FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups 
 ProfileProfile   PreferencesPreferences   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Forum index » Electronix » Misc
Need DMM with VTVM Equivalent Input Impedance
Post new topic   Reply to topic Page 1 of 1 [7 Posts] View previous topic :: View next topic
Author Message
Michael A. Terrell
electronics forum Guru


Joined: 24 Mar 2005
Posts: 2291

PostPosted: Tue Jul 11, 2006 5:04 am    Post subject: Re: Need DMM with VTVM Equivalent Input Impedance Reply with quote

Bill Shymanski wrote:
Quote:

hhc314@yahoo.com> wrote in message
news:1152152517.999276.288750@j8g2000cwa.googlegroups.com...
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.





Geo wrote:
On 4 Jul 2006 11:28:39 -0700, hhc314@yahoo.com wrote:

I'm still working with an old RCA VTVM for voltage measurements of
high-voltage, high impedance sources. I need something at least in
the
order of 5-10 Megohms/Volt.


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.
http://bama.sbc.edu/


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
Back to top
Bill Shymanski
electronics forum beginner


Joined: 04 Mar 2006
Posts: 7

PostPosted: Mon Jul 10, 2006 3:18 am    Post subject: Re: Need DMM with VTVM Equivalent Input Impedance Reply with quote

<hhc314@yahoo.com> wrote in message
news:1152152517.999276.288750@j8g2000cwa.googlegroups.com...
Quote:
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.





Geo wrote:
On 4 Jul 2006 11:28:39 -0700, hhc314@yahoo.com wrote:

I'm still working with an old RCA VTVM for voltage measurements of
high-voltage, high impedance sources. I need something at least in
the
order of 5-10 Megohms/Volt.



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
Back to top
Glenn Gundlach
electronics forum Guru Wannabe


Joined: 30 Apr 2005
Posts: 222

PostPosted: Sat Jul 08, 2006 4:49 am    Post subject: Re: Need DMM with VTVM Equivalent Input Impedance Reply with quote

hhc314@yahoo.com wrote:
Quote:
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! Smile 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.

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
Back to top
hhc314@yahoo.com
electronics forum addict


Joined: 11 Sep 2005
Posts: 88

PostPosted: Thu Jul 06, 2006 2:21 am    Post subject: Re: Need DMM with VTVM Equivalent Input Impedance Reply with quote

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! Smile 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:
Quote:
On 4 Jul 2006 11:28:39 -0700, hhc314@yahoo.com wrote:

I'm still working with an old RCA VTVM for voltage measurements of
high-voltage, high impedance sources. I need something at least in the
order of 5-10 Megohms/Volt.

The application is repairing and reconditioning Geiger Counters, where
the 800-1200 VDC power supplies become loaded down when more and a few
microamps are drawn from them.

So, the question is are there any DMMs with this very high input
impedance capability, or need I continue to repair and re-use 1950s era
VTVMs?
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
Back to top
Geo
electronics forum beginner


Joined: 13 May 2005
Posts: 26

PostPosted: Wed Jul 05, 2006 3:14 pm    Post subject: Re: Need DMM with VTVM Equivalent Input Impedance Reply with quote

On 4 Jul 2006 11:28:39 -0700, hhc314@yahoo.com wrote:

Quote:
I'm still working with an old RCA VTVM for voltage measurements of
high-voltage, high impedance sources. I need something at least in the
order of 5-10 Megohms/Volt.

The application is repairing and reconditioning Geiger Counters, where
the 800-1200 VDC power supplies become loaded down when more and a few
microamps are drawn from them.

So, the question is are there any DMMs with this very high input
impedance capability, or need I continue to repair and re-use 1950s era
VTVMs?
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
Back to top
default
electronics forum Guru Wannabe


Joined: 03 May 2005
Posts: 175

PostPosted: Wed Jul 05, 2006 12:26 am    Post subject: Re: Need DMM with VTVM Equivalent Input Impedance Reply with quote

On 4 Jul 2006 11:28:39 -0700, hhc314@yahoo.com wrote:

Quote:
I'm still working with an old RCA VTVM for voltage measurements of
high-voltage, high impedance sources. I need something at least in the
order of 5-10 Megohms/Volt.

The application is repairing and reconditioning Geiger Counters, where
the 800-1200 VDC power supplies become loaded down when more and a few
microamps are drawn from them.

So, the question is are there any DMMs with this very high input
impedance capability, or need I continue to repair and re-use 1950s era
VTVMs? (These serve the purpose quite well, but I would prefer a
digital readout rather than the analog readout that VTVMs generally
provide.)

Harry C.

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 http://www.vk2zay.net/article.php/9 for a simple
electrometer that could be incorporated as a front end for a less
sensitive meter.


----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Back to top
hhc314@yahoo.com
electronics forum addict


Joined: 11 Sep 2005
Posts: 88

PostPosted: Tue Jul 04, 2006 6:28 pm    Post subject: Need DMM with VTVM Equivalent Input Impedance Reply with quote

I'm still working with an old RCA VTVM for voltage measurements of
high-voltage, high impedance sources. I need something at least in the
order of 5-10 Megohms/Volt.

The application is repairing and reconditioning Geiger Counters, where
the 800-1200 VDC power supplies become loaded down when more and a few
microamps are drawn from them.

So, the question is are there any DMMs with this very high input
impedance capability, or need I continue to repair and re-use 1950s era
VTVMs? (These serve the purpose quite well, but I would prefer a
digital readout rather than the analog readout that VTVMs generally
provide.)

Harry C.
Back to top
Google

Back to top
Display posts from previous:   
Post new topic   Reply to topic Page 1 of 1 [7 Posts] View previous topic :: View next topic
The time now is Sun Jun 25, 2017 1:44 pm | All times are GMT
Forum index » Electronix » Misc
Jump to:  

Similar Topics
Topic Author Forum Replies Last Post
No new posts Impedance Roger Dewhurst Basics 4 Fri Jul 21, 2006 5:43 am
No new posts two output ports tied to 1 input?? thai.tony@gmail.com design 4 Wed Jul 19, 2006 9:20 am
No new posts Input impedance on an instrumentation amp spam.zachr@gmail.com design 12 Tue Jul 11, 2006 4:56 am
No new posts FS: Impedance Bridge Scott Dorsey Repair 3 Sun Jul 02, 2006 11:45 pm
No new posts STR DE845 Analog input problems 427Cobraman Repair 1 Sun Jul 02, 2006 1:41 pm

Copyright © 2004-2005 DeniX Solutions SRL
Other DeniX Solutions sites: Unix/Linux blog |  Unix/Linux documentation |  Unix/Linux forums |  Medicine forum |  Science forum  |  Send and track newsletters


Powered by phpBB © 2001, 2005 phpBB Group