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5A ammeter to 500mA ammeter?

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RDL2004

Member
Hi,

I know this is a simple question, but how would I convert a 5A ammeter to a 500mA ammeter ?

Would it be better to use a 500mA and convert it to 5A?

I am looking to have a dual range ammeter in a power supply project.

Thanks
 

eblc1388

Active Member
RDL2004 said:
Hi,

I know this is a simple question, but how would I convert a 5A ammeter to a 500mA ammeter ?
You can't.

RDL2004 said:
Would it be better to use a 500mA and convert it to 5A?
No.

RDL2004 said:
I am looking to have a dual range ammeter in a power supply project.
You can use a 1mA full scale meter and adapt it to display 500mA full scale or 5000mA full scale.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
eblc1388 said:
RDL2004 said:
Hi,

I know this is a simple question, but how would I convert a 5A ammeter to a 500mA ammeter ?
You can't.

Actually "can't" is a bit too strong!, "may not be able to" is more accurate. Depending on how the meter is designed you may be able to remove a shunt from it, giving a far more sensitive meter.

RDL2004 said:
Would it be better to use a 500mA and convert it to 5A?
No.

Definitely wrong this time, to convert a 500mA meter to 5A you simply place a shunt across it, it's resistance is calculated to pass 4500mA leaving 500mA to pass through the original meter.

RDL2004 said:
I am looking to have a dual range ammeter in a power supply project.
You can use a 1mA full scale meter and adapt it to display 500mA full scale or 5000mA full scale.

Yes, by adding shunts, calculated in EXACTLY the same way as converting 500mA to 5A :lol:
 

stevez

Active Member
I'd go for the 500 ma meter and add a shunt to get 5 A. You might also consider some other lower range (lower than 500 ma) and have two shunts -one for each of two ranges. Some math is involved. If you have a meter on hand and don't know it's resistance there are ways to measure it without ruining the meter. IM me and I'll see if I can find the page in my book that tells how.
 

Russlk

New Member
If you have a 500 mA meter, you can make it 5 A full scale by shunting 4.5 amps around the meter. You will have to measure the voltage drop of the meter to compute the shunt resistor.
 

eblc1388

Active Member
All very sweet talking about using current shunt, when one finally have to buy/make one.

Where can I find or build a 0.0364 ohm shunt which pass 4.5A to drop 0.164V that my 0.5A meter needs?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
eblc1388 said:
All very sweet talking about using current shunt, when one finally have to buy/make one.

Where can I find or build a 0.0364 ohm shunt which pass 4.5A to drop 0.164V that my 0.5A meter needs?

You build it with a thick piece of wire!, or a piece of metal plate!, or even a PCB track!. Or you can buy standard value ones (see RS Components), but it's more cost effective making your own.

In any case, you have to buy/make one if you're using a 1mA meter just the same!.
 

eblc1388

Active Member
Nigel Goodwin said:
In any case, you have to buy/make one if you're using a 1mA meter just the same!.

Of course it is not the same. With a 1mA meter, I can have the shunt at standard value(e.g. n resistors in parallel) and insert variable resistor to the meter coil to trim the meter to scale.
 

RDL2004

Member
Thanks for all the replies. I will do some searching on shunts to see if I can find more info on how to calculate the value of the resistor.

It seems it would be better to start with a 500 mA.

This power supply will be variable voltage probably around 2 to 25 volts range at up to about 3 amps, does that complicate things any?

I chose the 5A and 500 mA ranges because you can buy these types pretty cheap and the numbers on the scale would match up, I would just have to do some mental decimal point shifting.

I agree it might also be nice to add a lower range of 50 mA also.
 

Optikon

New Member
Nigel Goodwin said:
eblc1388 said:
All very sweet talking about using current shunt, when one finally have to buy/make one.

Where can I find or build a 0.0364 <a href="#">ohm</a> shunt which pass 4.5A to drop 0.164V that my 0.5A meter needs?

You build it with a thick piece of wire!, or a piece of metal plate!, or even a PCB track!. Or you can buy standard value ones (see RS Components), but it's more cost effective making your own.

In any case, you have to buy/make one if you're using a 1mA meter just the same!.

yes well, he doesnt talk about any accuracy requirements or other performance needs.. so yeah, these cheap ways will work, albiet very LOUSY / poor.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
eblc1388 said:
Nigel Goodwin said:
In any case, you have to buy/make one if you're using a 1mA meter just the same!.

Of course it is not the same. With a 1mA meter, I can have the shunt at standard value(e.g. n resistors in parallel) and insert variable resistor to the meter coil to trim the meter to scale.

:?: You can use "n resistors in parallel" just the same, although I wouldn't really recommend it. Adding a preset in series with the meter also isn't a good idea (even with a 1mA meter), better to use the correct value of shunt rather than 'bodge it' - which would increase the voltage loss across the shunt, and the power dissipation in it.

But, to go back to the 'just the same' quote, you 'could' add a variable resistor in series with the 500mA meter to make it adjustable, it just needs to be a lower value pot.
 

eblc1388

Active Member
Nigel Goodwin said:
But, to go back to the 'just the same' quote, you 'could' add a variable resistor in series with the 500mA meter to make it adjustable, it just needs to be a lower value pot.

You are venturing into something you don't know about. It all good in theory, until you need to find the pot.

A 500mA current meter has only 0.5 ohms resistance max. or less so if you want to insert a variable resistor to adjust the current for 10% in both ways, you need to find a variable resistor of 0.1 ohms, 500 mA current rating.

Just show us where we can easily find such a variable resistor, cheaply. No point paying many pounds or dollars for such a variable resistor, don't you say?

On the other hand, if a 1mA meter movement is used, one can use any garden variety variable resistor for the adjustment.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
eblc1388 said:
Nigel Goodwin said:
But, to go back to the 'just the same' quote, you 'could' add a variable resistor in series with the 500mA meter to make it adjustable, it just needs to be a lower value pot.

You are venturing into something you don't know about. It all good in theory, until you need to find the pot.

I was doing this almost 40 years ago!, at Grammer School.

A 500mA current meter has only 0.5 ohms resistance max. or less so if you want to insert a variable resistor to adjust the current for 10% in both ways, you need to find a variable resistor of 0.1 ohms, 500 mA current rating.

Piece of wire, slide another one along it, solder them together when you find the correct spot - cost?, nothing!.

It's not rocket science, we not even really talking electronics, we're talking first or second form (sorry year 7 or 8 now) physics!.

Just show us where we can easily find such a variable resistor, cheaply. No point paying many pounds or dollars for such a variable resistor, don't you say?

See above, cost ZERO!- although, as I said before, it's a clumsy and inefficient method, you should adjust your shunt correctly, so there's no need for such a pot.

On the other hand, if a 1mA meter movement is used, one can use any garden variety variable resistor for the adjustment.

But, again!, it's a poor idea - do the job properly with the correct value shunt.

If you happen to have a 1mA meter, fair enough, but if you happen to have a 500mA meter it makes no real difference - in either case you need a low value shunt, preferably without bodging a preset in series with the meter.

Incidently, the usual inbuilt series resistor in a 1mA (or similar) meter is to swamp variations in the resistance of the winding.
 

eblc1388

Active Member
Nigel Goodwin said:
On the other hand, if a 1mA meter movement is used, one can use any garden variety variable resistor for the adjustment.

But, again!, it's a poor idea - do the job properly with the correct value shunt.

If you think that's a poor idea, then I have no more to add to the thread. We can both keep our preferences on how to do things. :twisted:
 
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