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

Subject: and motor spike diodes? (Re: [DPRG] Motor noise caps)
From: R. Bickle rbickle at swbell.net
Date: Fri May 24 15:36:01 CDT 2002

William,

Schottky diodes are of a different construction than silicon diodes. This
permits them to have a much faster switching time than silicon diodes. In
the case of the DC motor flyback application, the longer that the diode
takes to switch, the higher the spike voltage will rise at the motor
contacts as a result of inductive discharge. I have seen these spikes go
over 200V on the scope with no diodes attached. In some cases, these spikes
can damage the MOSFETS or other drivers being used in the H bridge.

Just a guess here, but it may be that in your datasheet, the internal diodes
to not have fast switching times as schottky diodes do, and the engineers
put the extra diodes in for that reason.

A note on the motor capacitors. I have seen this done frequently to reduce
the transients generated by the motor's commutator. Of course, If one is
driving the motor with a PWM signal, then the extra capacitance will serve
to waste some power and round off the input signal to the motor. This will
of course depend on the value of the capacitor(s) and the frequency of the
PWM. Keep in mind that capacitive reactance is xc = 1/(2*pi*f*c). I suppose
you could watch the commutator noise with a scope and a DC supply, and
choose the capacitor values according to the duration of the noise.

Rick J. Bickle


-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org]On Behalf
Of William Harold Newman
Sent: Friday, May 24, 2002 3:12 PM
To: dprglist at dprg.org
Subject: and motor spike diodes? (Re: [DPRG] Motor noise caps)


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
PGP key fingerprint 85 CE 1C BA 79 8D 51 8C  B9 25 FB EE E0 C3 E5 7C
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