Digital Multimeters (DMMs) and Precision DC Sources

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SMU 4139 resistor

We are testing a DUT which has a DC-DC power supply function. Our test setup uses one 4139 SMU to supply input power to the DUT, and a second 4139 to act as a current load on the DUT output. The load SMU is operating in current mode, with a steady 4V output from the DUT, and the SMU syncing anywhere from milliAmps to 2A.  This usually works, however the load SMU often throws an Overload Protection error, forcing a reset of the SMU.  The SMU is being connected to the output of the DUT through either SPDT switches, or a switch matrix card.  It's not clear what is causing the random OLP errors.  I can run the test sequence one time, and it will work okay, then re-run it on the same DUT and the OLP might occur. Any thoughts or suggestions?

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Message 1 of 8
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If a moderator could change the topic name to "SMU 4139 Overload Protection"... that would be appreciated. Smiley Embarassed

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Hi,

 

I'd like to get more information around the overload protection instances. Are there any consistent ranges or time periods in which you see the OLP occur? Does it appear to be completely random or always occur after a certain amount of time or range of values?

 

Any other information would be great.

 

Regards,

 

Finch Train

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It always happens right away, when Initiate is called. But only if the SMU is connected to the DUT. If I do the Initiate BEFORE closing the matrix switch that connects the SMU to the DUT, nothing happens -- there's no load on the SMU, so no current is actually flowing. Then I connect the SMU to the DUT and perform initiate again, the OLP is thrown. Conversely if I start with the SMU in then off state, then connect the DUT, the first Initiate will trip the overload.

Now that I think about it, the Initiate vi doesn't actually fail. it is the wait for event vi immediately after initiate that causes the OLP to happen, or at least for the OLP state to become known to the system.  I should try to step through the code and pause after initiate to see if the SMU hardware is actually OLP'ing. 

My working theory is that there is an instantaneous surge when the SMU transitions to the running state in current mode. It seems like the SMU would limit the current and voltage because the limits are set low enough, I'm no where near a 12W syncing load (the max for 4139). If the OLP doesn't happen immediately, the SMU can pull 2A at 4V from the DUT all day long. It just has to get past the initial turning on.

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Hi,

 

That sounds like a valid theory. Do you have other equipment to measure the current when the switch completes the circuit? Based on your description, the surge from the Matrix sounds like a plausible problem.

 

Keep me posted on steps you've tried.

 

Regards,

 

Finch Train

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We are testing a possible solution, which is to open the "Output Connected" relay prior to Initiate and closing the relay just after Initiate. So far the results are positive (no OLP errors).

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Since no one has jumped in yet. The default state for the 4139 as far as I know is to be set in Voltage Ouput mode at 0V with the output connected. So until it is configured otherwise when initiated, that would present what appears to be a short to your DUT if it is expecting some type of load, which is likely why it trips the protection. 

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Larry Morgan
Senior Hardware Engineer
National Instruments
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I found this thread having a similar issue to the original post.

Two NI-4139 through switch matrix measuring simple resistance both in current mode.

One SMU as primary source (1mA) second SMU as a simulated sense line (1uA) to eliminate lead resistance.

 

I found that the resistance of the voltmeter portion of the SMU essentially limits the minimum current that can be applied on the sense SMU while still achieving the full 60V range.  You can try this yourself, float the SMU and source 1uA at 20V compliance.  You will get ~10V rather than hitting the compliance value of 20V.  A bit of math suggests ~6uA is the minimum current which can be sourced that still allows use of the full 60V range in the case of a large resistance or an open.

 

When I encounter an open circuit in my application at the original current levels the sense SMU potential of 10V allows the primary SMU current to flow through the source SMU voltmeter throwing the OLP error.

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