#99541

开云体育

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.

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

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

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

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.? 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.

?

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

Bertho

?

#99546

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

wn4isx

#99547

My 260-8 has these DC ranges: 0-1-2.5-10-25-50-250-500-1000V

?

I like the solution to the wrong termination resistance....in some ways I wish my HV probe was like yours, I own a Pomona Electronics Model 3157. This is a HV probe with build in meter. The probe designed for plug in operation presents a much lower load on the HV source, mine has a built in "sort of accurate" analog meter.

?

Back when I played with 6502, 6914, 6914A and 7404 image converter tubes, I had to use a borrowed electrostatic voltmeter, talk about high impedance....

[Rare and pricy!]

?

BTW Pomona makes extremely high quality test leads and oscilloscopes and even nicer "Pomona Blue Enamel 2397 Electronic Project Boxes"

?

But be prepared to pay premium prices.....

#99548

Roy,
The capacitor had an internal short.  It measured 240nF with a Q of zero.  It is supposed to be 1?F +/- 3%.
This was a very well-done service manual, 34 pages of pictures, schematics, disassembly procedure, and troubleshooting.
Well-done LG except that diode test.
The scope probe is Tektronix P6015 rated at 12kV.
Bertho

Show quoted text

-----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

#99549

开云体育

For very interesting reading, see the works of Marie Curie.

She was measuring nA currents 130 years ago.? It is incredible to see the detailed work done with the quadrant electrometer and piezo electricity.

The working conditions were also poor.

Bertho

?

Show quoted text

From: [email protected] <[email protected]> On Behalf Of wn4isx via groups.io
Sent: 20 December, 2024 13:19
To: [email protected]
Subject: Re: [electronics101] High Voltage Probes

?

My 260-8 has these DC ranges: 0-1-2.5-10-25-50-250-500-1000V

?

I like the solution to the wrong termination resistance....in some ways I wish my HV probe was like yours, I own a Pomona Electronics Model 3157. This is a HV probe with build in meter. The probe designed for plug in operation presents a much lower load on the HV source, mine has a built in "sort of accurate" analog meter.

?

Back when I played with 6502, 6914, 6914A and 7404 image converter tubes, I had to use a borrowed electrostatic voltmeter, talk about high impedance....

[Rare and pricy!]

?

BTW Pomona makes extremely high quality test leads and oscilloscopes and even nicer "Pomona Blue Enamel 2397 Electronic Project Boxes"

?

But be prepared to pay premium prices.....

#99550

Fifty+ years ago , I tried using my Simpson 260 series 3 on the 5000V range to measure a microwave oven.

When the mushroom cloud dissipated there wasn't much left other than the bakelite case.

At least I had sense enough not to be near the meter or wires when the circuit was energized.

Hugh Gilbert

Show quoted text

On Fri, Dec 20, 2024 at 1:18?PM wn4isx via <wn4isx=[email protected]> wrote:

My 260-8 has these DC ranges: 0-1-2.5-10-25-50-250-500-1000V

?

I like the solution to the wrong termination resistance....in some ways I wish my HV probe was like yours, I own a Pomona Electronics Model 3157. This is a HV probe with build in meter. The probe designed for plug in operation presents a much lower load on the HV source, mine has a built in "sort of accurate" analog meter.

?

Back when I played with 6502, 6914, 6914A and 7404 image converter tubes, I had to use a borrowed electrostatic voltmeter, talk about high impedance....

[Rare and pricy!]

?

BTW Pomona makes extremely high quality test leads and oscilloscopes and even nicer "Pomona Blue Enamel 2397 Electronic Project Boxes"

?

But be prepared to pay premium prices.....

wn4isx

#99552

Oooo mushroom cloud...so cool, my favorite man made cloud.

?

My first attempt at a quasi comp amp based on 2N3055 worked fine with Jefferson Airplain White Rabbit...right up until "Tell 'em a hookah-smoking caterpillar", made it halfway throiugh catepillar and became a genuine a firegod sacrifice with a nice, if rather small, mushroom shapped cloud.

?

I was at school using their scope to check for ultraonics....

?

My first decent self made, self etched PCB, amp, and power supply all went up nicely.

?

It was also so silent.... until the heat sensor over me went off.

?

This was way pre-smoke detector.

?

It was so much fun explaining to the firemen "Well you see it was an amplifier..." their looks were priceless.

?

And yea I really should have used a fuse in the V+ power rail.

?

The post mortum suggested both 2N3055 shorted.

?

Fuses are mandatory, not an option.

?

#99555

On Fri, Dec 20, 2024 at 04:19 PM, wn4isx wrote:

Oooo mushroom cloud...so cool, my favorite man made cloud.

I call it the genie was released. Problem with all these electronics is that once the genie escapes, it does not work its magic.

?

BTW, I designed up a zener diode checker using an LED driving circuit. Works well for those HV diodes. I am currently redesigning a HV CRT display power supply at work. The zener diode checker will work with zener diodes up to 130V breakdown. Furthermore, it is nice as it has different current ranges so one could also use it to reverse test a basic diode. I even used it to debug my LED Christmas light string. I will have to post it some day.

wn4isx

#99557

To check LED light strings make a hi-z JEFT front end for an AF amp, slide along the cord and the hum should drop when you reach the open bulb.

?

I once had a commercial device called a or the "Ticker" that used this principal. I loaned it and the person moved before returning it.

?

Never ever loan illreplaceable tools.

?

wn4isx

#99558

I just checked a non contact voltage detector on our ancient series Christmas tree lights.

[They were my wife's family, I'd toss them if it were left to me. I hate series lights!]

Anyway, the string was dead, as expected. The first pass didn't? catch anything. probably because the hot was on the 'cold' side. I reversed the AC plug and the 5th bulb was blown. Easy peasy.

Now if I can just get her to change to LEDs.......