Simulating 5mV with a fluke meter

[Pitt123]

I need to simulate a 5mV signal to a load cell input to simulate the output of the load cells why they are not wired up. My process fluke meter only has a mA output signal and does not have a mV output signal. Any ideas how I can simulate this desired 5mV signal at the load cell input.

Could I select an mA output on the meter and then calculate a resistor size to drop the 5mV across the resistor? For example if I output 20mA on my meter can I use 5mV/20mA = .25ohms and then use this voltage drop across the resistor as the input to the load cell input?

Could I use a 24V power supply with 2 resistors to create a voltage divider?

Do I need to take the input impedance of the load cell input into account when doing this?

 

[petersonra]

If the meter can't do 5 mV directly, probably the easiest solution is to use a 5 mA output driven into a 1 Ohm resistor.

Whether the input impedance of the load cell input will matter is another thing.

A lot depends on just what input you are trying to simulate. If it is an actual load cell, the bridge input might not appreciate your external voltage source.

I'd be inclined to buy a cheap load cell for this kind of testing. They are available for $20 or $30.

 

[jbelectric777]

I was gonna say what pete said, use a resistor. I got tired of PCB boards and soldering IC's on em, I took IC sockets and soldered tails off em insulated with 1/8" heat shrink, works like a charm !:grin:

 

[jbelectric777]

I mean Bob, sorry

 

[GeorgeB]

Could I select an mA output on the meter and then calculate a resistor size to drop the 5mV across the resistor? For example if I output 20mA on my meter can I use 5mV/20mA = .25ohms and then use this voltage drop across the resistor as the input to the load cell input?

That will work, but I'd probably use a 1 ohm at 5mA as others suggested, or 1.2 ohm at 4mA.

Could I use a 24V power supply with 2 resistors to create a voltage divider?

When I did some of this kinda thing in the early 80s, I took a D cell, a resistor to draw about 2 mA to a forward biased diode (to eliminate battery drop over time) giving about 700mV. I then took about 1.8k in series with a 100 ohm pot. That pot wiper adjusted to about 40mV, covering the 0-30mV range with my cell.

Do I need to take the input impedance of the load cell input into account when doing this?

Maybe, yes, no ... in my case, exact numbers were not the issue. The input Z on a voltage input is usually quite high. I watched the actual via a PLC; I trusted the PLC. If you are trying to CALIBRATE an input, my method is not good ... but then neither is the current source and resistor unless you know the real value ... you can use a meter in that case to measure voltage and calculate actual resistor value.

 

[Pitt123]

A lot depends on just what input you are trying to simulate. If it is an actual load cell, the bridge input might not appreciate your external voltage source.

Yes we are trying to simulate the load cells mV feeback into their respective input. Can you explain more the potential problem of the bridge not appreciating the external voltage source?

 

[petersonra]

Yes we are trying to simulate the load cells mV feeback into their respective input. Can you explain more the potential problem of the bridge not appreciating the external voltage source?

If you are applying this just to the input of the transmitter, its probably OK. I doubt it will give you any consistent reading, but for test purposes it should be OK.

I realized afterward that you had written in your post that it would only be used when the load cell was not connected.