# convert an voltmeter into ammeter?

Discussion in 'General Electronics Chat' started by Senz_90, Aug 22, 2013.

1. ### Senz_90 Thread Starter Member

Jul 11, 2013
70
0
anyone on this forum ever try to convert / modify some voltmeter into ammeter?? maybe convert an AC voltmeter into DC voltmeter.
im try to googling and got just result using resistor like shunt resistor with paralel or series way... but i did'nt understand enough how to do that...it just give me an arithmetic formulas that confused me becaus i don't know my voltmeter description. im beginner hobbyist so basic isn't very strong and please don't bothering me

please show the way if you ever do that..

i just got my idea because my city didn't sell any voltmeter or ammeter for DC electric !!! it got me confused.. and how to measure voltmeter description i.e μA of mine..

2. ### TheComet Member

Mar 11, 2013
88
12
You can put a tiny resistor in series with the rest of your circuit and measure the voltage over that.

Then you can solve for I in the formula: V = R*I (R=10m, V=measured voltage)

TheComet

3. ### Papabravo Expert

Feb 24, 2006
10,021
1,757
The basic problem is that a voltmeter typically has a high impedance, and is placed in parallel with the circuit under test. This is because you want to measure a voltage while drawing a minimal amount of current so as not to disturb the circuit under examination.

An ammeter by contrast has a very low impedance and is placed in series with the circuit under test. This will make using a voltmeter as a direct reading ammeter difficult. Nowadays most multi-meters have multiple probe input locations so that one meter can perform a number of functions. Like calculators they are available from multiple sources so cheaply(< \$10.00) that is hardly worth the effort to do what you are trying to do.

That said, if you know the impedance of the voltmeter and you measure the voltage drop across a known low impedance shunt, you should be able to calculate the current.

Jul 18, 2013
10,245
2,262
If it is a basic moving coil meter, then it should have a full scale current value on it some where, e.g. 10maFS.
If it is already set up as a voltmeter then it will already have a series resistor internally, so this would have to be removed/modified if converting to a shunt-ammeter.
The resistance of the meter itself can usually be ignored, for all intents and purposes.
The calculation for any shunt or series resistor is then calculated based on the FS current for the meter.
For AC measurements you would need to add a small bridge rectifier before the meter.
Max.

5. ### Senz_90 Thread Starter Member

Jul 11, 2013
70
0
impedance !! that is the thing that i wanna know, could u explain how to know the impedance of voltmeter of mine? yeah i don't really know basic electronics.. just a little bit a knowledge... and how to measure voltage drop across a known low impedance shunt like a resistor? please explain with a picture so i can understand clearly..if you don't mind hehehe

6. ### Senz_90 Thread Starter Member

Jul 11, 2013
70
0

first at all, great thanks for thecomet advice.. i'll try that way !!

yeah u are very right sir! i found that a resistor in series internally. so i'll have to remove it first? after remove that what value of resistor as shunt paralel with that meter? please give me a easy expalanation and some formula with picture to determine what should i do with a very" poor basic of electronics..

without known of meter resistance is that possible to calculate shunt resistor ?

7. ### Senz_90 Thread Starter Member

Jul 11, 2013
70
0
tiny resistor. what does it means? high ohm value of resistor or high wattage of resistor? yeah that is the basic formula Ohm's law that every electrician must to know...

8. ### TheComet Member

Mar 11, 2013
88
12
In the manual of your voltmeter should be specified what the internal impedance is. Usually it's around 1MΩ-10MΩ.

A tiny resistor means the resistance (in Ohm [Ω]) needs to be significantly less than the total impedance of the rest of the circuit. If you have a circuit that runs at 5V and draws 100mA, you will need to choose a resistance much smaller than for a circuit running at 5V and 100uA. Rule of thumb is factor 10.

5V @ 100mA gives you a total impedance of 50Ω, so you will need to use at most a 5Ω resistor in series to measure the current (just as an example). It's good to be as low as possible, but be aware that the lower you choose the resistor to be, the more error prone it is to your voltmeter.

TheComet

9. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,730
963
I guess you are talking about a panel mount type volt display meter.. And not a multimeter (commonly referred to as a voltmeter) as they have the ability to read current just by switching the leads and setting it to the right setting.

10. ### TheComet Member

Mar 11, 2013
88
12
Can you take a picture of your voltmeter and post it for us to see?

Jul 18, 2013
10,245
2,262
If you convert it to a ammeter you will only need a parallel (Shunt) resistor, otherwise you will limit the current you are trying to measure.
It is a simple formula, you have to obtain the FSC (Full Scale Current) number this is the current through the bare meter (typical <=10ma) , no resistors in place, then the shunt resistor can be calculated that allows FSC to flow when the rest of the current goes through the shunt.
Parallel resistance calculation.
PS: If you know the present DC voltage range for full scale, it is quite easy to calculate the current through the series resistor, Ohms law, this will give you the FSD current for the meter.
Max.

12. ### Senz_90 Thread Starter Member

Jul 11, 2013
70
0

here it is I mean.

but i get the picture from googling.. but u can trust me my voltmeter same as likes that picture no doubt.

hmm.. i understand a little now.. but not fully clear in my mind. sorry for my stupidness

i think the first thing i'll have tried is buy a small resistor and remove the resistor inside my voltmeter !!

File size:
39.6 KB
Views:
1,758
13. ### WBahn Moderator

Mar 31, 2012
17,446
4,699
Just WHAT is it you are trying to do? Convert a voltmeter into an ammeter, or convert an AC voltmeter into a DC voltmeter? These are two different things.

Begin by describe WHY you are wanting to do this in the first place and WHAT problem you have that you hope will be solved by doing this.

Jul 18, 2013
10,245
2,262
Hopefully it is not a moving iron (AC) meter but moving DC coil, but a small bridge fitted internally?
It is 250v FSD so what was the value of the series resistor?
It probably has had the original face covered up, so the FSD current could be also covered now.
Max.

15. ### Senz_90 Thread Starter Member

Jul 11, 2013
70
0

hmm.. i'll try to photo my meter as soon as i can.. it has a single dioda seriesly with resistor and i forgot to look the value.

i want try both of them mr WBahn
i think that very useful if i can make both of them functionally.. so i didn't need try to search everywhere on my around city tryin' to find DC ammeter and voltmeter.. note that i buy voltmeter can used on AC or DC. but it really has purpose using ond AC look from that scale...

16. ### WBahn Moderator

Mar 31, 2012
17,446
4,699
So are you trying to modify a panel meter?

What is the scale of the currents you are trying to measure? Are they AC or DC currents?

Jul 18, 2013
10,245
2,262
You either need to obtain the present value of the resistor, or to work out the FSD of the meter empirically to go any further.
And fortunately it is moving coil if it has a simple diode for AC.
Did you want to make it multi-scale or just one FS reading?
Max.

18. ### Senz_90 Thread Starter Member

Jul 11, 2013
70
0
my scale looks like this photo.. any way to describe the current and the resistance? i found googling that using wheatstone bridge but i didn't understand clearly..

u can see that i modify a little for the picture. just because my camera have a low resolution. hahaha

File size:
151.9 KB
Views:
72
19. ### Senz_90 Thread Starter Member

Jul 11, 2013
70
0
i have ever read FSD from googling that try measure resistance from galvanamoter, but i didn't understand clearly how to make my meter full deflection, did i need to make substitute resistance box to try it?

i just want to rescale it so can measure 1-30 v for DC voltmeter. and the two design is make it to be a ammeter, u know because i can't find any ammeter on my city ! it looks ridiculous thing, but u must trust me, even a ammeter i couldn't get. so i plan to buy 2 voltmeter like that and convert it:
1. 1-30v DC voltmeter
2. 1-25 ammeter

so what i must to do first at all?

please don't bother to looking my picture. so u can describe what i must to do sir.. for notes, a series resistor with diode insertly is 82k ohm after i open it and describe the value.