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R&S HMP4040 voltage spike at PSU turn-off
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electr_peter:
2017-2018 is 1-2 years newer model. HMP4000 series was made by Hameg from 2012 or earlier, this model is still made today by R&S. Predecessor of HMP4040 is Hameg HM7044.


--- Quote from: mahi on October 01, 2023, 07:27:25 am ---By the way, your diagram can't be right. You have a direct connection between the sense circuit and the outputs. In your diagram R+ and R- serve no (useful) purpose.

--- End quote ---
Yes, diagram above is incomplete and does not represent functional sense circuit at all, but it is the simplest circuit representation that matches impedance measurements across output and sense terminals in PSU OFF state. Resistive network is complicated by active circuitry (diodes and ICs), thus impedances look strange.

Actual circuit would be similar (but more complicated) to linked in your post above which in off state reduces to much simpler representation. Exactly what happens in off state is difficult to say without exact schematic. Also, your linked schematic gives a hint why shorting sense and output terminals can give more voltage/current.
SparkyBruce:
I checked my HMP2030 this afternoon and I also see the 2.3V ish output on all 3 channels after turning the unit off - even with the latest firmware.
I am collecting a second identical unit tomorrow and I will also check that.
This is all unloaded (apart from the 10M of the DVM).
electr_peter:
@SparkyBruce, can you also check date of manufacture and resistance across output (in OFF state)? I guess there are 10 kOhm on outputs.
mawyatt:
This is serious if across all these R&S power supplies, and not just a select rogue few.

If true then seems R&S needs to step up and issue a recall. As mentioned many times this can and will destroy modern small feature CMOS, as it produces gate oxide punctures which are non-recoverable and fatal!!!

Best,
electr_peter:
There is clearer estimated schematic of PSU with locations of sense and load discharge (?) resistors. Sense resistors are 1.5 kOhm and load discharge resistors are either 10 kOhm or 1.87 kOhm




--- Quote from: mahi on September 30, 2023, 04:43:39 pm ---The resistances between the output terminals on my HMP4040 are quite different from yours:
With sense terminal unconnected:
    between output terminals - 1.87 kOhm
...

--- End quote ---
By the look of PCB, 1.875 kOhm is four 7.5 kOhm in parallel. Source of PCB pic: "EEVblog #1174 - Rohde & Schwarz PSU Teardowns" at time 09:18




--- Quote from: SparkyBruce on October 03, 2023, 08:20:57 pm ---I checked my HMP2030 this afternoon and I also see the 2.3V ish output on all 3 channels after turning the unit off - even with the latest firmware.
--- End quote ---
Same as in posts above, just with HMP2030 instead of HMP4040. I believe this is HW related and FW would not affect it.
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