# Help me understand how electrical meters work

Hondaboi1729
Recently I asked why with a multimeter you should not measure the resistance of a fuse in a car while there was still power reaching the fuse but why it was ok to measure the voltage.
I was told the way resistance is measured is a small current is sent by the meter for the calculation to be made. with battery power still coming confusion would occur. However with voltage only the battery's current was relied on so it was OK.

This prompted me to try and find out how the three basic meters work.
For some very annoying reason google finds this a mysterious question and is yielding very few helpful results:

SO:
1) How do both an analogue and digital voltmeter work
2) How do both an analogue and digital ohmmeter work
3) How do both an analogue and digital ammeter work.

If I can understand these I can look at multimeters later. But first these three.
Please note I'm not asking about how to use them in a circuit. That is fine. sadly google keeps giving me this.
and please also note I'm not asking about galvanometers.
I'm asking about the internals of these meters and the physical explanation of how they carry out their function. I'd really appreciate detailed explanations and diagrams or really just a website which holds all the information I need.

Thanks very much,

Mr Hondaboi1729

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## Answers and Replies

Tom.G
Science Advisor
I'm not asking about galvanometers.
Is this because you already know about them or because you don't care about them?
The reason I ask is because analogue Voltmeters and Ohmmeters ARE galvanometers at there heart, and often Ammeters are too.

davenn and Asymptotic
Hondaboi1729
because galvanometers just detect current and don't give any other info... that's why I wasn't interested

davenn
Science Advisor
Gold Member
because galvanometers just detect current and don't give any other info... that's why I wasn't interested

then you misunderstand the use of a galvanometer
you should go do some reading on basic meters and their use

Asymptotic and phinds
Hondaboi1729
well I clearly can't find anything explaining a galvanometer in ammeter form or voltmeter form. Do you think I haven't tried or somthing? Or that I don't know what the use of the three basic meters are at this stage?

note I'm not asking about galvanometers
Actually, you very much are. Analog meters are built around them. Ponder upon the first paragraph in the Wikipedia article.

A galvanometer is an electromechanical instrument for detecting and measuring electric current. The most common use of galvanometers was as analog measuring instruments, called ammeters, used to measure the direct current (flow of electric charge) through an electric circuit. A galvanometer works as an actuator, by producing a rotary deflection (of a "pointer"), in response to electric current flowing through a coil in a constant magnetic field.

Because they are galvanometer-based, analog voltmeters, ammeters, ohmmeters, and so on are inherently current measuring instruments.

Digital voltmeters, ammeters, and ohmmeters are built around a op amp which measures differential voltage. The magic of how both current and voltage based devices can be used to measure voltage, current, and other phenomena is in the switchable scaling resistor networks that connect them to your test leads.

An excellent way to learn how multimeters work is to build one; Elenco markets both analog and digital multimeter kits. Closely study the model M-2666K digital meter data sheet (linked to the Jameco catalog page) because it has an overview of how voltage, current, and resistance meter scaling is done.

Edit: Elenco's overview would have been better had they added a paragraph or two showing the voltage divider and current shunt equations.

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Hondaboi1729
Ok so all analog versions of the ammeter and voltmeter are based on the galvanometer then. Which means they don't have their own power source like a battery (since galvanometers have no power source) as a digital version would or a multimeter would.
But an ohmmeter must have it's own power source for when testing so it must be somewhat different to a galvanometer surely?

The important thing I guess I should have shot to was: in the digital versions there is a power source- however this source doesn't have any involvement in the external circuit you're investigating if voltmeter or ammeter or in either of these modes on a Multimeter- this source is just for other stuff for within the meter like powering 7 segment display and other stuff I'm ignorant of.
For an either kind of ohmmeter whether analogue (I've read about them now Tom thank you)or digital or part of multimeter (yes I'll look into that Asymptotic) that's the only time power from the meter goes to the component?

If this last paragraph is correct I'm happy.

Tom.G
Science Advisor
For an either kind of ohmmeter... that's the only time power from the meter goes to the component?

If this last paragraph is correct I'm happy.
OK then. It's Happy Time!

Hondaboi1729
Hondaboi1729
you've said the same thing like three times now- it's getting boring. i know how basic current and voltage works- so no.

Averagesupernova
Science Advisor
Gold Member
Good luck.

jim hardy
Hondaboi1729
ok.

davenn
Science Advisor
Gold Member
But an ohmmeter must have it's own power source for when testing so it must be somewhat different to a galvanometer surely?

It still has a galvo. at it's core with a few additional components. The power source is just to supply a small voltage to the circuit/resistance under test

sophiecentaur
Science Advisor
Gold Member
2020 Award
well I clearly can't find anything explaining a galvanometer in ammeter form or voltmeter form. Do you think I haven't tried or somthing? Or that I don't know what the use of the three basic meters are at this stage?
I know that, if I google those terms, I will get hundreds of hits, some of which are bound to contain exactly what you want to know. If you first learn to use search engines properly and if you are prepared to do some actual reading (and take some time), you can find out all of that basic stuff and more. PF is not a source of pre-digested information for people who won't find out stuff for themselves.

Asymptotic, jim hardy, Averagesupernova and 1 other person
The Galvanometer is JUST an part of making an analog Ammeter, Voltmeter or Ohmmeter. If you look at an analog meter, ALL of the scales are on the same face, surely, at some level all of these things are utilizing the same core element. As such this is sometimes referred to as "the movement" of a meter, similar to a watch, the core element from which makes the device possible. The more precise the galvanometer, the more precise the resulting meter CAN be if the other components are properly selected and applied.

"how to make an ohmeter with galvanometer" -- leads you right to the info you are asking about.

Asymptotic
jim hardy
Science Advisor
Gold Member
Dearly Missed
It is nice to understand what's inside your test equipment. That way you don't blow it up nearly so often.

Basic analog ohm meter is a galvanometer.
E1 is typically a 1.5 volt AA cell.
R2 adjusts for run down battery.
R1 is the reference resistor to which unknown Rx is compared..
Non linear meter scale calculates ratio of Rx to R1, shows Rx .

http://electricalacademia.com/instr...ts-and-working-principle-ohmmeter-definition/

R1 will be the value written at the center of the analog scale. Can you figure out why ?

cnh1995 and Asymptotic
jim hardy
Science Advisor
Gold Member
Dearly Missed
Can you figure out why ?
And,
can you figure out what happens if you try to measure a blown car fuse that's got 12 volts from the car battery ? Or a blown fuse in your microwave that's got 120 volts from the wall ?

old jim

Asymptotic
Ok so all analog versions of the ammeter and voltmeter are based on the galvanometer then. Which means they don't have their own power source like a battery (since galvanometers have no power source)
Mostly, but not necessarily true.

There are limits on how sensitive a mechanical meter movement can be built, and a galvanometer introduces what may be an unacceptable amount of electrical load into the measured circuit. An example of a class of instrumentation that bridged the gap to digital was the VTVM (Vacuum Tube Volt Meter). These used a high input impedance vacuum tube amplifier (a powered circuit) to provide impedance matching, and often to boost weak voltage signals to drive a galvanometer.

Averagesupernova
Science Advisor
Gold Member
Who knew there was so much to know about electricity!?!?!?!

dlgoff, davenn, jim hardy and 1 other person
And,
can you figure out what happens if you try to measure a blown car fuse that's got 12 volts from the car battery ? Or a blown fuse in your microwave that's got 120 volts from the wall ?

old jim

A savvy enough meter ought to get good and pissed off and give you alarmingly nonsensical readouts until you abruptly yank your probes from the circuit aghast at the mistake you have just made. A cheap one might blow up. Be careful with meters.

jim hardy
Science Advisor
Gold Member
Dearly Missed
A cheap one might blow up
An analog one is vulnerable on low ohms scales. Here's why:

For simplicity assume
R2 is set to zero.
E1 is a 1.5 volt AA cell
R1 is ten ohms
meter is ideal, its R is zero
so when Rx is zero,
current is 150 milliamps and that gives full scale meter deflection.
Power in R1 is I2R = 0.152 X 10 = 0.225 Watts, not quite ¼ Watt

If Rx becomes ten ohms then current is 1.5volts / 20 ohms = 75 milliamps, half as much as before giving half scale deflection.
Power in both R1 and Rx is now 0.0752 X 10 = 0.056 Watt, not quite 1/16 of a Watt apiece.

If Rx becomes instead of resistance a source of +12 volts
then current becomes (12-1.5)volts /10ohms = 1.05 amps, a 7X overload for the meter
and power in R1 becomes 1.052 X 10 = 10.5 watts . It's probably a 1/4 Wat resistor, soooo.....

That's why you see so many analog meters with the low ohms resistor burnt up.

cnh1995 and Asymptotic
That's why you see so many analog meters with the low ohms resistor burnt up.
With 480V, typically you hear a loud bang then find a saucer-eyed tech surrounded by debris, holding a pair of blackened test probes ....

krater and jim hardy
jim hardy
Science Advisor
Gold Member
Dearly Missed
With 480V, typically you hear a loud bang then find a saucer-eyed tech surrounded by debris, holding a pair of blackened test probes ....

I hope he was wearing gloves and a face mask and an asbestos apron.

.

480 makes a real loud bang, you'll think a cannon went off..

krater and Asymptotic
I hope he was wearing gloves and a face mask and an asbestos apron.
No, this was circa 1981, before the need for arc flash protection was stressed. Fortunately, no one was injured (well, except for a touch of sunburn, and picking out a half dozen shards of partially melted copper from his arms).