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0-600 vac to binary

The most sensible of the suggestions here, however, with 240k input and the threshold at say 12v for reliable detection of 24v it only takes a couple of hundred pF coupling from a line carrying 560v to turn it on.

A 25:1 input range is over the top really. If I was monitoring a 560v ac line I wouldn't expect to be told it was active when there was only 24v ac on it.

I think it need some 'ranges'. Resistor links or options or whatever on the live side would make the job easier but it could be done with adjustable thresholds on the isolated side.

Reply to
nospam
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Consider adding a current source (or constant current diode) inside a bridge rectifier in series with your optoisolator. This won't help you for dissipation, but it will help to bring your dynamic range under control.

Reply to
Ralph Barone

Actually it might. A 1 mA current source will draw the same current at 600 VAC which will be only about 600 mW.

Also, you can use a single series diode and only draw current on the positive excursions of the AC line, cutting power in half. The difficult part may be getting a device that can withstand the 900 volts peak of 600 VAC.

I also like the idea of generating narrow pulses to the opto when voltage is present.

And it should be possible to design a triac or SCR circuit to produce perhaps 10 VDC to a 10k resistor and the opto, for all values of applied voltage by using phase firing. That would also be rather efficient and inexpensive.

Paul

Reply to
Paul E. Schoen

so you'll know in advance if it's a 600V or a 24V circuit ? you could design in a neon-based opto-isolator for the voltages above

100v and use the LED based one for the lower voltages.
Reply to
Jasen Betts

How about a PTC thermistor on the LED so that it can live through having the 600V connected. Beware that the PTCs that claim a 600V rating on the front of the data sheet can really only stand about

100V.
Reply to
MooseFET

To safely detect 560 VAC, your circuit board will have to be insulated for 1 kV, and there are safety considerations that will constrain your optoisolator choice.

The impedance problem (600V doesn't match a low-power optoisolator with LED input) can be dealt with using a primitive switching supply, i.e. a relaxation oscillator. A PUT or diac plus low voltage capacitor can be run up in voltage using a resistor and diodes, at very low current. When threshold is reached, the device fires, discharging the capacitor (through your optoisolator's LED) at a goodly (2 mA?) current.

Thus, the AC input will generate a pulse train into the optoisolator, roughly indicating the AC voltage by its frequency. Either multiplex fast, so as to catch the pulses, or latch the pulses (and clear the latches each scan cycle).

My preference would be for a safe distance from the fire-resistant fused grounded box that has 560VAC input to the logic box that sniffs for the logical presence of power.

Reply to
whit3rd

Use a fusible resistor and it will auto-set to 600VAC (once!). ;-)

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Please don't top-post, it's terribly rude.

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Reply to
Bob Larter

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