OK, I have a PM128 (Data sheet).
PM128: 1mA and an impedance of 10 MΩ
The adjustable (volts and amps) power supply is rated to give 18v at 1 amp. I couldn't get the right transformer and I have a 12.6 1.2 amp transformer feeding the PS. (It really should have a 14-16V center tapped 1A transformer, but I couldn't find one at the time.)
When I use the method here, I get an answer of 18.01 Ohms, and 18W for a resistor.
When I use an algebraic method:
Vshunt=Vmeter
(s- shunt m-meter)
IsRs=ImRm
Is = Itotal - Im = .999A
Rs=(ImRm)/Is = .001A(10,000,000Ω)/.999 = 10,010 Ω
Now, considering that the impedance for the PM128 is so high, I would think that it could be ignored?
But wait! There's more!
I have the PS connected to terminals.
I have the PM connected to those same terminals.
And I have a switch that will put a 1 Ω - 10 Watt resistor as a shunt to the PM.
Here's where it gets weird.... I attach a MM to the terminals to see if the PM is accurate.
Voltage is accurate.
Current on the other hand, when the current dial is maxed out shows .91A. BUT between 0 - .91A, the PM shows about twice as much as what the external MM shows in current. AND when I turn the MM to measure volts, it shows the same thing as the PM.
Now I understand that when taking current or any other measurements of a circuit, you're adding to the circuit. So, maybe I'm adding a couple of shunts to the circuit and that's why I'm getting such different readings?
Or, is it a design issue?
Any input would be appreciated.
P.S. For the record, I think I'm misunderstanding something really basic here and it's not getting through the concrete here (taps head).
Thank you!
PM128: 1mA and an impedance of 10 MΩ
The adjustable (volts and amps) power supply is rated to give 18v at 1 amp. I couldn't get the right transformer and I have a 12.6 1.2 amp transformer feeding the PS. (It really should have a 14-16V center tapped 1A transformer, but I couldn't find one at the time.)
When I use the method here, I get an answer of 18.01 Ohms, and 18W for a resistor.
When I use an algebraic method:
Vshunt=Vmeter
(s- shunt m-meter)
IsRs=ImRm
Is = Itotal - Im = .999A
Rs=(ImRm)/Is = .001A(10,000,000Ω)/.999 = 10,010 Ω
Now, considering that the impedance for the PM128 is so high, I would think that it could be ignored?
But wait! There's more!
I have the PS connected to terminals.
I have the PM connected to those same terminals.
And I have a switch that will put a 1 Ω - 10 Watt resistor as a shunt to the PM.
Here's where it gets weird.... I attach a MM to the terminals to see if the PM is accurate.
Voltage is accurate.
Current on the other hand, when the current dial is maxed out shows .91A. BUT between 0 - .91A, the PM shows about twice as much as what the external MM shows in current. AND when I turn the MM to measure volts, it shows the same thing as the PM.
Now I understand that when taking current or any other measurements of a circuit, you're adding to the circuit. So, maybe I'm adding a couple of shunts to the circuit and that's why I'm getting such different readings?
Or, is it a design issue?
Any input would be appreciated.
P.S. For the record, I think I'm misunderstanding something really basic here and it's not getting through the concrete here (taps head).
Thank you!