DPRG mailing list archives: [DPRG] Motor backlash / Back EMF

Thu Jan 24 21:32:34 CST 2008

On this topic of motor backlash / bouncing ground -- I'm confused due  
to recent experimentation :)  Btw, this thread was really great  
stuff, thanks!   I've been working on a first bot build which  
naturally is going slowly and roughly. :)  The source of confusion is  
a BS2 resetting about 50 ms after it turns on a mosfet to drive a motor.

Details: BS2 on a proto board powered by 9V batt, small brush DC  
motor (mabuchi FA130) powered by a 3V source (2xAA).  I'm using a  
single NPN MOSFET (IRF510A if that matters) to power the motor.

* One motor input is connected to +3V, and the other input to the Drain.
* The Gate is hooked to a pin on the BS2 and is also pulled down to  
ground thru a 10M resistor (err... I think so... maybe I took this  
out?).
* The Source going to a common ground
* The wires between the motor and the board are long (4").   The  
wires from the AA pack to the board are also long (5")

I've tried running the Source pin to ground at the 3V battery input  
to the board, to the 9v batt (-) terminal, to the BS2's Vss pin, etc.  
None of these mattered; the BS2 reset shortly after attempting to  
power up the motor (versus turning it off as described in the  
thread).  It would not reset with the motor disconnected but the 3V  
pack in place.  The scope showed a voltage dip across the 3V pack as  
one might expect but didn't see any dip on the 9V batt. Wasn't sure  
what to look for as far as a bouncing ground until after I fixed it  
and came back and re-read this :)

The fix was a 100uF electrolytic cap across Vdd to Vss on the BS2.  
(Vdd is regulated).  I had a .1uF on hand for decoupling but it  
didn't do a thing...  I have looked at the circuit (accursed wire  
wrap) over and over and can't find any wiring goof ups...

Does this sound more like I've goofed the wiring and something is  
drawing too much current from Vdd (I don't see how but...)?

Or is it some mystical inductive / EMF voodoo causing an apparent  
brown out / bouncing ground / etc. ?

I'm at a loss to explain what is going on and thought I'd ask the  
gurus :)

Many thanks!

Michael
---
Michael Shimniok - 1986 Jeep Grand Wagoneer "Troubled Child" - FSJ  
Blog @ http://tc.wagoneer.net/

On Jan 7, 2008, at 6:01 AM, Triffid Hunter wrote:

> On Mon, 7 Jan 2008, DeltaGraph at aol.com wrote:
>
>> OK, Chuck,
>> I think I am getting clued in.
>>
>> So really more than IR drops due to loads we are talking inductive  
>> drops or
>> increases of voltage V=Ldi/dt depending on increasing or  
>> decreasing  current.
>>
>> So for an instant, when relay opens and current in motor circuit  
>> decreases
>> very rapidly , Scott's ground lead looks like a battery supplying  
>> at least  a
>> few volts to the ground wire -- got it.
>>
>> If per chance it supplied 5 volts, the voltage supply might drop  
>> to zero --
>> assuming no capacitance at processor.
>>
>> This brings up a question. Could a reasonable capacitor say 10 uF  
>> tied to
>> processor (or real close)  protect processor from the bounce?
>
> That's exactly what decoupling capacitors are for. Also remember  
> that cmos chips only take low currents on average - that low  
> current is taken in very very short sniffs of up to a few amps, and  
> they *need* 100nF or so of high frequency capacitor as close to  
> their power pins as possible to keep their voltage good. Even an  
> inch or two of track introduces enough inductance to cause the chip  
> issues. Ceramics are excellent for this, and MKTs should be  
> acceptable. Electrolytics simply don't have the high frequency  
> response to work here, although they work well in parallel with  
> something that does.
>
> A star grounding system can help too - where each separate  
> section's ground is connected to a single physical point, so all  
> voltages everywhere are relative to that point, and high current  
> sections have no effect on the ground leads of low voltage ones.
>
> Remember, wires at right angles to each other have almost no  
> coupling, whereas parallel wires have (relatively) good coupling.  
> Keep this in mind when running any high current wires. Also,  
> twisting wires with their corresponding grounds helps keep noise  
> down as the currents (and hence fields) cancel.
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