[oopic] Re: H-Bridge issue...maybe.

> The part a bout the MOSFET and relay seems like the most solid
solution.
> Would I be correct in assuming that you were suggesting that the
relay would
> be an open switch when either the OOPic was off, or when the OOPic
was
> starting up, including the 500 delay (which I have included after
seeing it
> so highly recommended here by you and others). Then I would close
the relay
> via the MOSFET, and leave it closed all the time that the OOPic was
running.
> This could be accomplished using a logic level MOSFET, which I
probably have
> lying around. Is there any particular relay you would recommend?
>
> I'm inclined to actually go in that direction first simply because
it seems
> more definitive than using the resistors to bias the input. Perhaps
that's
> not the best reason to choose one technique over another. The
resistors
> would certainly be cheaper, but neither solutions is actually
expensive, so
> I tend to favor the definitive one. Is that wrong?

I guess I am nervous about a situation where the H-Bridge needs an
active signal to STOP the motor. The H-Bridges we have discussed
before all stopped when the PWM stopped. Or, failing that, we could
apply the brakes by suitably wiggling the 2 control lines.

So, yes, I am thinking that the relay (with an adequate contact
rating) will always control the 'master' power to the H-Bridge. This
implies that there could be other things in the circuit that might
drop out the coil. Maybe a panic button/switch?

You need to decide on a coil voltage and since you don't have 12V, it
will probably have to be 5V. As a result, you want the coil current
as low as possible. And don't forget the reverse biased 1N4001 diode
across the coil to limit the kickback voltage.

I started at DigiKey and things didn't look easy. I went to Mouser
and they have a much nicer way to look through relays that are
actually in stock. Still better would be to get a paper catalog (or
even a .pdf) and search until what you like is what they stock.

Richard

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