# Digital Multimeters (DMMs) and Precision DC Sources

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## How does current limiting in the SMU work?

Hello,

I am sending arbitrary voltage signals across a DUT (which has unpredicably varying resistance) and measuring the current with an SMU. I just have a general question about how the current limit function works, physically, in the circuit. I've attached a simple circuit diagram, and I'll describe it as best I can, and hopefully my question will be clear.

I am sourcing using a DAQ 6368, measuring current using an SMU 4141 (just using it as an ammeter, voltage is set to 0V). I also use a DAQ input channel to measure the actual resistance across the DUT. The DUT, as I said, has variable resistance, so occasionally when the resistance drops to be very low, I reach compliance current on the SMU. This gives rise to a situation which I'm having trouble understanding:

The resistance of the DUT has unexpectadly dropped to ~100 ohms. I am applying 5V across the DUT, and my voltage probe confirms that I am in fact sending 5V across the DUT. This should mean ~50mA of current, but the max current output of the 6368 is ~5mA, so that is not possible. The SMU, meanwhile, has a current limit of 100uA, and it is reading 100uA.

What is happening in this scenario:

Does the SMU have an internal resistor which engages at the max current or something? If this happens then why is there still not a linear response to more voltage (the current remains at 100uA regardless of voltage).

The SMU cannot just be dumping extra current to ground because the DAQ cannot source that much current.

I've read that the SMU also can source an opposing voltage to ensure voltage drop across the internal circuitry is 0V, but the DAQ input still reads 5 V.

So what is going on here?

Thanks!

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## Re: How does current limiting in the SMU work?

oops, I found a typo, should say:

I also use a DAQ input channel to measure the actual voltage across the DUT (including the SMU, though)

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## Re: How does current limiting in the SMU work?

According to your diagram, you're not measuring the voltage across the DUT, but across the source.  When the SMU reaches its current limit, it adjusts its output voltage so as to keep the current limited.  So if it's measuring 100uA, you can be pretty sure it is no longer sourcing 0V.

So if I understand what's happening, you have a 5V source, which you measure.  10mV of that is across the 100Ω DUT (100µA), and 4.99V is across the SMU.

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