Clarification on VOM's

David G

Well-known Member
There have been several posts on using a VOM, so here is my stab at it.

To measure amps, you want 0 resistance in the meter so it does not effect the reading. The holes that are marked amps and common provide this.

To measure voltage, you want infinite resistance in the meter so it does not effect the reading. The holes marked volts/resistance and common provide this.

To measure resistance, the meter provides a voltage and looks at the current required to determine resistance. The holes marked volts/resistance and common provide this.

You will usually not damage a meter if measuring current with the probes in the voltage holes, but can if measuring voltage with the probes in the current holes.

You should never have voltage on a circuit when measuring resistance.

The meter must be selected to the correct position that matches the holes or it will not work correctly.

The meter usually has in internal fuse that protects it.
 
When I was taking automotive electrical courses at the local community college, all the VOM's there had an inline ATO-type fuse holder soldered into one of the leads. The head of the automotive department told us that was because, if you're working with a VOM after regular store hours and you 'pop' a fuse, it's a lot easier to find an automotive fuse, even at your local convenience store, than it is to find the internal fuse. Blowing the internal fuse might shut down your work until the next day. I'm guessing this was the voice of experience speaking.

Following his example, my VOM now has an automotive ATO-type fuse soldered into one of the leads.
 
Ahhhhhhhhhhh, give me my old trusty reliable Simpson 260 ANALOG VOM any day lol.

As we learned wayyyyyyyyyyy back in the sixties at Purdue, an "IDEAL" Ammeter has Zero ohms resistance while an "IDEAL" Voltmeter has Infinite resistance.

Sure, I have a new fangled DIGITAL meter, but for old tractors with noisy arcing sparking brush generator DC systems, give me my old Simpson 260 Analog meter.

John T Too darn old engineer
 
I have a Fluke and a Simpson 260. Outside in the sunlight I like the Simpson 260 because it easier for me to read.
 
Analog VOMs are rated by "ohms per volt". A 20,000 ohms/volt meter has an input impedance of 40,000 ohms on its 2 volt scale. Low enough for significant errors when working with high impedance circuits. Digital millimeters typically have 10 megohm input impedances, making it a non-issue most of the time.

If you use the "diode check" range on a DMM to measure continuity, be aware that the reading is in VOLTS, not OHMS.
 
I still have an old Vacuum Tube Voltmeter somewhere, but its probably junk after all these years....

John T
 
JohnT,
I put my 260 to rest 40 years ago. When I worked in the electronics department at USNAD Crane, that was the meter of choice.

I found a old analog Radio Shack meter at Goodwill for a few bucks. Most of the time I use the cheap, free gift, HF EVOM. The kind of work most of us do, who cares about accuracy, internal resistance, loading a circuit? Besides, who hasn't heard the popping sound a meter makes when you try to measure 220v when meter is on the ohm scale? I simply just pitch the free HF meter and get another one.

I sometimes like HF digital ammprobe over my old analog probe. It the digital does a better job of measuring starting amps, peak amps, of an electric motor.
 
I didn't know you worked at Crane, I was there as an electrical engineer on and off from the mid sixties to my 1991 retirement.

John T
 
JohnT,
As a student at ISU, I cooped two semester there. Fall of 68 and another semester in 69. I worked in the electronics calibration lab with many old retired navy sea dogs. Got to calibrate some of the best electronic equipment money could buy.
George
 
When I was teaching electronics at Ferris State U, we always kept a stock of spare fuses. Amazing how many fuses the students blew. Other instructor (electrical engineer) and I could never figure how the blew so many. Have forgotten what the bench meters we had were. Think maybe students were putting meters in circuits, and spinning the dial when energized...
 
AMEN right back at ya........ BSEE from good Ol Purdue in 1969/70, then JD from its arch rival IU in 1994 lol

John T
 
Leaking batteries ruined my Simpson.

bought a Radio Shack meter,

still use it.
a172334.jpg
 
Yup.. when I was in votech in 1970 we had RCA VTVM meters that you had to plug into a 117 outlet. Ohhhhh they were such a spiffy thing to use. When I worked for Xerox we used voms and when I was trained on the new engineer copiers I got a weston digital. Hot doggies!!! I have a Fluke 179 for ME only and at work I use the HF digital that sells for around $29.oo full price and is sometimes on special. It works just fine. Still have my simpson at home. If I remember correctly they still make them don't they? I think I saw that somewhere.
 
Last summer I stumbled on to the box where I stored my old military issue Simpson 260. It's still like new. I opened the battery compartment to make sure I had not left any batteries in there to corrode it to junk. No batteries, but gold plated battery connections in there I had forgotten about that.COOL. :)

For a long time I have used cheap but serviceable meters for most jobs. Don't feel nearly as bad if I make a mistake and the meter goes POOF or CRUNCH. :)
The cheap "cheese grater" battery testers with their big analog voltmeter and clamps to keep the leads attached to the battery posts make a very good electrical / charging system diagnostic tool for little money.
 
(quoted from post at 11:44:24 10/23/14) Leaking batteries ruined my Simpson.

bought a Radio Shack meter,

still use it.
a172334.jpg

Some where I have one like that.
That might the one with a cracked lens, I dropped my hard hat on it. It laid between the seats in the service van I was using. I climbed in, forgot it was there, and just dropped my my hard hat.

Dusty
 

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