Current Switch

Google "Current transformer".

Good Luck! Rich

You can use a current transformer, like this:

CT (Current transformer) -- |//| AC ------|//|-----[Load]---+ +--|//|--+ | | -- | | | | | AC ------------------------+ | | +--[Bu]--+ (Bu = burden resistor) | | +--+ +--+ | | ---------- | Bridge +|---+----> To comparator input |Rectifier | | | | [C] | | | | -|---+--- Gnd ----------

The current transformer produces an output current proportional to the current in the primary, which is one of the wires carrying AC to the load that passes through the CT. The CT is connected to a burden resistor, and a voltage is produced across that resistor. You can rectify & filter that voltage and run it to a comparator, then adjust the reference voltage so the comparator switches when you want it to. Use the output from the comparator to control whatever you want to. It's a few bucks worth of parts - or you can buy one ready made:

Ed

Wouldn't it be better to position the burden resistor on the output of the bridge rectifier - it still provides a working load for the secondary of the transformer?

I figure the resistor has a lower probability of going open than the bridge. What would be the benefit of the resistor after the bridge vs. before it?

Ed

The idea behind burden-after-the-bridge is that the constant-current output from the transformer absorbs the error from the diode drops. A following RC filter is all that is needed for accurate sensing.

The value of the burden has to be reduced to allow for the extra 1.5V required by the bridge. There is a slight ambush though because the voltage the secondary actually has to sustain looks like a 3V pk-pk square wave, with the burden half-sines riding on top of it. This increases the core loss in most CT's and reduces the accuracy of the current ratio, especially at low currents.

Thanks! For this application, and assuming high enough current where the bridge then burden setup avoids the low current innaccuracy, would it be any more accurate at detecting and switching at some threshold than the burden, then bridge, then comparator setup? I'd like to read more on the technique - any tips on what I can Google/ what text to read ... ?

Ed

Bridge-before-Burden will be more accurate, but in either case passive rectification would only be used these days for simple indicating instruments.

If there are already pcb and power facilities for a comparator then it makes more sense to simply add an opamp precision rectifier. This gets the best of both worlds... the CT can be run at a low secondary voltage, and the precision rectifier takes care of diode errors.

In fact though I dropped using CT's some time ago, preferring instead to use the LEM closed-loop Hall effect modules. The standard 1000:1 modules have about a 130 Ampere-Turn maximum input, so about 5 turns through the hole would bring it into the 20A full scale range. There used to be a version of the LEM that included an RMS rectifier.

One of the interesting things about most LEM models is their impressively-fast transient response and high frequency response. Another thing, a bit sobering, is to see the pair of power transistors mounted on the package (of some models) and realize they're actively canceling the hole-current through the LEM, and then consider the rather high +/-15V power-supply currents that can result as the LEM cancels the current peaks, while delivering the cancellation current as an output for your use (current-output models).

For example, the LA 305-S can handle 500A-peak, and with a 1/2500 conversion ratio, it can deliver a 200mA secondary-signal output and draw up to 220mA from the power supply. It has a 1us small-signal risetime and it can accurately follow a 100A/us primary-current slewing rate.

A 12-0-12 200mA supply is no problem, nor apparently is the dissipation in the LEM module. The thing to watch out for though is the dissipation in the burden resistor. 130mApk is 91mArms, and at 9.1Vrms output this would be 0.8W dissipation. It usually requires several

Why build it is often cheaper to buy. See:

found with this google search string:

"current sensing switch"