How can I design an isolated, wide-range digital logic input compatible with AC and DC voltages from (say) 5V-300V?

Question

There are some excellent suggestions on how to create a wide-voltage range digital logic input on this question.

The question is very similar to my needs. I'd like to be able to detect the presence of a voltage (either AC or DC of either polarity, from, say, 5V-300V or thereabouts) and have the input go to a digital input pin of a microcontroller. I particularly like the second solution that suggests current-limiting resistor -> zener diode clamping... but...

In my understanding, an input that can take such high voltages should really be isolated (by a transformer / optoisolator) from a normal low-voltage circuit. How could I create an isolated input like this? Is the limited current and voltage of an input circuit like this enough to drive the input of an optoisolator?

And furthermore, how can I cope with DC voltages of either polarity and AC voltages on that input? I assume that I can put a bridge rectifier straight after the input, but that will cause a decent voltage drop before anything else has happened. Then I know there will be a capacitor in there somewhere to smooth out the input for AC, but I don't understand where it should go, how big it should be and how it will affect switch-on and switch-off times when it's just a plain DC voltage input?

Bonus points if the solution can go down to 3V (or even lower) instead of just 5V. Further bonus points for use of only cheap, standard 'jellybean' parts.

Answer 1

Your specifications lack detail, but let's assume the following:

1. A single threshold voltage, the same for both positive and negative voltages.
   For example, if the threshold is 4V, then anything between +/- 4V input will give a zero, and anything more than +/- 4V would be a one on the digital output.

2. Vin can be +/- 4V to +/- 300V minimum

3. Input is isolated with no ground reference

4. No input power supply needed

You could try a circuit like this:

^{simulate this circuit – Schematic created using CircuitLab}

I've used a very similar circuit to provide zero crossing detection on mains voltages. The input threshold should be a little under 4 V with this opto.

The CPC3982 is a depletion mode FET that works as a constant current source for the opto once the input voltage is high enough. R1 should be selected to give the low current opto 500 uA. This limits the power dissipation in the FET to around 150 mW at 300 V input. When the voltage input is low (under 5 V) the FET will be turned on. At about 2-3 V the opto current will be low enough to register a zero on the digital ouptut, though this will depend on the value of R1. You can increase the low voltage threshold by reducing R2.

Answer 2

A circuit that I have used in the past is a simple BJT. This will detect positive DC voltages, or AC voltages as a series of pulses.

^{simulate this circuit – Schematic created using CircuitLab}

Note that Q1 and D1 together keep the base voltage to within ±0.7 V, even if the input swings to -300 V for any reason.

R1 sets the collector current at saturation. This, combined with the minimum current gain of Q1, determines the minimum base current. Minimum base current and minimum input voltage determines R2.

Note that R2 must be able to withstand the full input voltage. Use multiple resistors if necessary.

C1 is optional. It provides a low-pass filter that helps mitigate switch bounce as well as ESD. 10 nF gives a time constant of 10 ms, but you can vary this as needed for the application.