken
                  • Roy J. Tellason
                    ... What you re seeing there is a perfect demonstration of the fact that the battery has internal resistance , a concept that I remember stumbling over a bit
                    Message 9 of 12 , Sep 1, 2007
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                      On Saturday 01 September 2007 21:04, Ken Stuempges wrote:
                      > The supply voltage definitely drops each time the coil is activated.
                      > I was surprised at that since I am using a battery, but it definitely
                      > drops. The scope shows the battery voltage dropping at the frequency
                      > of the 555 oscillator. I'll measure the total drop, as I remember is
                      > is a volt or two.

                      What you're seeing there is a perfect demonstration of the fact that the
                      battery has "internal resistance", a concept that I remember stumbling over
                      a bit back when I was first getting into this stuff... (WAY back. :-)

                      > When I added the capacitor, it definitely smoothed out the voltage drop on
                      > the battery, and pretty much eliminated the spikes. Unfortunately I do not
                      > have any more caps of various sizes to try, will have to wait till Tuesday
                      > for that.

                      Something smaller in value than the electrolytic (_in addition to it_) would
                      be good, the value is not at all critical and it can be salvaged out of
                      whatever junk electronics you might have handy.

                      > I will take a digital photo tomorrow and post it - the circuit itself
                      > is on a small board, about 1 1/2 by 1 inch, - BUT - I have
                      > connecting jumpers all over the place, since the rest of the
                      > arrangement is not final. I have long jumper leads going to and from
                      > the coil, to and from the IGBT and battery, etc. This might be
                      > contributing to my problem. I kind of wondered about that, but did
                      > not think it would cause major problems.

                      It might indeed, since such wiring would have lots of distributed inductance.
                      Going to short and direct wiring may alleviate your problem as well.

                      > Based on your reply, I will clean up the wiring, close to what the
                      > final arrangement will be, and life test it some more and check the
                      > waveforms. Also, I will size the inter-connect wires to a larger
                      > gage, etc. Unfortunately, when the circuit might fail is totally
                      > unpredictable at this point. I'll check what the supply voltage looks
                      > like with some of these changes also.

                      Are you still seeing a drop with the added capacitor across the supply lines?
                      I agree with John's post about locating that (and the other cap when you add
                      it) close to the coil and emitter terminals. Layout in something like this
                      _will_ have some significant effects.

                      --
                      Member of the toughest, meanest, deadliest, most unrelenting -- and
                      ablest -- form of life in this section of space,  a critter that can
                      be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
                      -
                      Information is more dangerous than cannon to a society ruled by lies. --James
                      M Dakin
                    • Ken Stuempges
                      ... John: I posted some pictures in the photos section under The Hermits Stuff The first shows the circuit board. The IGBT is vertical with heat sink, the
                      Message 10 of 12 , Sep 3, 2007
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                        --- In Electronics_101@yahoogroups.com, "John Popelish" <jpopelish@...>

                        > Have you got a digital camera to photograph this construction?
                        John:
                        I posted some pictures in the photos section under "The Hermits Stuff"
                        The first shows the circuit board. The IGBT is vertical with heat
                        sink, the two brass studs are the Vcc and Ground connections. The
                        Led's and other 8-pin dip are some unrelated stuff. Eventually I plan
                        on potting the board, so only the top of the studs and leds will be
                        exposed. The IGBT will be bent over so just the heat sink is above
                        the potting material.

                        I tried placing an electrolytic cap across the Vcc and Ground studs,
                        and a 47 Mike Cap seemed to eliminate any voltage overshoot. I did not
                        add a small ceramic cap yet. Also not sure what voltage rating I
                        should use on the electrolytic - 25V with 32V surge seem OK?

                        The other two photos show some jumbled up wiring, which is how I have
                        been testing the board. The final version will be very compact, with
                        short lead lengths, except for the leads that will "make and break"
                        the negative connection to the battery. The leads will be about 4
                        inches long, and 20 gage or so.

                        I do not have a very substantial ground plane on the board, and as yet
                        do not have a de-coupling cap on the 555. I didn't think the cap
                        would be necessary, but probably will add one. Also will try to
                        increase the amount of copper for the ground plane.

                        Thanks
                        Ken
                      • John Popelish
                        ... If the caps are successful in eliminating serious supply spiking, then they need to be rated for just a bit more than the supply voltage. I think, for a 6
                        Message 11 of 12 , Sep 3, 2007
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                          --- In Electronics_101@yahoogroups.com, "Ken Stuempges"
                          <ozarkshermit@...> wrote:

                          > I posted some pictures in the photos section under "The Hermits Stuff"
                          > The first shows the circuit board. The IGBT is vertical with heat
                          > sink, the two brass studs are the Vcc and Ground connections. The
                          > Led's and other 8-pin dip are some unrelated stuff. Eventually I plan
                          > on potting the board, so only the top of the studs and leds will be
                          > exposed. The IGBT will be bent over so just the heat sink is above
                          > the potting material.
                          >
                          > I tried placing an electrolytic cap across the Vcc and Ground studs,
                          > and a 47 Mike Cap seemed to eliminate any voltage overshoot. I did not
                          > add a small ceramic cap yet. Also not sure what voltage rating I
                          > should use on the electrolytic - 25V with 32V surge seem OK?

                          If the caps are successful in eliminating serious supply spiking, then
                          they need to be rated for just a bit more than the supply voltage. I
                          think, for a 6 volt battery, 10 to 25 volt capacitors are fine.

                          > The other two photos show some jumbled up wiring, which is how I have
                          > been testing the board. The final version will be very compact, with
                          > short lead lengths, except for the leads that will "make and break"
                          > the negative connection to the battery. The leads will be about 4
                          > inches long, and 20 gage or so.
                          >
                          > I do not have a very substantial ground plane on the board, and as yet
                          > do not have a de-coupling cap on the 555. I didn't think the cap
                          > would be necessary, but probably will add one. Also will try to
                          > increase the amount of copper for the ground plane.

                          The point of the capacitors is that the current pulses to the coil
                          also magnetize the space between the two battery leads (since the
                          output current also encircles that space). When the current is
                          switched off, not only does the coil produce a large voltage kick, the
                          inductance of that magnetized space between the supply leads also
                          generates a kick. Having a local place for the sudden changes in
                          supply current to go, reduces the kick from that loop inductance,
                          because its current changes slower. But you can also reduce it by
                          reducing that loop inductance, itself, by reducing the area inside
                          that loop. One easy way to do that would be to loosely twist the red
                          and black supply clip leads together, instead of just laying them out
                          on the bench.

                          If you will turn this circuit off by breaking the supply line (while
                          the output current may be in full swing) you have to deal with the
                          energy in the coil being dumped back into the supply rails and passing
                          through the 555 and opamp while the supply voltage is reversed.
                          Adding a Schottky diode (i.e. 1N5818) across the rails would help
                          detour this reverse current out of the ICs (which look like a diode
                          when the supply voltage reverses). Schottky diodes conduct at about
                          half of the forward voltage as silicon junction diodes, like the ones
                          built into ICs, do. The capacitors should do most of the work of
                          absorbing the load's stored energy, and if they are large enough,
                          eliminate the need for the diode.

                          Keep in mind that the added capacitors will also suck a large slug of
                          current when the supply is first connected, so the switching device
                          will have to handle that inrush, and may set an upper limit on how
                          much rail to rail capacitance you can add.

                          I doubt that not having a ground plane has anything serious to do with
                          the operation of this circuit.
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