Need DMM with VTVM Equivalent Input Impedance

Many of the digitals are in the range you want. Ten meghoms is common for DVM's for the 200 mv ranges

I have an el-cheapo 3-1/2 digital panel meter input impedance is listed at greater than 100 megohms and the sensitivity is 200 millivolts full scale. That's 500 megohms per volt . . .

Velleman components makes it I don't see a model number - this one works on 9 volts and has an LCD display. 500 megohms per volt . . . You'd just need to buy a few high value resistors and make a divider. Bare meter for ~$10, with a few more parts it would do the trick.

Check out

for a simple electrometer that could be incorporated as a front end for a less sensitive meter.

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All DVMs (even expensive bench instruments) that I have worked on/with have high impedance on the 2v and below ranges only. The higher ranges are fed into a precision resistive divider which presented 10Mohms to the input source. Most of them (that I recall) had a maximum input voltage of 1kv.

A high voltage probe e.g.from RS Components has a fixed Input Impedance 1000 M Division Ratio 1000:1 For use with DMM 10 M Imp.

Otherwise how about an electrostatic meter - if they are still made - no electronics just a moving needle...

Geo

Thanks guys, but it sounds like continuing to work using VTVMs remains even today my best option for relatively accurate measurements of high-impedance, high-voltage devices.

I had thought that there were some DMMs with a high impedance FET based front end, but so far I haven't been able to find one -- something that more or less emulates the impedance of the control grid in a vacuum tube, and no current needs to be drawn from the circuit under test. I had hope that there were some DMMs that employed a balanced voltage bridge approach, but still haven't been able to find any short of lab grade (reads $3,000) instruments that work this way. These would be perfect for the application, but sadly my budget is $100 or preferably less! :-) Thankfully, VTVM technology and a good supply of 6SN7s gets the job done, at least until I run out of 6SN7s! :-)

Thanks guys for trying to help, and some great ideas.

Harry C.

Geo wrote:

high

snipped-for-privacy@yahoo.com wrote: > Thanks guys, but it sounds like continuing to work using VTVMs remains > even today my best option for relatively accurate measurements of > high-impedance, high-voltage devices. >

You got me curious enough to go find some schematics of VTVMs, notably the RCA WV-98C Senior Voltohmyst. Similar to yours? It has an input load of 11 meg regardless of voltage setting so it isn't 5 or 10 meg/volt. What I read is what I remembered from 30+ years ago. Basically it's the same loading as my Fluke 8060A meaning very little on low voltage ranges but not so good on the 1KV scale. I was measuring some G2 voltages on Sony monitors this week (around 500V) and the 11 meg load messed up the readings. The Fluke HV probe (1000:1) fixed that problem. I'm sure you'd have similar issues with the VTVM.

I.E. get yourself any decent DMM (even a radio Shack) and you'll be pretty good to go.

GG

the

10 megohms per volt? I thought a VTVM just had a 10 megohm input impedance, period - not "per volt". All the voltage multiplier probes I'd read about worked with a so-called "10 Megohm" input meter.

Bill

They do. 10 Megohms on all AC ranges, and 11 Megohms on all DC ranges. You can find several VTVM schematics on BAMA.

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