AC Current variable trip circuit

[Seekonk]

I was just thinking how a three pin TL431 makes a nice precision relay driver.  It trips at about 2.5V  and the ACS712 would be at 2.5V with a 5V supply.   Add a pot and you have something.

[timb]

I was just thinking how a three pin TL431 makes a nice precision relay driver.  It trips at about 2.5V  and the ACS712 would be at 2.5V with a 5V supply.   Add a pot and you have something.

Not a bad idea. You might even go with a SSR or Triac for a quicker response. You'd still need a circuit to latch the output, otherwise, when the current limit hits and the relay comes on, the current would drop turning the relay off and around and around we go.

[fuzzoli]

I was looking at this from the CR Magnetics website.  The full document can be found here:

http://www.crmagnetics.com/Assets/ProductPDFs/Precision%20Rectifier%20Circuit%20for%20CT%20Signal%20Conditioning.pdf

My thought was to start with their circuit which converts the CT output to a DC signal.  The output of their circuit would feed a comparator to set the trip point, followed by a 555 to set the trip "delay".  There would obviously be a need for some "glue" parts, but my thought was for the core sense/trip circuitry to be micro-free to minimize the risk of failure of the most critical part (i.e. the part that stops the current from flowing).

...more to come...

-Frank

[timb]

Here's a circuit using the ACS712: http://home.cogeco.ca/~rpaisley4/AcDcBreaker10.html

The guy uses it as a breaker for model railroad stuff, but if you use a 120/240VAC relay it should work fine.

[Seekonk]

I was just thinking how a three pin TL431 makes a nice precision relay driver.  It trips at about 2.5V  and the ACS712 would be at 2.5V with a 5V supply.   Add a pot and you have something.

Not a bad idea. You might even go with a SSR or Triac for a quicker response. You'd still need a circuit to latch the output, otherwise, when the current limit hits and the relay comes on, the current would drop turning the relay off and around and around we go.

Details, details.    I had a manager who said his best education Was working at a traveling carnival one summer.  We would go into meetings with a customer.  The customer would ask if the machine could do this.  He would respond, "You could make it do that."   The customer heard what they wanted to hear.  The correct interpretation was .....YOU could make it do that, he wasn't going to do it for you.  One of only three brilliant men I have known in my life.

[sarepairman2]

is there some kind of magnetic material with a sharp trip point?

I.E. saturating a 1:1 transformer with a DC pulse, blocking the AC signal?

[T3sl4co1l]

If you like short circuits, yes...

Tim

[dom0]

Either way, you can't switch AC off by saturating a reactor. If the reactor is in series, saturating it reduces the inductance to ~nil, "switching it on". And if the reactor is in parallel you blow stuff up.

Anyways, fast switching isn't exactly a virtue of reactors anyway, at least not for 50 Hz reactors.

[T3sl4co1l]

You'd have to have an L network (one in parallel, to short it out quickly, another in series to open circuit), and then some active circuitry to do the steering (or at least a carefully magnetized core on the one).

Tim

[Ian.M]

Current transformer monitoring the Variac output + burden resistor feeding a precision rectifier, then either a peak detector if you need a very fast trip or a low pass filter to average over a few cycles.  Next you need a comparator (or two if you want to be able to trip on excessive peak and average current at the same time) with a pot with a pointer knob and a 0-10 scale providing the reference voltage.  The comparator output(s) then drive the RESET input of a D flipflop, with a debounced pushbutton to apply power connected to its clock input and '1' on its D input so that holding in the button cant prevent tripping,  The flipflop should control a two pole contacter that isolates the Variac input.

I see little point in using a true RMS chip.  If you need that level of trip current accuracy, you'd do better to go digital and replace everything between the current sensor and the contacter with a MCU.

[Seekonk]

OP wants say a 5A trip.  At last someone mentions what does that mean.   Is it peak, average, RMS, follow a fuse thermal profile, fast or slow.   Just about nothing is resistive today.  Look at the scope image for a E27 LED lamp.  Notice the spike as a capacitor charges up.  This short duty cycle is typical of most electronics today.  Despite CE regulations I find many good size wall warts are half wave rectification.  So you turn a pot till it shuts down and back off.  Nothing precision there and within the scope of a hobbyist if he has the knowledge to repair anything.

Another idea, use another TL431 to generate 2.5V and use a pot voltage divider to create a 0-2.5V reference.  Then feed that as a reference to a current transformer sense.   Then look at that with another TL431 that triggers a SCR and relay.