Current Measurement

Repeat your tests in the most simple possible setup.

As your results differ so widely, there is probably just a missreading somewhere. (Known as 'finger trouble' in my circles...others doubtless have another name for it!)

When in doubt ...simplify.

My instinct is to believe your 1ohm test result. But how did you measure the volts drop across it?...with a scope I hope.

Digital multimeters are not much use at v low signals which are not stable DC.

What type of actuator, how long does it operate for, what is the manufacturers spec'...What volts is it being driven from.... Is it a DC actuator...what is it's DC resistance.

Let us know how you get on.

PS.

I often use the 1ohm resistor as a quick test, (12v DC motor gearbox and electronic control cct type stuff)

Maybe try a Digital multimeter in series for a direct readout of current.

Del

Tricky to use a resistor to measure current. The resistor adds more resistance to the circuit and reduces the overall current. In your case, however, reducing the resistance also reduced the current. Strange.

My knee jerk answer would be to say that there's something wrong with your set up:

• you've probably misread your meter
• you've misread your resistor (that happens to me, especially with color coded ones)
• something wrong with your connections
• your meter's become like a brain dead person. It's still ticking and breathing but dead to the world.

However, you mentioned that you're using sense lines that you didn't have before. I don't know exactly what you mean but I'm going to hazard a guess.

Your other sense lines are getting part of the current and starving your sense resistor. 'Try taking them off first and repeating your measurement?

It's also possible that your current may have an AC component. That may result in wide differences in DC measurements between a large and a smaller signal - especially a signal that small.

Check the tolerance of the resistors used.

Low value resistors do not have good tolerance.

Take few sets of readings with different resistors

and if possible with different multimeter.

Try a known value of current.You will be able o guess the reliable components and multimeter.

Firstly, how did you measure 4 microvolt !!?

Secondly, with 1 Ohm resistor if the current is DC you should have 0,1 volt drop on it

and so 0,01 volt on the 0,1 resistor ,that you say you don't

obviously , something wrong is with your measurement or something you didn't explain well.

Thankyou very much for your help guys. Eventually I found that it was an error on my part as I left the sense terminals commoned together with the output terminal on the PSU, always something simple. And as stated the sense line and the power line shared the current between them.

Now you see why we call it 'finger trouble' !

Use a 4-wire kelvin connection. Your resistors and lead wires are too close in resistance values to accurately measure the voltage drop.

See: http://en.wikipedia.org/wiki/Four-terminal_sensing