I'm building a small circuit for measuring high(ish) voltages with an oscilloscope. It's based on an isolation amplifier that measures the voltage across a measurement resistor (r3 below), attenuated by dividing with R1 or R1+R2, depending on the range selection. I've scribbled down a picture of the relevant part of the circuit below.
I'm planning to use a toggle switch for SW1, but notice that they have maximum AC and DC voltage ratings. Now, I understand the difference between the two ratings (arcs self-quench more easily with AC), but I'm not sure what properties I need from SW1 for my example.
Specifically, R1 is a series chain of resistors adding to about 450kOhm and R2 is much larger. I want to be able to measure 700V pk-to-pk AC (British line voltage if the probes are out of phase). I'm more ambivalent about DC ratings: I don't think I have a 500V DC source lying around… (Incidentally, I'll be using a panel-mount switch for SW1, so the terminals on the PCB can be well separated.)
The question (finally) is what ratings I need for SW1. I assume I'll need ≥1kV insulation strength to protect the user, but how do I characterise the AC/DC voltage rating? I mean, if SW1 is fully open, it will have almost the entire input voltage across it (because R2 >> R1), but R1 = 450k means that, even if V is 1kV, the maximum current that can flow is about 2mA: I assume that arcing won't be a huge problem…
However, I'm not sure how to express that in terms of switch ratings: can anyone tell me what I'm missing?
simulate this circuit – Schematic created using CircuitLab
You should use a switch rated for at least the max voltage and current you expect to see (700VAC and 500VDC in this case). Using a relay is probably the way to go to isolate from the higher voltage. Having a higher current rating shouldn't matter, but here are some options that look like they meet your spec w/ relatively low current ratings: