Vom?

[JJWalecka]

I was asked a question recently...


A VOM measures what?


Any input would be appreciated.

Justin J. Walecka

 

[mdshunk]

volts, ohms, milliamps

(Is this a trick question somehow?)

 

[roger]

Volts and Ohms.

Roger

 

[JJWalecka]

Not a trick.... inane qestion.

Thank you Roger

Justin Walecka

 

[pcmike]

Not a trick.... inane qestion.

Thank you Roger

Justin Walecka

On voltage settings, the tested voltage causes a current across a known resistor ? the voltage is then ?derived?.

On resistance settings, a known voltage causes a current through the tested resistance ? the resistance is then ?derived?.

On ampere settings, the current in the leads is shunted through a known resistance ? the shunted current is the ?derived? value.

Therefore, VOMs measure current.

 

[Davis9]

Here is a site I found with a quick search, hope this helps.


http://www.maxim-ic.com/glossary/index.cfm/Ac/V/ID/328/Tm/VOM

Tom

 

[roger]

On voltage settings, the tested voltage causes a current across a known resistor ? the voltage is then ?derived?.

On resistance settings, a known voltage causes a current through the tested resistance ? the resistance is then ?derived?.

On ampere settings, the current in the leads is shunted through a known resistance ? the shunted current is the ?derived? value.

Therefore, VOMs measure current.

PCmike, welcome to the forum, but I must say Huh?????

As far as any measurements are concerned they are "derived" but this doesn't mean that a Volt or Ohm measurement leads to a conclusion of current.

Roger

 

[tallgirl]

PCmike, welcome to the forum, but I must say Huh?????

As far as any measurements are concerned they are "derived" but this doesn't mean that a Volt or Ohm measurement leads to a conclusion of current.

Roger

Sure they do -- an ampmeter uses an electromagnet working against a spring. The field is generated by the current flowing through the electromagnet and it's strength is known by the amount of torsion in the spring it is working against.

So ... volts and ohms are "derived" from the current flowing through the meter.

 

[roger]

So ... volts and ohms are "derived" from the current flowing through the meter.

Absolutely, that is what I already said, all (any) measurements are "derived" but the measurement is not current is it?

Roger

 

[tallgirl]

Absolutely, that is what I already said, all (any) measurements are "derived" but the measurement is not current is it?

Roger

Well, I think that technically it is. I'm assuming this is an analog VOM since all the digital VOMs I've seen over the years are called DVMs, not VOMs. The only thing you can measure with an analog meter and no additional parts is amperage, typically in the small number of milliamps range.

If you do like I did when I was a kid and take apart an old analog meter, you'll see that there is an electromagnet between a pair of permanent magnets arranged about a shaft, and a spring is attached to that shaft to hold it at "0" and counteract any forces applied to the shaft. When current flows through the meter it creates a magnetic field (remember that current creates magnetic fields, not voltage) and that creates torque which is counteracted by the spring. The torque is directly related to the current and unrelated to the voltage.

 

[roger]

Julie, your problem is the "assume" (in the first person) you're trying to defend. :grin:

Roger

 

[tallgirl]

Julie, your problem is the "assume" (in the first person) you're trying to defend. :grin:

Roger

Oh, definitely -- if this isn't an analog meter it could be directly measuring something else, including voltage.

 

[roger]

Oh, definitely -- if this isn't an analog meter it could be directly measuring something else, including voltage.

Which would probably be "derived". :wink:

Roger

 

[NoVA Comms Power]

Which would probably be "derived"

Agreed.

My Fluke DMMs both pass 1 microamp per volt when measuring VAC.

(i.e. 120 VAC creates a flow of 120 microamps thru the meter)

 

[charlie b]

The question had to do with what was measured, not the mechanism by which it was measured. OK, so current will cause a magnetic field to exist around the current path, and that field can be detected, and internal fancy electromagnetic stuff causes a needle to move. That does not mean that the VOM is measuring current. The fact that one physical parameter is detected and another is calculated does not alter the fact that this device is measuring voltage and resistance. That is why it is called a Volt Ohm Meter.

 

[winnie]

When I was growing up, my mom would call them VTVMs, well after they started to be built with transistors...
(Vacuum Tube Volt Meter)

I don't think that it is fair to say that the device measures current, even if the mechanism by which the meter _indicates_ is because of current flow. There are any number of mechanisms by which the quantity that you want to measure can be converted into a quantity that is easy to perceive, and current creating a magnetic field and acting against a spring is but one of these.

Voltage can be measured by the direct physical attraction of the electrostatic field, eg. in some forms of oscilloscope, or electrostatic action against a spring.

Current can be measured by its magnetic field, or by voltage induced in a magnetic field (hall effect), or by the temperature change of a resistor.

I am certain that there are other mechanisms.

Understanding the mechanism that a particular measurement instrument uses is very important, because it help to understand what other influences will corrupt the measurement.

-Jon

 

[brian john]

Winnie:

In your case that is actually a VOM (very old meter)

 

[al hildenbrand]

(very old meter)

:smile: LOL :grin: