Keithley 2420, Voltage overload in current mode - Page 1
Forum section: electronics / repair

author: Aheld
author_rank: Regular Contributor
timestamp: March 02, 2025, 04:44:39 pm
content: Hi Forum,I have a Keithley 2420 device which works fine at first view.But, if I sink 1 or 2 Amps for example with a short between output hi and low. It works fine only for while.The Fan speed goes up, direct after switching on the output.The device is driving the current, only for approx 2 minutes, then I receive the failure code "overvoltage detected" and I observed U211 with not normal temperature behavior (picture attached).At my last test, I observed that the current decreases continuously while the temperature at U211 is increasing.It is clear that I'll replace U211 (AD712) device.Is this kind of failure already explained some where, or any additional hints on this failure mode?Br.Axel

author: Kleinstein
author_rank: Super Contributor
timestamp: March 03, 2025, 08:43:16 am
content: When sinking the 1-2 A, what is the voltage ?There is a limit to what external voltage can be there as there will be quite some internal power loss. There is the obvious external power and additional the lower internal supply voltage. So the problem would be simply overheating from limited cooling. The manual should give a limit to the allowed external voltage - the error message could indicate too much power to handle over a longer time.It is a bit unusual that a simple OP-amp gets hot, but I doubt that this part is actually the one that gets too hot. It would be more the main heat sink to get hot. The somewhat warm OP-amp could be normal. Before replacing it, I would check the temperature with more normal operation.I would consider a small heat sink glued to it.Another point to maybe check could be if there is something oscillating. The combination of a regulated voltage source (e.g. lab supply) and electronic load (SMU in sink mode) can sometimes oscillate if one or both of them are on the edge of stability: the PSU may not like a high capacitive load and the electronic load can look capacitive. Similar the other way around the load may get unstable with an inductive source and a lab PSU circuit can look inductive - sometimes worse than a real inductor.

author: voltsandjolts
author_rank: Unknown
timestamp: March 03, 2025, 09:32:19 am
content: Quote from: Aheld on March 02, 2025, 04:44:39 pmYou placed a short circuit on the 2420 output, then set the 2420 to output 1 or 2 Amperes, into the short. This is the worst case for a psu, because if using a linear regulating element, it must dissipate a lot of power (depending on transformer tap, or pre-reg smps output). Not sure what the rating for the 2420 would be in this case. There may be nothing wrong with it.

author: Aheld
author_rank: Regular Contributor
timestamp: March 04, 2025, 11:47:12 am
content: Hello Kleinstein, hello voltandjolts,thank you on your responses, let me check you points and I'll come back.Regarding the direct short at the output:I agreee that this is a worst case for the PSU, but I'm woking with Keithley SMU over years, mainly on semiconducter characterization in comparable configuration.SMU is driving high current and the current comes always back to the SMU channel. From my point of view SMU should be able to handle this mode.I found the idea nice with the small heat sink. I think I still have one self glueing type from a Raspberry controller.I'll try it an see how the behavior changes.Br.Axel

author: voltsandjolts
author_rank: Unknown
timestamp: March 04, 2025, 01:12:42 pm
content: Well, true, Keithley literature suggests it should be possible...