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From a bespoke board I'm receiving 18vdc that I need to switch. This 18vdc will usually be +18vdc but at times can be -18vdc. The load is .35A. The polarity is being changed via a H-Bridge and unfortunately it's not possible to do the switching before or using the H-Bridge (I can't change the existing board).

I'd like to switch with a logic level (5vdc ideally) signal, and I've only got a single GPIO to play with. At present the best plan I can come up with is to use a non latching EMR, but would really like to avoid this due to noise which will be factor.

Switching at no more than a 2-3Hz at most.

Is there another solution I'm overlooking?

chip
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  • can you send it though a bridge rectifier to normalize the polarity? you can use a DC-DC converter after the bridge if 2 junction drops is a deal-breaker. – dandavis Jun 18 '20 at 16:49
  • @dandavis unfortunatenly not, the load is an electromagnet, and I need to keep the polarity as is. Hacking at an old automation system... – chip Jun 18 '20 at 16:53
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    you could put in your own hbridge to mimic the polarity after the bridge, wouldn't take any "smarts"; an opto and hex inverter could switch direction. I assume triacs are out. Maybe you could fallback to the EMR only for negative, using a FET most fo the time. – dandavis Jun 18 '20 at 17:02
  • Or put the "switch" *before* the H-bridge. – rdtsc Jun 18 '20 at 17:43
  • @rdtsc: what kind of switch? do you know of something we don't? – dandavis Jun 18 '20 at 21:19
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    Dremel a notch out of the +18v trace going to the H-bridge, scrape off the resist, and solder in a suitable PEMOSFET. Tie a 100k from +18v to gate, and 1k to an NPN collector. Emitter to ground, base to 1k to GPIO. Low = H-bridge power off. – rdtsc Jun 18 '20 at 23:16

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