How can I trip the current at arbitrary, adjustable values ?

A project log for Mains protection box

Work more safely with mains power

Yann Guidon / YGDESYann Guidon / YGDES 04/22/2019 at 20:137 Comments

The mains input has a standard 2A circuit breaker, which should be able to sustain inrush current from the 400VA isolation transformer. Let's say it is a last resort for dramatic failure and also serves as a simple input on/off switch.

But 500W is a very dangerous power and a lot of damage can occur before the 2A protector trips. I want an electronic circuit breaker that reacts fast, at a current I dial at will (under 1A). Even at 10mA so I can deal with small PSU modules that can handle 1W.

This is quite out of the realm of the "electric devices and supplies" that equip your house and that you can readily buy at a nearby store. I'd love to use existing products for 1) speed of implementation 2) compliance with safety rules 3) cost (I don't think I can make cheaper than others).

There are various types of current sensors :

In addition to the manual switch, the current can be stopped by

Safety would dictate that both are connected in series, you never know :-P

Then comes the other uncomfortable question of : how do you power these switches ?


Yann Guidon / YGDES wrote 04/23/2019 at 13:27 point

I've found the right search term :-D "Current Monitoring Relay"
I've found some hits on eBay but the prices are ... significant.
The dials also don't look to be designed for many changes, only to set or adjust a few times at most.

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Yann Guidon / YGDES wrote 04/23/2019 at 12:51 point

The idea to "hack" a standard differential circuit breaker is exciting, though these devices rely on the input voltage to be "nominal", while I expect to work at very low voltages, which will not provide enough energy for the circuit breaker to trip.
The Hall effect and current transformers seem to be the only remaining choices, which is sad because I'd love to mount standard modules on DIN rails  :-/

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K.C. Lee wrote 04/22/2019 at 22:21 point

If you are dealing with AC, use a current transformer for sensing current.  Play with the shunt resistor and a pot on the secondary side and may be use that to trip the GFCI breaker.  :)

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Ted Yapo wrote 04/22/2019 at 20:19 point

Can you add a circuit after a commercial GFCI that draws a fraction of the current through the ground lead to trip it at a specific, low point? Just an idea, not thought through at all.

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Yann Guidon / YGDES wrote 04/22/2019 at 20:45 point

There is an idea but... I suspect that my earth connection is flawed. I don't want to rely on external devices to work.

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Ted Yapo wrote 04/22/2019 at 22:05 point

Oh, I meant adapt a commercial GFCI as your switch instead of a relay or triac.

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Daren Schwenke wrote 04/23/2019 at 06:16 point

That's a pretty good idea.  You would only need to handle 3ma to trip it. A non-zero crossing triac output optocoupler like the MOC3023 with a series resistor would more than suffice to trip the GFCI and ultimately give you a 3 component, isolated failsafe.

Along similar lines for DC, just drop a hefty mosfet across your power rails and trip whatever protection you have there (fuse or breaker).  Creating your own short circuit to save the rest, with a device that can handle hundreds of amps for a split second.  The latter was my failsafe for an ill-conceived 700W 12v DC bed heater I had concocted. 

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