question/caution note to usage of linear psu

[harp]

I have UNI-T UTP3315TFL (2025 model) and just want to measure some zenner diodes directly with it. Set it to 12v and 7mA and connect BZX85C9V1 to it... shown on display 8.91v at 7mA... that seem fine to me. But, I guess I have in box a larger zener, let set it up to 16v, same current. Relay inside clicking to bigger range over 15v. When I test the sameone zenner accross probe, it clicks back to lower range, but displayed about 0v. The zenner was samehow killed by switching winding on 7mA limit... It was a 1W zenner, and can take up much higher voltage and current, so what is hapened realy, am I invent new psu quality test, or do some fullish thing?

[stj]

i always wondered about professional psu;'s that use multiple transformer taps.
i never expected them to change taps on the fly!!

[harp]

I am not get it... the switching between taps is done before rectifier, and not interference with setted current limmit. If it generate a high voltage spike by induction, there is much control electronic on the way before get to the output. I am a bit worry, because, if it connected to some circuit (that has to be repaired) and start to switching between taps for any reason, it can silently kill protection diodes in mosfets, ICs, makeing them shorted........................... I like to hear from different users and psu (linear) model, if they can perform above zener test and worrylessly swipe a full voltage range at fixed 5mA limit with no damage a diode, to dismmiss a possibiility to only one issue with my model/these model from UNI-T?

[stj]

can you do it again and capture the signal with a scope?

[harp]

I just kill another 9v1 zenner at 5mA, that say, it is repetable indeed. I have no digital scope so I can do only a yt review, capture execution on video clip... so guys be aware of that. I think it is only issue specificaly with zenner and zener properties to conduct at some point and too slow regulation (or too fast letting go) that control current. When it switch to lower tap, current limit is fast release in gap while induction surge hit zenner and slowly initiate again when is diode yet damaged........................ This topic is for someone who has needed equipment (on mind eevblog), I beleave that is no likely manifest in real circuit... but... not happy at all...

[m1ch43lzm]

When testing zener diodes use a resistor in series with the + output of the PSU, 1k or 2.2k, you can use lower resistor values if you want to test at a higher current
Haven't tried just relying on the current limit, as my bench PSU doesn't have that precision

Of course the zener diode will "break down" once you exceed its rated zener breakdown voltage

Maybe the current limit is too slow to react if you start at a higher voltage, you can test what happens if you do the opposite: start at lower voltage and slowly increase the voltage until you notice an increase in current and the voltage stabilizes