[Amps] meter shunt
Carey Lockhart
kc5gtt at gmail.com
Thu Aug 25 09:12:34 PDT 2011
now lets say i have a callabrated meter and i want to check plate current
against the amps meter. is the shunt still required in order to get a semi
accurate reading? say going from my b- to ground or to my bias circuit
input. i am using a pnp with a zener for biasing. i have the meter switched
for HV and Ip. so one or the other leg of the meter is going to ground. and
then the bias circuit pulls from ground.
Carey
On Thu, Aug 25, 2011 at 6:25 AM, <TexasRF at aol.com> wrote:
> **
> Hi Carey, there are number of ways to design meter shunts. A method used by
> many is to install a low value resistor in series with the circuit and
> measuring the voltage drop across the resistor with the meter.
>
> Meter over current protection is easily added by shunting the low value
> resistor with opposite connected diodes. That clamps the voltage to +/- .7
> volts or multiples of that with diodes in series.
>
> Starting with a 1 ohm resistor and a target of 500 mA full scale for
> example, the voltage would be I X R. At 500 mA, there would be .5 volts.
> Your meter being 1 mA, needs a resistor in series to limit the current to 1
> mA when there is .5 volts present. Normally that would be .5 / .001 = 500
> ohms. But, the meter has resistance that has to be included in the 500 ohms.
> Personally, I determine the series resistor experimentally while measuring
> the voltage drop with a known good DVM.
>
> The 1 ohm resistor needs to handle full current reliably and for a long,
> stable life should be rated for several times the current expected. At 500
> mA that would be .5 X .5 X 1 or .25 watts. 2 to 5 watt resistors would give
> plenty of head room and are small enough to fit most anywhere.
>
> At a lower current, like 10 mA for example, the low value resistor is not
> exactly low value. If you want to use the diode protection scheme, then a
> value of about 50 ohms will have a voltage drop of .01 X 50 = .5 volts. The
> meter resistor then would be .5 / .001 = 500 ohms less the meter resistance.
>
> The meter and it's resistor are in parallel with the low value resistor.
> That means the 1 mA of current flowing through the meter is not flowing
> through the low value resistor. So, the 500 mA example would actually be
> 499 mA full scale and the 10 mA example would be 9 mA full scale. To
> compensate, the meter resistor would need to be adjusted to a slightly lower
> value for accurate measurements.
>
> All of this is just using Ohm's Law; no rocket science at all!
>
> 73,
> Gerald K5GW
>
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> In a message dated 8/24/2011 10:47:28 P.M. Central Daylight Time,
> kc5gtt at gmail.com writes:
>
> i have a 1ma meter that i would like to rescale and use with a little gi7 2
> meter amplifier. what size shunt should i use or what is the calculation i
> should use?
>
> Carey, kc5gtt
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--
Carey Lockhart, KC5GTT
Boerne, Tx. 78006
www.kc5gtt.com
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