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and motor spike diodes? (Re: [DPRG] Motor noise caps)

Subject: and motor spike diodes? (Re: [DPRG] Motor noise caps)
From: William Harold Newman william.newman at airmail.net
Date: Fri May 24 15:12:17 CDT 2002

I have no answer to Clay's question, just a slightly related question.
Looking at the datasheet for the SN754410 "quadruple half H-driver"
chip, the circuit diagram on page 2 shows it as having built in diodes
to short motor voltage spikes to VCC2 and GND, but then the
"Application Information" diagram on page 6 shows it being used with
extra, external diodes.

Is it important to use the external diodes? If so, why aren't the
internal diodes sufficient?

Also, I think I dimly remember a recommendation somewhere (for another
H bridge, but presumably the same principle) that such diodes should
be Schottky diodes. Why? And would the diodes built into the SN754410
be Schottky diodes?

On Fri, May 24, 2002 at 12:12:37PM -0700, Clay Timmons wrote:
> A question to my fellow robot builders.
> 
> I've often seen a capacitor placed across motor terminals
> which is supposed to help suppress the high freqency noise
> coming from the sparky little motor brushes.  I'm looking
> for opinions on this...
> 
> 
> Which is better; a single cap across the motor terminals
> or two caps, one from each motor terminal to the motor case?
> 
> What size caps should be used?
> 
> Are such caps appropriate for motors being drvien with PWM?

-- 
William Harold Newman <william.newman at airmail.net>
"When I am picking problems to work on, ones that stumped John von
Neumann go at the bottom of the stack." 
  -- <http://www.daviddfriedman.com/laws_order/index.shtml>, ch. 8
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