Re: [beam] Re: BBPV Foolery
- 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 -----From: J Wolfgang GoerlichSent: Saturday, August 16, 2003 5:25 PMSubject: [beam] Re: BBPV FooleryThank 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.
Adding a few more letters between us, eh? :)
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> 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 thatMakes sense. I am assuming that I can build this on a breadboard
> the components in this simple circuit all work together and drive
> the motors hither and yonder.
instead of protoboard to achieve the same objective.
> The BBPV is a little vulnerable to damage if a motor with too lowThank you for the explanation. That makes sense, and may be behind
> resistance is connected to the first driver stage. (...)
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 processThank you. We will try that when time next allows.
> 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.
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 that Solarbotics is sold out as well. I do have ACs,
and I can order the HCTs. Any recommendations which flavor to use as
J Wolfgang Goerlich
- 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
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.
----- Original Message -----
From: "J Wolfgang Goerlich" <jwgoerlich@...>
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
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