Measuring EHT

Hi all,

On this scope I've been working on there are voltages present (according to the schematic Arfa kindly provided me with) of up to 14kV. Does one need a special voltmeter to measure such levels or can I use a work- around of cobbling together say 20 x 1Meg resistors in series and measuring the voltage across one of them with a regular DVM? My (hobby) background has been in messing around with typically

Reply to
Cursitor Doom
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Scope tubes are very high impedance. You would need to know exactly how mu ch current the tube pulls in order to calculate what the voltage drop shoul d be across any resistor. Although there is but a small voltage drop acros s the resistor, there is still a big voltage potential from what you're mea suring and any ground. You also can't put the resistors from the HV lead to ground for the same reason (high impedance, low current HV trans). Maybe you can borrow a HV probe from a TV repair shop. I have three of them and rarely use them anymore.

Reply to
John-Del

Those voltage levels are intimidating. You don't really want to use just series resistors at tha voltage level. Most of the 1/2 to 1 watt resistors are only rated for about 300 to 500 volts across them. Even with 20 in series you are pushing it over the limit. They do make high voltage probes for the meters. As most digital meters are around 10 megohm input, they are probably universal probes for the high voltage.

Reply to
Ralph Mowery

I picked up HV probes from eBay over the years. They are usually pretty cheap...and handy for our shop. Put then we still service picture tube based monitors for arcade games...

$35 and up on eBay right now:

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And Tinyurl version:

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John :-#)#

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Reply to
John Robertson

Your query had me rushing off to see if I still had the nylon rod strung with multiple large resistors that I used to use as a probe. But I don't think I was measuring quite so high EHTs.

And as others have said, an EHT supply may not deliver 1mA. It only has to charge the tube's capacitance and deliver leakage current. It may supply enough current to measure with a sensitive ammeter and give voltage by calculation. And multiple measurements with different loads should give an indication of the regulation.

But as other have also said, don't exceed the individual resistor voltage limit. So quite a lot of resistors may be called for!

Mike.

Reply to
Mike

It's usual to use a HV probe that contains series resistors in the tip area, and a safe-to-hold ground shielded area for the handle, and the input resistance of the meter to complete a voltage-divider. So, aside from some calibration and packaging, you've got the right idea. Always attach the ground wire from the probe first...

Reply to
whit3rd

In order for him to do that, he'll need about 600 meg worth of resistors to keep from swamping the HV supply. But rigging series resistors to ground wasn't what he was suggesting. Reread his original post.

Reply to
John-Del

Use the meter to read microamps.

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Reply to
Adrian Tuddenham

Many people seem to overlook the voltage rating of a resistor because power dissipation is not being exceeded. For example a 1/2 watt 10 MegOhm resistor would dissipate 1/2 watt at 2240 volts, but might have a dielectric breakdown at 350 Volts.

As John Robertson suggested, look for used high voltage probes on E-Bay. RCA, Eico, Heathkit and others made them. Be sure you pick one that is rated for your voltage.

Fred

Reply to
Fred McKenzie

The 'microamps' function might fail open (there's usually a fuse), which would be unfortunate. A 100:1 or 30:1 divider is easily arranged, using the voltmeter's V scale resistance, and will take my Fluke to 100 kV (if I were to require that); 1000:1 is excessive.

Good HV resistors are fat things, a centimeter or so diameter is recommended.

Reply to
whit3rd

Remembering that at college (in the 1960s) we had an electrostatic voltmeter (which may have even been an entirely passive device like a calibrated electroscope) I had a google for that title.

I see that it is possible to buy a non-contacting voltmeter measuring up to 20kV. I didn't look for the price but I guess it is not in the hobby range...

Mike.

Reply to
Mike

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