How to use a millivoltmeter :( - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st October 2013, 09:51 PM   #1
mskl99 is offline mskl99  United States
diyAudio Member
 
Join Date: Jul 2011
Location: Connecticut
Default How to use a millivoltmeter :(

This is really back to basics. I have used nothing but digital multimeters and panel meters in the past, but decided to buy 2 analog panel millivoltmeters to display anode currents in a tube amp, because they look cooler; hopefully one of them is shown below. I measure the input impedance at 13 ohms in both of them.
Am I missing something? Should there be some kind of circuitry in front of this thing to increase the input impedance to maybe the megaohm range, where it would be useful. Could they both be defective?
I tried writing the manufacturer, but of course got no response.
Thanks in advance for your help. This is embarassing.
Mike
Attached Images
File Type: jpg photo.JPG (131.7 KB, 231 views)
  Reply With Quote
Old 21st October 2013, 10:04 PM   #2
diyAudio Member
 
Join Date: May 2011
Location: Silicon Valley
Perhaps they are actually ammeters, which would explain the low input impedance.

Perhaps fullscale, 100% needle deflection represents 100 microamperes rather than 100 millivolts.

You could rig up a variable power supply, a 100K series resistor, and a couple of DVMs, to tickle the meter and find out. If I were you, I'd make a 3 column table: V_across_meter, I_thru_meter, Front_panel_indication. Then I'd plot the data in Excel and see whether one of the curves is a straight line that passes through the origin.
  Reply With Quote
Old 21st October 2013, 10:31 PM   #3
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

Fundamentally all meters are ammeters, deflection is related to passed current.

Fundamentally the lower the current required for full scale, the better the meter.

Your missing some fundamental simple electronics. Series high value resistor
to turn it into a voltmeter. Parallel low value resistor to turn it into a ammeter,
and some very tricky details if you want to measure any sort of high currents.

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
  Reply With Quote
Old 21st October 2013, 10:58 PM   #4
mskl99 is offline mskl99  United States
diyAudio Member
 
Join Date: Jul 2011
Location: Connecticut
Thanks for your help. I've never used anything with needle-deflection, except for a VTVM decades ago in college. I can't imagine this is a good meter, but I'll put a large resistor in series and calibrate it, I guess.
The meter is being used to measure voltage across a 1 ohm resistor.
  Reply With Quote
Old 21st October 2013, 11:32 PM   #5
diyAudio Member
 
Refugee1's Avatar
 
Join Date: Jul 2012
Location: Worksop
On safety grounds I would use them to measure the currant at the cathode as the zero adjuster screw may end up live or may not be insulated to withstand the HT voltage.
All the safety warnings that come with mains powered bench volt meters both digital and analogue apply to the meter itself.
If you are measuring currant you just allow for the resistance of the coil in the meter and use ohms law to work out the shunt resistor value. Also you could just calibrate it with a preset pot.
  Reply With Quote
Old 22nd October 2013, 01:12 AM   #6
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

The meter insulation in good for (floating) 1000V.

Yes you just slap a big resistor in series with it and calibrate.

It may have its current rating printed on the back.

FWIW predigital the specification that mattered for your
meter was Kohm/V. 3K was basic, 10K decent and 20K,
a 50uA full scale meter movement, the quality standard.

Not that it matters much measuring the voltage across
a 1R resistor, but it matters a lot across a 1M resistor.

This was the business when I was young :

Click the image to open in full size.

http://www.shinjo.info/frank/instrum...MkI&IIuser.pdf

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 22nd October 2013 at 01:28 AM.
  Reply With Quote
Old 22nd October 2013, 02:52 AM   #7
diyAudio Member
 
dchisholm's Avatar
 
Join Date: Mar 2011
Location: St Louis, Mo
The ARRL "Radio Amateur's Handbook" (revised and published every year) used to have a pretty good section about the classic D'Arsonval meter movement, how to determine the sensitivity (true range) of an unknown meter movement, calculate series/shunt resistors for use as voltmeters or ammeters, etc. To be honest, I haven't looked at one since the turn of the millenium but they are fairly common in public libraries as well as college and university libraries, and used book stores. I suspect any of the annual volumes from about 1940 up to 2000 or so would have the information you need.

Dale
  Reply With Quote
Old 22nd October 2013, 04:15 AM   #8
diyAudio Member
 
sofaspud's Avatar
 
Join Date: Nov 2005
Location: San Antonio
What needs to be done is to determine the sensitivity of the meter (ie, x amps = full scale), then determine the shunt resistance required to create the new scale.
I don't remember off the top of my head the test procedure for determining the sensitivity, but it isn't difficult. Most meters seem to use a standard, eg. 50μA or 100μA. I'll see if I can post a good test procedure.
I doubt you'll need a series resistance. The meter operates via current through its coil; it is only the scale that is calibrated and marked for voltage.
What range of current are you looking to display?
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
  Reply With Quote
Old 25th October 2013, 07:41 PM   #9
mskl99 is offline mskl99  United States
diyAudio Member
 
Join Date: Jul 2011
Location: Connecticut
Thanks. At 7.8ma, the needle deflects fully to 100mV, and I measure 100mv across the unit with an accurate digital voltmeter. So that makes the input resistance about 12.8 ohms (100/7.8), I think.
I want to measure (milli)voltage across a 1 ohm resistor (at the cathode). I don't want to put this unit in series and measure current directly.
But I still haven't figured out how to do it with this beauty. Putting a large series resistor in there seems like it would reduce the current so much that there would be no needle deflection at all.
I'll look at allaboutcircuits.com or something similar to see how to get the input impedance up, although I guess putting 12.8 ohms in parallel with 1 ohm is not the worst thing in the world.
  Reply With Quote
Old 25th October 2013, 07:52 PM   #10
mskl99 is offline mskl99  United States
diyAudio Member
 
Join Date: Jul 2011
Location: Connecticut
I guess I could always put a 1.085 ohm resistor in the cathode circuit instead of 1 ohm, so that with 12.8 ohms in parallel => 1 ohm.
That seems sort of crude and non-portable, though.
Mike
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Analogue Millivoltmeter Scale Snappa Equipment & Tools 8 22nd July 2013 12:25 PM
Audio millivoltmeter clivel Parts 12 3rd August 2011 08:52 PM


New To Site? Need Help?

All times are GMT. The time now is 07:33 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2