09-06-2024, 03:17 PM
Obviously measuring the negative supply is also less simple. One approach is an isolated power supply for the digital meter and reference the PSU 0V to the negative output, then measure the system 0V rail. A situation where a mechanical meter is simpler.
The LM3900 can connect to negative voltages. So can a regular op-amp with suitable biasing. The LM3900 has four "Norton" amps in it. I might use the fourth with a 99 M Ohm (3 x 33 M Ohms) input as buffer to measure the "Breakdown" voltage out, which may be 700V via 4x 1M resistors. Regular common leaded resistors are only 200V rated. Could be the voltage rating of the trimming spiral. Resistors are not just about power rating, tolerance, noise and temperature stability, because they have a breakdown voltage. I bought a bunch of 33M resistors to make an EHT probe. I forget how many kV each they are. The theory was that three is 99M so nothing for a 1M DMM and a bit more than 1M final for a 10M, 11M or 20M input DMM.
I've some basic LED voltmeters coming on Monday (tomorrow) too. I'll see what input resistance they are.
The LM3900 can connect to negative voltages. So can a regular op-amp with suitable biasing. The LM3900 has four "Norton" amps in it. I might use the fourth with a 99 M Ohm (3 x 33 M Ohms) input as buffer to measure the "Breakdown" voltage out, which may be 700V via 4x 1M resistors. Regular common leaded resistors are only 200V rated. Could be the voltage rating of the trimming spiral. Resistors are not just about power rating, tolerance, noise and temperature stability, because they have a breakdown voltage. I bought a bunch of 33M resistors to make an EHT probe. I forget how many kV each they are. The theory was that three is 99M so nothing for a 1M DMM and a bit more than 1M final for a 10M, 11M or 20M input DMM.
I've some basic LED voltmeters coming on Monday (tomorrow) too. I'll see what input resistance they are.
