开云体育

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Roy J. Tellason, Sr. via groups.io
Sent: 20 December, 2024 11:24
To: [email protected]
Subject: Re: [electronics101] High Voltage Probes

On Wednesday 18 December 2024 08:30:23 pm Bertho wrote:

There have been several comments on high voltage measurements and
probes buried in various messages.

While fixing my microwave, I needed to measure the high voltage output
from the transformer.

I have never seen the need to measure that. And I've worked on a few of those.

As many have mentioned, a regular DVM cannot be used at that high voltage.

Nope. But surprisingly a Simpson 260, at least as old as the one I have, will measure up to 5000V. I've never used those particular input jacks.

I have a 1000 to 1 high voltage oscilloscope probe, but it is designed
for a 1MΩ load, not the 10MΩ of the DVM.

I have a "HV Probe" bought back when I wanted to measure the "2nd Anode" voltage in CRT type TV sets, back when I was working on those. It'll measure up to 40KV. A rather impressive-looking beast. :-)

Adding a 1.11MΩ resistor across the input to the DVM compensated for
the input resistance.

Right.

I double checked measuring the 120AC. The transformer output was 2,380VAC .

As usual do not do it if you really do not know what to do.



It was a bad high voltage capacitor.

Bad how? Open circuited? Changed value?

Surprisingly, the very detailed
service manual had a surprising error: It described testing the high
voltage diode in forward and reverse with a regular DMM using the
diode test mode.

I think I've encountered *one* microwave that came with a schematic. None at all with anything that could be described as a service manual.

It will read open both ways since the forward diode drop is typically
8V because of the stacked diodes.

I'd wondered what it would take to forward bias one of those...

--
Member of the toughest, meanest, deadliest, most unrelenting -- and ablest -- form of life in this section of space, a critter that can be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James M Dakin