Keithley 2450 SMU, Power supply spark at Turn-ON (fixed) - Page 1
Forum section: electronics / repair

author: HighVoltage
author_rank: Super Contributor
timestamp: May 17, 2017, 02:44:35 pm
content: My Keithley 2450 SMU scared me today, when I turned it on.I saw a flash and heard a spark at power turn-on, really loud and really bad.So, I took the instrument apart to look for the problem and it seems like the problem was found right away at the power supply.Right at the 230V input protection there is a part RT1 and it looks like a varistor / VDR but I am not sure.This part definitely is broken and needs to be replaced.It also seems all writing on the part is burned away.Anyone here having a better idea of what that blown component is and why it would fail in a relatively new instrument (2 years old)Thanks for any help.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 17, 2017, 02:46:18 pm
content: More detailed picture

author: HighVoltage
author_rank: Super Contributor
timestamp: May 17, 2017, 02:59:59 pm
content: OK, PSU removed from the instrument.Similar to Keysight, it seems Keithley is not making their own PSU.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 17, 2017, 03:02:07 pm
content: How interesting, this PSU is available from Farnell:http://de.farnell.com/xp-power/ecs100us12/psu-offen-100w-12v/dp/1738297?mckv=sOUpHKACu_dc|pcrid|88251738663|kword|ecs100us12|match|p|plid|&CMP=KNC-GDE-GEN-SKU-MDC-XP_Power&DM_PersistentCookieCreated=true&gclid=CPa8pPiY99MCFYaT7QoduF8E6Q

author: HighVoltage
author_rank: Super Contributor
timestamp: May 17, 2017, 03:12:19 pm
content: Taking a screen shot from the PSU datasheet, the part is market with an NTC 10

author: HighVoltage
author_rank: Super Contributor
timestamp: May 17, 2017, 03:15:32 pm
content: Is it possible that this broken part is a EPCOS 10 Ohm NTC Thermistor?

author: dr.diesel
author_rank: Super Contributor
timestamp: May 17, 2017, 03:55:07 pm
content: Quote from: HighVoltage on May 17, 2017, 03:15:32 pmYup, I'd say so!If you end up replacing it I'd suggest testing/loading the PS outside of the 2450, just in case there are other issues with the PS.

author: pmcouto
author_rank: Unknown
timestamp: May 17, 2017, 04:03:07 pm
content: Yes, it's possible.10R NTC makes perfect sense in that PSU.

author: pmcouto
author_rank: Unknown
timestamp: May 17, 2017, 04:09:47 pm
content: NTC usually explodes when there’s a short circuit on the primary side of the SMPS.I would suggest to check it carefully, starting with the bridge rectifier and switching transistor.Also, check the fuses – One of them is probably open.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 17, 2017, 05:03:39 pm
content: OK, thanks for the confirmation.I ordered some 10 Ohm NTC and they should arrive in a couple of days.Interestingly, although there was this explodes NTC, the 2450 still turned on.But I will inspect the PSU carefully and take some measurements.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 19, 2017, 12:27:02 pm
content: The new NTC arrived today.The old one had about 10 mm diamterThe new one has about 14 mm diameter NTC 10D 13Although the new one is much larger, it fits well on the PCB.The rest of the power supply showed no problems.I could even load it to 8A with my DC load and everything seems perfect.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 19, 2017, 12:33:34 pm
content: Now I measured the inrush current on the 230V input side to the PSUInrush Current, MAX =49.6 AIdle current of the instrument =718 mACurrent at a 1A DC output of the instrument:1.3AI am surprised by the high inrush current, but may be that is normal?

author: HighVoltage
author_rank: Super Contributor
timestamp: May 19, 2017, 02:51:06 pm
content: Well, I thought to replace the NTC in myKeithley 2460 SMUbut that one looks very differently inside.Actually it looks like a completely different generation of SMU.Here are some pictures.

author: Gyro
author_rank: Super Contributor
timestamp: May 19, 2017, 03:59:33 pm
content: If you power cycle the unit too quickly the NTC doesn't have time to cool to ambient, so increases the inrush current significantly. I'm not sure if this was the problem with the original NTC but I've had it before when I've cycled SMPSs too often during testing. Do it too many times and I think it becomes a fatigue failure mode.