carl seyersdahl wrote:
>
> I am repairing an FL2100B in which the powersupply went south, and I have
> found what appears to be the meter shunt burned out. (IP) It is a
> wirewound unit , and the manual doesn't tell me what it's value is.!!
> Can anyone give me a clue ??? I'm guessing it is likely to be .1 ohm , as
> some others are.
> Any help out there?? Thanks !!
> carl / kz5ca
If the resistor has burt out, there is a good chance the mater has too,
so I would check that first.
The value depends very much on the meter you have - its FSD and internal
resistance as well as the new FSD you want for the meter. However, if
you can find out:
a) The FSD of the meter in amps (say 0.001 if 1 mA) - call it Im. That
would be easy to measure.
b) The internal resistance of the meter in Ohms - call it Rm. Again it
is easy to measure.
c) The current you want the meter to read in Amps - call it Iw.
you can do it all with Ohm's Law.
Your meter without it's shunt is a volt meter with a full scale voltage
of Vm = Im * Rm.
Your shunt needs to take the difference (excess current) to between the
meter's current range and that of FSD you want, whilst at the same time
developing a voltage Vm across it.
So your shunt resistance (call it Rs) needs to take the current (call it
Is) o
Is = Iw - Im
As you need to develop Vm across this shunt. So the resistance of the
shunt Rs is:
Rs = Vm/Is = (Im * Rm) / (Iw - Im)
The power dissipation of the shunt will need to be Is^2 * Rs.
As an example. You want a current of 100 mA FSD, using a meter with an
FSD of 1 mA and a resistance of 2500 Ohms.
Vm = Im * Rm = 0.001 * 2500 = 2.5 V
Is = Iw - Im = 0.1 - 0.001 = 0.099 A
Rs = Vm/Is = 2.5/0.099 = 25.3 Ohms.
Power dissipation for shunt = Is^2 * Rs = 0.099*0.099*25.3 = 0.24 W
Hence unless you are measuring lots of current, the dissipation of the
shunt is not the high. However, the wire-wound design might be for arc
protection. Since it seems not to have worked, perhaps putting a higher
power one back might not be a bad idea.
I expect you will find other ways of working out the shunt resistance on
the web.
--
"The day Microsoft makes something that doesn't suck is probably
the day they start making vacuum cleaners." -Ernst Jan Plugge.
Dr. David Kirkby,
Senior Research Fellow,
Department of Medical Physics,
University College London,
11-20 Capper St, London, WC1E 6JA.
Website: http://www.medphys.ucl.ac.uk/~davek
Author of 'atlc' http://atlc.sourceforge.net/
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|