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

Re: [beam] Re: BBPV Foolery

Expand Messages
  • wilf rigter
    Hi Wolfgang, I always socket my chips just in case of such fingertrouble . I suggest you build and test the BBPV on a protoboard to prove that the components
    Message 1 of 8 , Aug 16, 2003
    • 0 Attachment
      Hi Wolfgang,
       
      I always socket my chips just in case of such "fingertrouble".
       
      I suggest you build and test the BBPV on a protoboard to prove that the components in this simple circuit all work together and drive the motors hither and yonder. 
       
      The BBPV is a little vulnerable to damage if a motor with too low resistance is connected to the first driver stage. That can cause the logic level on the second driver paralleled inputs (connected to first driver outputs) to be near the switching threshold. That can cause up to 200ma to flow through the second stage output stage and the circuit can selfdestruct from internal thermal overload.   
       
      A slightly different design uses two inverters in series to process the photo input stage and uses the first and second inverter outputs to drive the respective parallel inputs of 2 groups of 3 inverters used as motordrivers. A small layout drawing is attached.
       
      The addition of the cap changes behaviour in different light conditions like the BeamANT.
       
      wilf
       
      ----- Original Message -----
      Sent: Saturday, August 16, 2003 5:25 PM
      Subject: [beam] Re: BBPV Foolery

      Thank you for that, Wilf. It appears I have a bad chip. Would this
      problem be related to when we melted the photodiodes? Or is it more
      likely to be heat related? I ask because I am debating whether or not
      to buy a DIP socket.

      J Wolfgang Goerlich


      > Check: connect the midpoint of the two series PDs of the BBPV
      > through a small (100ohm) resistor to 240 pin 20 and one motor
      > should turn. Then connect the resistor to 240 pin 10 and the other
      > motor should turn. If OK the PDs are the problem. If not OK the
      > chip is the problem.


      Post Script:
      > wilfred
      Adding a few more letters between us, eh? :)




      To unsubscribe from this group, send an email to:
      beam-unsubscribe@egroups.com



      Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.
    • J Wolfgang Goerlich
      ... Ok, I will use a socket. ... Makes sense. I am assuming that I can build this on a breadboard instead of protoboard to achieve the same objective. ...
      Message 2 of 8 , Aug 17, 2003
      • 0 Attachment
        > I always socket my chips just in case of such "fingertrouble".

        Ok, I will use a socket.

        > I suggest you build and test the BBPV on a protoboard to prove that
        > the components in this simple circuit all work together and drive
        > the motors hither and yonder.

        Makes sense. I am assuming that I can build this on a breadboard
        instead of protoboard to achieve the same objective.

        > The BBPV is a little vulnerable to damage if a motor with too low
        > resistance is connected to the first driver stage. (...)

        Thank you for the explanation. That makes sense, and may be behind
        the problem as the IC does get pretty warm when running. For motors,
        we are using the Solarbotics Gm6. These measure ~400 Ohm. Is this low
        enough to cause a problem?

        > A slightly different design uses two inverters in series to process
        > the photo input stage and uses the first and second inverter
        > outputs to drive the respective parallel inputs of 2 groups of 3
        > inverters used as motordrivers. A small layout drawing is attached.

        Thank you. We will try that when time next allows.

        J Wolfgang Goerlich
      • J Wolfgang Goerlich
        Hello Wilf, I see you used the 74HC240 in the BBPV2 circuit, while Solarbotics used the 74AC240 in the original BBPV. Unfortunately, I am out of HCs, and I see
        Message 3 of 8 , Aug 17, 2003
        • 0 Attachment
          Hello Wilf,

          I see you used the 74HC240 in the BBPV2 circuit, while Solarbotics
          used the 74AC240 in the original BBPV. Unfortunately, I am out of
          HCs, and I see that Solarbotics is sold out as well. I do have ACs,
          and I can order the HCTs. Any recommendations which flavor to use as
          a replacement?

          J Wolfgang Goerlich
        • Wilf Rigter
          Sorry, I meant the generic HC/AC240 chip. The 74AC240 is the preferred type with higher available output current and a relatively symmetrical switching
          Message 4 of 8 , Aug 17, 2003
          • 0 Attachment
            Sorry, I meant the generic HC/AC240 chip.

            The 74AC240 is the preferred type with higher available output current and a
            relatively symmetrical switching threshold (~Vcc/2).

            The high output current makes the motor driver more efficient and powerful
            while a symmetrical input threshold matches up with the balanced
            photobridge output signal near Vcc/2 resulting in straight motion. If the
            240 threshold is lower than Vcc/2 (and it is for HCT/ACT type) one or more
            diodes or a LED can be placed in series with Vcc end of the PD bridge to
            lower the balanced PD bridge output voltage correspondingly. The best way
            to check this is to rplace the PDs with a 10K pot and adjust the pot until
            both motors run at equal speed and then measure the resistance ratio of the
            pot (each end terminal to the middle terminal). That will tell you whether
            some threshold offset compensation is required.

            The GM6 resistance measured with an ohmmeter is about 8 ohms. With Vcc=5 and
            no mechanical loading, the current is 88 ma and the effective motor
            resistance is about 57 ohms.

            From Eric Seale's test data shown here
            http://www.solarbotics.net/library/circuits/driver_24x_comp.html

            the output resistance of a AC240 inverter with Vcc=5V and output
            current=90ma is about 10 ohms. With 4 in parallel you get abut 2.5 ohm.
            With a 8ohm there should be no problem with the 8 ohm loaded AC240 output
            driving the input of the next stage attached to the same point. The problem
            may be that the BBPV without the feedback resistor oscillates at >>1MHz and
            now winding capacitance and especially a cap across the motor winding can
            cause muach higher AC currents to flow with a higher voltage drop. Also
            resonance can boost the voltage at the output until the protective internal
            clipping diodes operate with more power dissupation and the chip can quickly
            selfdestruct. The modified BBPV2 isolates the motor driver inputs and
            provides a positive feedback cap (0.22 uf) to limit the frequency to < 1KHz.
            So the motor drivers are always driven with low frequency Vcc or 0V signals
            minimizing the chip heating. The trade of is one fewer inverter in the
            parallel motor drivers so that the effective output resistance is 3.3 ohm
            instead of 2.5 ohm.



            wilf

            ----- Original Message -----
            From: "J Wolfgang Goerlich" <jwgoerlich@...>
            To: <beam@yahoogroups.com>
            Sent: Sunday, August 17, 2003 9:22 AM
            Subject: [beam] BBPV2 -- HC Versus AC


            > Hello Wilf,
            >
            > I see you used the 74HC240 in the BBPV2 circuit, while Solarbotics
            > used the 74AC240 in the original BBPV. Unfortunately, I am out of
            > HCs, and I see that Solarbotics is sold out as well. I do have ACs,
            > and I can order the HCTs. Any recommendations which flavor to use as
            > a replacement?
            >
            > J Wolfgang Goerlich
            >
            >
            > Yahoo! Groups Sponsor
            > ADVERTISEMENT
            >
            >
            >
            >
            > To unsubscribe from this group, send an email to:
            > beam-unsubscribe@egroups.com
            >
            >
            >
            > Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.
            >


            ---
            Outgoing mail is certified Virus Free.
            Checked by AVG anti-virus system (http://www.grisoft.com).
            Version: 6.0.504 / Virus Database: 302 - Release Date: 7/24/2003
          Your message has been successfully submitted and would be delivered to recipients shortly.