• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

ID a zenor diode

Thread starter #21
if you reverse polarity connected the battery to the output
The adapter is new, never used.
The only thing I did was test the voltage to make sure the center pin in the plug was positive and that's when I discovered the low voltage, about 1.8 instead of 3vdc.
The vendor has refunded the purchase price so I am just trying to gain some experience working on it.
There are four connections on one side of the transformer and two on the other but I'm not sure what I should be seeing on any of them.
I looked for the reference numbers Hi Pot 66015620, RO1 SC22, 155-10A, c828-03 but didn't have any luck.


Nigel Goodwin

Super Moderator
Most Helpful Member
I pulled it out of circuit and used a digital multi meter with a capacitor function.
That's not testing it at all, as I said the only way to test them is with an ESR meter.

When capacitors go high ESR (which is almost always how they fail) the capacitance doesn't usually change, or at least not enough to be out of spec, so a capacitor meter, a capacitance bridge, or the capacitor range on a multimeter will show a faulty capacitor as OK.

But assuming it's actually like the circuit you posted?, it will work perfectly fine without the zener in circuit - it's only there as a safety measure, to go S/C, if the output goes too high.


Active Member
digital multi meter with a capacitor function.
? both polarities - what you should check is use 20kΩ region and alernatingly charge it in opposing directions (while doing so note that you won't discharge it with your at worse about 18kΩ skin resistance) - what you should see is ohm-meter showing up some resistance that fast fades to infinity (. 1 on LCD) - if it doesnot fade to ∞ then there's a DC path through it in that direction
It's a component which commonly fails in these PSU's (because they use a cheap lo-spec capacitor), and failure of that capacitor will blow the crowbar zener.
what Mr. N. said is that the cap smoothens* out a current peak from TF (so the zener has to deal with now* already a way lower load) -- so if the cap has too high ESR or is "dead" then all of the previously said doesn't occur and your zener is on it's own

+ sketching a schematic at least for your own use is advised -- is just something to help you to be shure of what you're doing and what effect to expect from it (those supplies may have their own specific peculiarities - and you shouldn't do anything based on an assumption what the schematic may look like)
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
what Mr. N. said is that the cap smoothens* out a current peak from TF (so the zener has to deal with now* already a way lower load) -- so if the cap has too high ESR or is "dead" then all of the previously said doesn't occur and your zener is on it's own
It's far worse than that, the electrolytic is a vital part of the entire PSU, with the capacitor high ESR the output voltage rises (considerably), this will cause the zener to do it's job and go S/C.

At least this PSU has a crowbar, most don't - numerous items are destroyed when their PSU goes high for this reason. If anyone has ever repaired the PSU's in Grundig/Goodmans Freesat boxes an identical capacitor fails, the PSU goes high, and there's about a 50% chance that it will destroy something vital, and make the box scrap. The annoying thing is that there will have been fault symptoms for a few months before it dies altogether, if customers brought them in when symptoms first appeared (before the ESR gets too bad) then a simple capacitor replacement would cure them.
Thread starter #27
My meter only has a 40k range but at that the capacitor will show a fluctuating reading then fade to 0.
Reversing the leads is the same.
Thread starter #29
I'll order an esr meter and retest the cap, it's something I should have anyway.
I really appreciate all the input.

Nigel Goodwin

Super Moderator
Most Helpful Member
Thread starter #32
I used the esr meter to test the cap and compare it to a new cap and there was a difference so I replaced the cap and zenor, the 3 volt adapter is now producing 3.54 volts dc.
The cap still seems excessively hot but I'm going to heat sink it and keep a close eye on it.
Thread starter #33
The capacitor is generating too much heat, something else is causing the problem so it's headed to the garbage after I pull and test the other components.


Active Member
i am not doing such kind of repairs and tests and the reason here is quite simple
it requires good and sophisticated lab equipment that i simply can not afford all at once (or just for once)
the missing lab equipment can be substituted by years of experience in this specific or related field - so you could match the simple test data to a possible fault condition of the circuit - and you know the simple measure to spot it - i'm more into digital than power el.-cs

what you describe suggests that your output winding may generate HV spikes (too heavy energy I/O) or the isolation does not hold at input (?failed suppressor ?failed lacquer and insulation . . . or your High Side puts too much energy into TF for the Low Side to manage . . . or the combination of prev.)
so you basically should be able to measure/display your input and output 100kHz wave forms with 100ns spikes and be able to tell wheather such is ok or indicates a failure of some sort - and what is the actual signal respect your scope/probe setup

i personally see no reason why to invest 1000-s of € to be able to fix a bad homework of some commercial market spoiler - while i can see by few € how some other brand manages - which can be predicted by cust./owner feedback info avail at www ?

if it were my thing i'd simply detached it to spears -- the simple reason is - ordering the replacement components that usually is iC or TF *** see below *** would cost more than buying a new one

if you like to learn something from it (detachment) - draw the circuit up , identify all components , parametrize the TF (you need also parmetrize the core - or most impossible - get the asian -- or why not also a western -- manufacturers datasheet, by few cryptic digits if any on the TF marking label . . .) . . .
. . . however if you manage the prev. you can redesign or reassemble the thing - see how it's doing ← ← even if all prev. goes smooth and without fatal (to components and/or PCB) mistakes what time does it still take - - can you actually spend that if you can't predict sure success . . .

for example i had 2x of https://www.radiomag.com.de/product/adpv18b-12v-1-5a-es18b12p1j-tischaufstellungnetzgerat_19508.html suddenly one of them dropped it's output to Zero (nothing) *** as previously *** all other components were ok after un mount but the switch and TF -- which had !!! !!! a factory repair on one of it's U cores [ ]←glued together at corner !!! !!! - the other one likely does not have that fix coz it's still operational -- i still would recommend the thing (just put the candle in the church you won't need to buy too many before getting a "stable" one)
Last edited:
Thread starter #35
Is there a method to test this transformer if I don't know the voltage in or out?
There are four pins in and two that are for the output supply.
It belongs to a 3vdc ac/dc wall adapter.


Latest threads

EE World Online Articles