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Author Topic: Measuring voltages 2000 to 3000 DC range. Is divider needed?  (Read 4636 times)

Posts: 199

« on: August 23, 2012, 02:41:53 PM »

I have seen in discussions online that it is not advised to measure voltages over 1000 CD.  Suggested was to use a voltage divider and use a voltmeter in the 1000 V range.
I have old Simpson Voltmeters that have a 6000 DC setting.  And I have been measuring these high voltages with it for 30 years. 
In the interest of safety should I change the way I measure these?
Or is the voltage divider used because modern voltmeters have the highest range of 1000 Volts?

Posts: 3822

« Reply #1 on: August 23, 2012, 03:17:30 PM »

Or is the voltage divider used because modern voltmeters have the highest range of 1000 Volts?

Of the high voltage probes I've seen they tend to come in two flavors:  The large self contained probe with hook attachment suitable for slipping under the cup of a CRT 2nd anode lead that has a small meter in the handle  - or -  the almost as large VTVM / DMM HV probe with a 10 meg glass resistor in the handle.  The second flavor gave a 10X range expansion so a 1kV VTVM became a 10kV VTVM.

In both cases the probes were large and extremely well insulated, which would be my primary concern about using the Simpson... Make sure the probes you're using are stout enough to handle the voltage.  For all I know you're still using the original probes which were rated for the meter but after 30 years I suspect they've been replaced.

BTW:   That's one reason I like good quality probes made with a high flex silicone wire.  Excellent insulation properties and they don't burn when kissed by a soldering iron.

Never change a password on a Friday                

Posts: 199

« Reply #2 on: August 23, 2012, 09:07:51 PM »

On these high voltages I usually use clips to make the connection when the power is off.  Then make sure the wires have some spacing and double check the wiring. .  Then bring up the power with a variac.  I try to stay away from the circuits and read the meter from a foot away.
Then power down before taking off the clips. 
I would guess that the readings are off as the meter is so old.  But sometimes I read the voltage with 2 or 3 of the old meters and take an average.  Sometimes I use a voltage divider and put a new DVM to read half or a third of the voltage if I want a better check on the accuracy.
But I did want to see if there were some other safety reasons not to measure the high voltages.
Thanks, Norm

Posts: 9304


« Reply #3 on: August 24, 2012, 05:47:21 AM »

Around 1970 or so I used a power supply filter method shown in the W6SAI Handbook, with a resonant choke. It was a 3500 volt supply for a pair of 3-400Z or 3-500Z.  The supply was in an old tube tester cabinet below the RF deck.

When I would turn the supply on, the tubes would arc and blow the line fuses. When I disconnected the tubes it would arc someplace else.

I connected a suitably rated meter to the supply output using new HV CRT anode wire. I was on a rubber mat, and kept one hand behind me, and turned the supply on. My fingers were on the metal power toggle grounded to the panel about a foot or two from the wire.

The HV wire blew in two inside the cabinet, and came out of the cabinet and attached itself to my wrist. I actually watched the second hand of the clock go around a few times as I tried to recall how long I had before my brain shut down from lack of fresh blood. When I was safely past the limit, I figured I would live and asked my wife to drive me to the hospital.

I had first through third degree burns, including a half-dollar sized hole in my wrist.

It turned out the resonant choke was "ringing" upon charging and multiplying the voltage. This can happen in a high-Q resonant circuit excited by pulses. I wrote Orr explaining it, and like other bad ideas in his Handbook he ignored it, not adding any corrections or cautions.

Since that time, when a wire flew out of the supply and pasted to my wrist, I've been reluctant to be within reach of anything with HV exposed....especially alone.

I understand QST and the editor of QST thinks it is OK to tell people to run high voltage with covers off while measuring things, but there is a case every few years where some fool gets inside an amplifier with a meter, often being a self-declared "experienced technician", and gets hurt or killed. Sometimes it is with a clip-in connection, but most often it is holding a test probe.

I have seen idiots on forums tell people to measure electrolytic voltages where both meter leads are above ground!

In truth, if you are really a good tech with common sense, almost any problem can be tested and located WITHOUT measuring HV, and without having HV on. In the very rare case where something with over several hundred volts has to be measured live, I would be damn sure I was well away from anything that could reach me or find a path to me. I would also have someone else who knows what to do around.

73 Tom

Posts: 353

« Reply #4 on: August 24, 2012, 07:18:57 AM »

I have a X100 HV probe I use.
Better safe than sorry.

Posts: 2152

« Reply #5 on: August 24, 2012, 09:36:45 AM »

I have a Simpson 630 meter that can read 6000 V directly.  I also have a NEW set of high quality silicone insulated HV leads that can one can screw an alligator clip on to the end of the probe.  No offense to Radio Shack, but this is not the place to use their replacement VOM leads.  I cannot count the number of amps I have either built or worked on since the 1960's. 

Knowing and understanding the potential danger, when reading HV on the Simpson 630:

1.  Make sure the HV circuit is at zero potential.  (If you have to ask "how" stop here)
2.  Sit the Simpson 630 meter on the bench and connect the leads to the 6KV jacks on the meter.
3.  Connect the (-) lead lead from the meter to ground (or HV -) via an alligator clip.
4.  Connect the (+) lead from the meter to the HV source via an alligator clip.
5.  Make sure the meter leads are separated adequately and that they are not touching anything that moves or that can get hot.
6.  Turn on the HV.
7.  Read the HV on the Simpson meter. (Do not touch or move the meter)
8.  Turn off the HV and watch the Simpson meter go to zero.
9.  Verify that the HV is at zero potential. (See #1 above)
10. Remove the leads.

Dick   AD4U

Posts: 3822

« Reply #6 on: August 24, 2012, 10:03:23 AM »

One of the HV probes I have is a Heathshkit model 336 (yes, even I own a little piece of Benton Harbor nostalgia, but that doesn't mean I can't make jokes about it...) and out of curiosity I looked it up on the web. Turns out it's a 100x multiplier (not 10x as I recalled) and includes an interesting caution on the assembly sheet: Even though your VTVM might go higher, the probe itself is rated only to 30 kV. Anything above that level presents a risk to the user. The kit is essentially a 1090 meg resistor with mounting hardware and a red wire.

BTW:  In reading this thread it now becomes clear why those probes are so large... It's somewhat like measuring HV with a 10 foot pole that keeps stray fingers where they need to be. Well away from the Tesla demonstration section of a chassis.

Never change a password on a Friday                

Posts: 851

« Reply #7 on: August 24, 2012, 11:00:56 AM »

since I got decked by CRT stored voltage on a TV back in the 70s, I do my "smoke testing" a 6-foot extention cord or more away.  if it's B+ ranges, I also have the probes resting on a sheet of Plexiglas.

let the first fireball, if any, fritter itself away long before it could find me.

doesn't mean I can't get kicked making adjustments on a "working" system, as I took a nasty path right-hand to left-palm to a grounded outlet box back in 10th grade off an old 3-band radio.  but an hour or two later, after I woke up and crawled out from behind a file cabinet on the other side of the room, I decided that a little extra insulation covering potentials if I'm working near something interesting was going to be my plan henceforth.

just saying a 400 volt potential is eminently more than enough to flow a 30+ mA death current on dry skin.
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