On Mon, Apr 05, 2010 at 05:26:12PM +0000, Andy Pugh wrote:
> On 5 April 2010 13:38, Erik Christiansen <[email protected]> wrote:
>
> > A) Is it safe to interrupt Motor Power to my 7i40s, with hairy great
> > inductive motors on their outputs? (OK, mostly resistive under load,
> > but then back EMF might also be deadly if V+ is open-circuited?)
>
> I don't know how relevant this is to your situation and the 7i40s but
> I found that an effective way to prevent relays exploding on my
> Robotwars robot (3 x 24V 750W motors, peak current 300A) was to wire
> conventional (and inexpensive) bridge-rectifier modules into the
> circuit with the AC terminals across the motor, and the +ve and -ve to
> the batteries. That way any voltage > Vcc + 0.6V is shunted back into
> the supply lines.
Ouch!
Here we only have the big filter caps in the motor supply. They'll shunt
a transient, but the field energy of a rotating machine is what the
automotive bods call a "load dump". On just a 12v system, that goes up
to 80v for tens of milliseconds, unless absorbed.
Nearly 30 years ago, I built a "super-transorb" for an automotive
manufacturer, to do that. It wasn't much more than a (e.g. 20v) zener,
base to emitter resistor, and a fat bipolar power transistor across the
supply. One does the whole system.
I could supplement the 7i40 overvoltage protection with one of them.
Erik
--
The surest way to corrupt a youth is to instruct him to hold in higher
regard those who think alike than those who think differently.
-- Nietzsche
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