[GreenKeys] Milliamp meter shunting...

[Bob kb8tq]

Hi

All that and we still have not gotten into ballistic galvanometers :)

Bob

> On May 6, 2017, at 9:05 PM, drlegendre . <drlegendre at gmail.com> wrote:
> 
> Ken,
> 
> As you might have concluded, a meter's scale markings may not indicate the actual "rating" of the meter. You can have a meter marked for "RF Watts X1000" which by no means directly measures 1000s of RF watts.. but rather, as a current driven by a voltage across some external low-impedance shunt / detector mechanism. 
> 
> But mainly, it's important to understand that +all+ analog meters are +current+ meters, in some form. This should be obvious, in that it requires a given current through the meter winding to produce a certain amount of deflection. Analog mechanical "volt" meters are really just high-impedance ammeters, with the best requiring as little as 50uA to drive full-scale. 
> 
> So you can see how such a very sensitive movement might be viewed as voltage-driven, being that it will still give an acceptable reading even if the source impedance is relatively high and the resultant current quite low. 
> 
> 
> 
> On Sat, May 6, 2017 at 1:37 PM, Ralph Mowery <rmowery28146 at earthlink.net <mailto:rmowery28146 at earthlink.net>> wrote:
> It would be hard to tell without knowing the model of the meter.
> 
>  
> 
> Some of the meters that require shunts will have on the front of the scale near the bottom in very small words something along the line of meter full scale 1 ma or FS=1 ma. Or whatever the meter movement actually is.
> 
>  
> 
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> 
>  
> 
> From: GreenKeys [mailto:greenkeys-bounces at mailman.qth.net <mailto:greenkeys-bounces at mailman.qth.net>] On Behalf Of Kenneth Gartland
> Sent: Saturday, May 06, 2017 1:49 PM
> To: greenkeys
> Subject: [GreenKeys] Milliamp meter shunting...
> 
>  
> 
> Hi guys...
> 
>  
> 
> I have a couple of old Simpson ma meters rated 
> 
> at 100 ma...
> 
>  
> 
> I went to use them to test a known 40ma cct.
> 
> the other day, and both pegged at over the scale limit...
> 
>  
> 
> Do these older meters require some type of "Shunt" 
> 
> across the terminals to get an accurate reading?
> 
>  
> 
> I know the newer meters can be directly connected in
> 
> series with the circuit load...
> 
>  
> 
> I used to know this kind of stuff, but too many brain farts
> 
> these days to remember!
> 
>  
> 
> Tks for help on this issue!
> 
>  
> 
> Cheers!
> 
>  
> 
> Ken
> 
> VE3-HMQ
> 
> 
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