What is the normal way to measure high voltages with a multimeter? I have been thinking about the problems with some meters and how to harden them. Plugging the probes into the wrong connectors of the meter or having the meter on the wrong setting may cause a problem, not to mention not all meters may actually meet the ratings marked on their cases. It seems with a little circuit, we could address these problems.
Obviously, I have made my own HV probes in the past for an oscilloscope. There I was looking for wide bandwidth in the several MHz. For a multimeter, looking at mains, I would think even 1000 Hz is more than enough. This should really ease the design.
The following article talks about high voltage probe designs:
http://repairfaq.cis.upenn.edu/sam/hvprobe.htm
In here they show a simple attenuator:
To design the voltage divider, the input impedance of the meter must be taken into account. There is a minor but significant difference between DMMs and VOMs.
DMM: Z-in is usually constant, often 10M ohms.
VOM: Z-in is the voltage range (full scale) times the ohms/volt rating of the meter.
Here is the basic circuit:
High Voltage <------/\/\/\/\/\---------+-------------> + to DMM/VOM
R1 | |
\ \
R2 / R3 /
\ \
/ /
| |
Ground Clip <-------------------------+-------------> - to DMM/VOM
What I don't like about this approach is if R1 arcs, bad things may happen.
This article is more about using a meter on mains. He considers an arc condition and uses fuses and GDTs to control it. He warns about the need for a good return path.
http://www.marcspages.co.uk/tech/5103.htmWhat I again don't like about both of these approaches is the you could swap the in and out of the attenuator, just like you could swap the leads of the meter. In both of these circuits, this may cause problems.
I have two thoughts about this. Using the second article, the design could follow a standard T attenuator. This would allow you to swap the in and out with the same performance. What I don't like about this is we still have a low impedance path to the meter. Maybe this is a good thing, but I would rather have the meter isolated. It seems like using a balanced T attenuator would be a good solution. Voltage ratings could be set high enough to handle a single point failure. You would be giving up noise and resolution but I am not sure it would be important when looking at high voltages.
The center could be made from multiple R's to handle an open condition (current to meter could still be very limited) and clamped to limit the voltage to the multimeter.
I scribbled some calcs and checked the parts. Seems like it could be fairly inexpensive to make. I don't like that the resistors are not isolated, but the multimeter itself is not isolated.