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.