I have a 1000 turn current clamp that claims it can measure "up to 5A".

But when I test its output in the 10A range, it's still spot on. Same at 15A. I don't have a load to measure higher.

What is the electrical theory behind a limit a current clamp can measure linearly? Is it core saturation?

 

There is resistance in the 1000 turns. This causes an error in the reading. That should be linear. The heat from this loss will cause problems including the resistance to change. Thus over seconds the reading will change a little.

Core saturation should be non linear. The (Voltage X Time) will cause the core to stop functioning.

 

I measured the clamp and it has 50 ohms of internal resistance. So let's say 20A is flowing through the clamp, and there is a 88 ohm burden resistor. There will be 5 volts across the burden resistor.

0.5w will be dissipated in the winding of the clamp, and 0.28w across the burden resistor. Is my math right?

So the resistance change (due to heating) in the winding and the resistor is what will mess up the linearity when measuring high currents?

 

Rethinking: The transformer is 1000:1 so 20A=20mA. The 20A is constant so the 20mA is constant. (assuming no saturation) The secondary is a "current source". It will force 20mA into the burden resistor. It does not matter what the internal resistance is. (20mA into 88+50 ohms) You can only see 20mA & 88 ohms.

Lets say Burden Resistor=88 and Internal Resistor =50. From the outside you will see 20mA on 88 ohms. You will not see 20mA on 138 ohms. From the primary side you will see almost twice the voltage you would normally see.

I have wondered why the core saturates sooner than I predicted. It is because the transformer must make enough voltage to drive the burned resistor and the internal resistance.

Now I am saying the internal resistance causes problems but I think it is not a cause of non linear errors. (with out saturation) You should only see constant current into burned resistance.

 

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3