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

[harp]

The initial idea was to use psu as measure device, not only as power source. The whole purpose of haveing a current limit is to prevent damaging circuit, and I use and explore that option... if the current realy stay at 5mA level all the time I dont think that while switching taps there can be enough surge power to damage 1w diode........................... So function of CC is no acomplish his task for fine electronic, I see that on that way...

[harp]

For these who want to test your linear psu, this is setup: set the voltage of psu al least to pass to one higher transformer tap (or simply to 30v) then the testing zener is, and set fixed current at 5mA. For these who have 3 digit display, you can anyway set fine current using a external dmm with caution. Example and procedure for 9v1 zener is: set voltage to 19v, current to 5mA, and connect zener across the probe (anode to ground), then read voltage on display, it will be about 9v, write on papers all digits and disconect probes. Again connect probes and read voltages on display - if the zener was damaged in this proces, the reading will no to be same, every time is lower and lower... In this example is using regular transformer for psu with taps on 8v, 15v, and 23v.

[stj]

if the tap changes on the fly then you will see a big increase or decrease in input voltage of the linear regulator section.
the regulator output will reflect that until the feedback makes it compensate for the change.
i guess the regulator feedback isnt fast enough to save the diode.

if it was my design the microcontroller wouldnt allow you to change tap while the output is enabled.
maybe it doesnt have a microcontroller and the tap relays are controlled by linear feedback.
that would be bad!

[harp]

I go with extend testing and kill some regular LED diode at "5mA". Now I can conclude, it is not issue about switching between taps and induction, but inertion of regulation circuit to do real "constant" current. I tested in first range on 7.6v (without switching occure), and noticed that led initialy briefly flash much stronger that can on 5mA. Then I set it to 10v, and visualy is the same even switching occure. LED stay undamaged. Then I setup to 30v, and led instant die after initial flash...................... as many as I watch lab psu review, in all is pay attention to not overshoot in voltage while switching taps, but never seen a measurement of overshoot in current... what is consider good, normal, poor I dont know...

[m1ch43lzm]

https://www.eevblog.com/forum/projec...-diode-tester/

Hi @johnwa. Nice little project. I am an incurable electronic gadget man but for zener testing I use my power supply set to a constant current of a few mA. Starting at zero I wind up the volts until CC sets in and reveals the zener voltage.
enut11

(For anyone else who wants to try this, make sure you follow enut11's advice exactly, and wind the voltage up slowly from zero - many power supplies have poor transient response and/or excessive output filter capacitance, which could otherwise result in damage to the DUT)

This looks like the correct answer, by the time your PSU switches from Constant Voltage to Constant Current, the zener (or the LED) dies by the initial voltage/current spike; what you need is a constant current supply with a high enough compliance voltage for the DUT

Most linear bench PSUs have a big output cap around 10000uF or more

[harp]

So, is that mean that smps lab psu are affected too with this issue?

[stj]

that depends on the design,
cheap ones are constantly variable and better ones have a linear regulator between the smps and the output.

[Agent24]

Have a good read through https://www.eevblog.com/forum/testge...ower-supplies/ and you will see why you keep blowing zeners.

Best practice is to use a resistor in series with a known voltage, and forget about the current limit on the PSU.

Or build a dedicated zener diode tester, like this one: https://www.siliconchip.com.au/Issue...r+Diode+Tester

[stj]

or build a 300v+ one like mine!

[harp]

"and forget about the current limit on the PSU."... and for me that is the only point of haveing lab psu in use........................... Have someone schematics of that psu UNI-T UTP3315TFL, I have noticed that they did a improvement in meantime by ommitting vr6 and vr8 in 2025 selling model... and flip over top to220 element... How hard can be that do a propper CC function, I dont think thay pay any attention at all for startup detail in firstplace... so perhaps it can be easely fix?

[Agent24]

"and forget about the current limit on the PSU."... and for me that is the only point of haveing lab psu in use............

Well of course you can use the current limit for other things... just not testing zeners, apparently.
Did you read the other thread?

[harp]

Yes I read eevblog link, and see there is the same issue even with branded psu. Everything point to electrolitic capacitor on output, so that seems for that ammount of energy isnt controlled... ridiculus... for my perspective it is a mayor flaw in design, and wonder how and why that problem isnt yet consider by (analog) psu designer, even on cheaper one psu... Now I have to worry if my psu has silently destroy some components while "repairing" something.................................. Everything starts with try to test zenner, but now it really isnt it, it is about psu, has he deliver what supposed to do all the time, partialy or just in some condition...

[harp]

Some overcurrent testing............ https://m.youtube.com/watch?v=RdT-xWC4h4k

[Agent24]

They do talk about some high end supplies having no output capacitors and even extra transistors to pull down the ouput voltage faster when needed, but those are big $$$$ gear.