Loading ...
Sorry, an error occurred while loading the content.

To Robert Moro, Re: [SeattleRobotics] Questions about DC supply and motors

Expand Messages
  • Carlos F Lopez
    Hi Robert, thanks. Just a question, you mean to use the LM2940 instead of the 7805? I read it protects the circuit, which is nice, but a drop of voltage is no
    Message 1 of 10 , Jul 4, 2005
    • 0 Attachment
      Hi Robert, thanks. Just a question, you mean to use the LM2940 instead
      of the 7805?
      I read it protects the circuit, which is nice, but a drop of voltage is
      no exactly desireable. My acid battey gives 6V when fully charged, with
      the 7805 directly, the logic has about 4v, which is why I want the
      booster so that the 7805 can give to the circut cosntantly 5V even when
      the battery maybe would be something discharged and giving just about 4V.

      The idea is to set the booster first connected to the battery, to have a
      voltage of about 9V when the battery is fully charged. To set the 7805
      at the booster output to have 5V constantly up to the case when the
      battery would be giving 4V and the booster maybe about 7V. (When the
      input voltage of the 7805 is less than 7V, then the output voltage is
      less than 5V).

      I know I'm complicating this too much, but the idea is to have a logic
      that would be working for a while. Maybe the LM2940 is enough, it'd
      start giving me about 5.5v for the logic which is nice, until the
      battery discharges about 0.5V, that is enough time for the robot to run.

      Just asking about your experience, is this last point true for the
      LM2940? If it is then I think I'd use it and make my life easier.

      Thanks for that advice. :)



      Robert Moro wrote:

      >On 6/29/05, Carlos Federico <loscf79@...> wrote:
      >
      >
      >>I'm using an acid battery of 6V 1.3AmH. Motors are feeded from the
      >>battery, through an L293D. The logic is feeded through a 7805.
      >>
      >>As many is guessing, I have issues to feed in this way my logic, the
      >>7805 it's meant to be used with an input of at least 7v. So I was
      >>thinking in using a DC to DC boster with the MC34063.
      >>
      >>I tried with a 9v battery for the logic, it didn't last long just one
      >>minute. So maybe the booster is the option. I'm thinking in setup the
      >>MC34063 for 9V 1A at the output (the chip supports 1.5A peak)
      >>
      >>
      >
      >There is no need to use a booster. Use a 1A Low Dropout Regulator like
      >the LM2940 instead.
      >http://cache.national.com/ds/LM/LM2940.pdf
      >
      >
      >Visit the SRS Website at http://www.seattlerobotics.org
      >Yahoo! Groups Links
      >
      >
      >
      >
      >
      >
      >
      >
      >
    • Carlos F Lopez
      Thanks for the big help guys. Every motor draw about 0.5A when they start, then the number lowers at about 0.3A, so maybe the circuit has a short somewhere.
      Message 2 of 10 , Jul 4, 2005
      • 0 Attachment
        Thanks for the big help guys.

        Every motor draw about 0.5A when they start, then the number lowers at
        about 0.3A, so maybe the circuit has a short somewhere. The 7805 has the
        correct polarity. What it worries me is that the logic circuit works
        fine fine, the robot works fine when I use the external power supply,
        but the 7805 is heating up too much and of course the consumed current
        seems to be too much.

        When I try to use the acid battery and do the same process, to connect
        every polarity cable when everything is ready I do have also a spark,
        but smaller. That was before I do the ON/OFF switch. Now that the
        switch is there, the spark could not be seen of course, but the problem
        should be there anyway.

        I'm about to check the circuit or even better, to do the whole circuit
        again to add more elemnts, I need more sensors and I still has an
        unsused port in my MCU, so it's a good opportunity.

        Thanks Tom,
        Carlos


        Tom Capon wrote:

        >Triffid is right--the H-bridge chip (L293D) cannot by itself provide
        >the variable speed control you are looking for. You need to have a
        >microcontroller send a PWM (pulse width modulation) signal to the
        >L293D, and by modifying that signal in the program running on the
        >microcontroller you can make the motor run at different speeds. Do a
        >Google search for "PWM motor control" and read whatever you can find
        >about it.
        >
        >The spark occurs because there is probably a short circuit somewhere
        >in the robot. You sometimes get sparks when directly connecting a
        >lead-acid battery to a motor that draws a lot of current, but unless
        >you're motor controller starts up in with the motor in the on state,
        >the spark is a very bad sign. Espeically since it takes at least 2 or
        >3 amps to make a spark at all, and the L293D itself probably cannot
        >handle a motor that draws that much current, if it's the motors
        >causing the spark.
        >
        >When you test it with the external power supply, do you connect it the
        >same way as you connect the batteries? Or do you bypass some part of
        >the circuit (like the On/Off switch)? It might be part of the battery
        >hook-up circuit that is causing the short.
        >
        >How much current do you're motors draw? If it's under 1.5 amps per
        >motor, then you know there is a short because the motors themselves
        >could not cause the spark.
        >
        >Make sure you have the 7805 connected with the correct polarity, and
        >that it is actually regulating the voltage to 5V. If they get in
        >backwards they fry, but you might inadvertently get a voltage out of a
        >fried one.
        >
        >Good luck! --Tom Capon
        >
        >On 6/30/05, Triffid Hunter <triffid_hunter@...> wrote:
        >
        >
        >>Carlos F López wrote:
        >>
        >>
        >>>You're right, it's 1.3Ah, sorry.
        >>>
        >>>And for the logic something's strange. I've checked the circuit and it
        >>>looks fine, and it works fine, but I still do feel something's wrong.
        >>>When I test the circuit with an external power supply it works fine, but
        >>>if I switch ON the robot first and then connect VCC and GND (in any
        >>>order, first one cable and then the second), when I connect the second
        >>>cable a spark comes out. Any idea when this happens? BTW, the 7805 heats
        >>>up too much but everything works fine, it has a heatsink anyway. I'm
        >>>about to add more elemtns to the circuit, maybe I'm going to remake
        >>>again the whole PCB.
        >>>
        >>>
        >>yep you have either a short circuit on your board, or you have an ic shorting out either cos its dead or due to design fault..
        >>
        >>
        >>
        >>>About the H Bridge, I'm still reading about them. I don't know if it's
        >>>possible what I want to do with this chip. Or, is there another way to
        >>>make such tasks? To control the speed of the motors for nice curves
        >>>rather than just stop one motor. Maybe there is a way to make that with
        >>>the L293D but I still don't know how.
        >>>
        >>>
        >>find out all about "PWM". when you've wrapped your head around that, try "PID" ;)
        >>
        >>you can implement a simple analog voltage to pwm with a 555 and a dual op-amp if you want, though its easy to do it in software too ;)
        >>
        >>
        >>
        >>>Thanks.
        >>>
        >>>
        >>Visit the SRS Website at http://www.seattlerobotics.org
        >>Yahoo! Groups Links
        >>
        >>
        >>
        >>
        >>
        >>
        >>
        >>
        >>
        >
        >
        >Visit the SRS Website at http://www.seattlerobotics.org
        >Yahoo! Groups Links
        >
        >
        >
        >
        >
        >
        >
        >
        >
      • Tom Capon
        One other thing: Do you have filter capacitors on both the unregulated and regulated sides of the 7805? If you have a large ( 200uF) filter capacitor, it
        Message 3 of 10 , Jul 4, 2005
        • 0 Attachment
          One other thing: Do you have filter capacitors on both the
          unregulated and regulated sides of the 7805? If you have a large (>
          200uF) filter capacitor, it can draw a large amount of current at
          startup when it is charging to full capacity. That could cause the
          spark.

          You should have some filter capacitors, probably a 100uF and a 0.1uF
          on both sides of the 7805 (with appropriate voltage ratings) would be
          good. Much more than that and you might see the excessive startup
          current, less than that and your circuit may be subject to
          interference from noise on the power supply lines. You could even
          drop it to 22uF and 0.1uF; usually battery powered devices use smaller
          filter caps because A) they don't need to filter rectified AC power
          and B) it draws less power on startup, minimizing losses.

          Did you try measuring how much current (with a mulitmeter) is actually
          going into the circuit, both with external supply and batteries?

          7805's are by their nature somewhat inefficient, and it is normal for
          them to require a heatsink to operate up to their design current
          capacity. Test the current and check it with a datasheet just to make
          sure there isn't a problem, though.

          Remember there is also the issue of regulator drop-out voltage: the
          7805 will not operate well (if at all) if it is not supplied at least
          7V. This could be a cause of it heating up, and could explain the
          difference between external and battery operation.

          On 7/4/05, Carlos F Lopez <loscf79@...> wrote:
          > Thanks for the big help guys.
          >
          > Every motor draw about 0.5A when they start, then the number lowers at
          > about 0.3A, so maybe the circuit has a short somewhere. The 7805 has the
          > correct polarity. What it worries me is that the logic circuit works
          > fine fine, the robot works fine when I use the external power supply,
          > but the 7805 is heating up too much and of course the consumed current
          > seems to be too much.
          >
          > When I try to use the acid battery and do the same process, to connect
          > every polarity cable when everything is ready I do have also a spark,
          > but smaller. That was before I do the ON/OFF switch. Now that the
          > switch is there, the spark could not be seen of course, but the problem
          > should be there anyway.
          >
          > I'm about to check the circuit or even better, to do the whole circuit
          > again to add more elemnts, I need more sensors and I still has an
          > unsused port in my MCU, so it's a good opportunity.
          >
          > Thanks Tom,
          > Carlos
          >
        • Tom Capon
          The LM2940 would be used in place of the 7805. What the 0.5V dropout means is that it only needs an input voltage 0.5V greater than the regulated output
          Message 4 of 10 , Jul 4, 2005
          • 0 Attachment
            The LM2940 would be used in place of the 7805. What the 0.5V dropout
            means is that it only needs an input voltage 0.5V greater than the
            regulated output voltage. So instead of needing 7V input like the
            7805, the LM2940 only needs a 5.5V input. This would work much better
            with your 6V batteries--the regulator would still give the regulated
            5.0V until the battery voltage dropped below 5.5V. The battery should
            supply about 6.3V normally, so you will have at least 0.7V of headroom
            for the battery to discharge.



            On 7/4/05, Carlos F Lopez <loscf79@...> wrote:
            > Hi Robert, thanks. Just a question, you mean to use the LM2940 instead
            > of the 7805?
            > I read it protects the circuit, which is nice, but a drop of voltage is
            > no exactly desireable. My acid battey gives 6V when fully charged, with
            > the 7805 directly, the logic has about 4v, which is why I want the
            > booster so that the 7805 can give to the circut cosntantly 5V even when
            > the battery maybe would be something discharged and giving just about 4V.
            >
            > The idea is to set the booster first connected to the battery, to have a
            > voltage of about 9V when the battery is fully charged. To set the 7805
            > at the booster output to have 5V constantly up to the case when the
            > battery would be giving 4V and the booster maybe about 7V. (When the
            > input voltage of the 7805 is less than 7V, then the output voltage is
            > less than 5V).
            >
            > I know I'm complicating this too much, but the idea is to have a logic
            > that would be working for a while. Maybe the LM2940 is enough, it'd
            > start giving me about 5.5v for the logic which is nice, until the
            > battery discharges about 0.5V, that is enough time for the robot to run.
            >
            > Just asking about your experience, is this last point true for the
            > LM2940? If it is then I think I'd use it and make my life easier.
            >
            > Thanks for that advice. :)
            >
            >
            >
            > Robert Moro wrote:
            >
            > >On 6/29/05, Carlos Federico <loscf79@...> wrote:
            > >
            > >
            > >>I'm using an acid battery of 6V 1.3AmH. Motors are feeded from the
            > >>battery, through an L293D. The logic is feeded through a 7805.
            > >>
            > >>As many is guessing, I have issues to feed in this way my logic, the
            > >>7805 it's meant to be used with an input of at least 7v. So I was
            > >>thinking in using a DC to DC boster with the MC34063.
            > >>
            > >>I tried with a 9v battery for the logic, it didn't last long just one
            > >>minute. So maybe the booster is the option. I'm thinking in setup the
            > >>MC34063 for 9V 1A at the output (the chip supports 1.5A peak)
            > >>
            > >>
            > >
            > >There is no need to use a booster. Use a 1A Low Dropout Regulator like
            > >the LM2940 instead.
            > >http://cache.national.com/ds/LM/LM2940.pdf
            > >
            > >
            > >Visit the SRS Website at http://www.seattlerobotics.org
            > >Yahoo! Groups Links
            > >
            > >
            > >
            > >
            > >
            > >
            > >
            > >
            > >
            >
            >
            >
            > Visit the SRS Website at http://www.seattlerobotics.org
            > Yahoo! Groups Links
            >
            >
            >
            >
            >
            >
            >
          Your message has been successfully submitted and would be delivered to recipients shortly.