Help Identifying values of a few diodes

Thread Starter


Joined Mar 1, 2011
Ive had an issue of an electrolytic cap dropping its guts over my heater control board. Its caused the main transistor 2sd1525 to fail. After replacing the capacitor and transistor, cleaning the board off I re-fitted it back on the car. All was bueno until A day after, Where I got the same symptom. Fault code 'high starting current' I suspect the fan unit and a replacement is on its way to me. But it seems that the electrolyte has corroded a lot of the board, and in my OCD state, would prefer to renew some components or lift them to clean underneath to get the electrolyte, silkscreen off the board.
I cannot Identify the values of the diodes, Attached is the schematics kindly drawn up by another forum member who had issues with his.

The main ones are D2, D3, ZD1, and ZD2.
ZD2 is a 1AZ20, but I cannot find a supplier for that diode.
Any help would be greatly appreciatedCircuit%20Diagram%20Blower%20power%20stage.jpg Circuit%20Board.JPG


Joined Sep 9, 2010
I would suspect that's a 20v zener rated to 1A.

In my opinion, you're likely to create more problems messing around. Replacing the electrolytic caps makes sense but otherwise I'd leave it alone unless you can see outright damage.

ian field

Joined Oct 27, 2012
I would suspect that's a 20v zener rated to 1A.

In my opinion, you're likely to create more problems messing around. Replacing the electrolytic caps makes sense but otherwise I'd leave it alone unless you can see outright damage.
I'd just clean it with neat dish washing detergent and a stiff paint brush. Scrubbing off the detergent will take out any contaminants.

Making sure its completely dry before putting back in service takes a little effort, but well worth while.


Joined Nov 30, 2010
rated to 1A.
Disagree. 1 amp would be 20 watts. That sucker is between two 10k resistors. Seems like any 1/4 watt or 1/2 watt zener would do. Likely something from the 1N5200 series.

Still, if it ain't broke, don't fix it.


Joined Feb 11, 2016
yea D3 has many K 1/4 watt on each side (check on that though, that they are all cheap kind). it is getting the output of an opamp i think (some do have high output or serve as amplifiers, switches, voltage regulators - likely a regulator)

but a diode is just NP of some rating so put in a power supply diode - or whatever high rating diode that seems to fit that you have. if the bias voltage (0.6V) is too high the opmamp (which appears to have 10V) won't pass through the diode and it wont work, if it does it will

the diode breakdown voltage shouldn't matter you dont want it to break down since opamps dont accept current at the output (the resistor back from 1 to 3 is to allow opamp to operate in linear mode as voltage regulator i think)

i doubt D3 is designed to break down (allow current V toward op amp) and bias the opamp in such a way to alter the total bias of the opamp somehow: it's counter-intuitive someone might design that in as "a need" rather than doing something plain (like a typical input back to opamp input)


Joined Feb 11, 2016
i also agree with what someone about said

if this D3 is (not a power diode) and is small: it also may not be what is causing the problem

ZD2 is a 1AZ20, but I cannot find a supplier for that diode. must be zener

if in doubt use ones that are similar to what else is on the board

you said replace capacitor but there are many.


likely if the capacitor was in bad physical shape it got overheated

so which capactor? and what is supposed to limit (ie, by offering resistance or by regulating voltage) to that capacitor

obviously if you stuck a megaohm resistor in line with the capacitor to protect it, it would not overheat (and also wouldn't supply the circuit with much of a capacitor)

another issue here is if the capacitor is beign shorted to ground (by the car, by bad soldering, by bad part, anything)


Joined Feb 11, 2016
did you do the ohm check on those coil xfrms? if they are (shorted) that might be an isue.

also that symbol on the printed board - that's not a typical diode symbol, and it is near an IC. it might be a shottky symbol? though i dont see how it'd make that kind of difference.

someone above said "zener" perhaps that is the symbol did you check? (zener can also act as a voltage regulator with resistors, an opamp as a switch rather than an amp - i havent diagnosed it thoroughly). if it's a zener acting so it might make a big difference.

the size of it would fit a typical zener - though that's guessing

i still think:

(1) the car not the board might be causing an issue. cars have say 20 computers. (and in german cars, have 5 networking protocols or more shared on at least a few different busses) ie some cars have power regulation computer boards that steer voltages on demand (if damage, it could steer the wrong thing in the wrong direction - it has relays, cpu, voltage regulators to use the same wires for completely different purposes; not a simple relay) (body control module). or you might have a short in the vehicle. and did you loose a ground connection (a bcm can drop a ground for exmaple).

(2) at the point of disecting the diagram for it's "intended operation", on such a small board, you may as well protect failing components with resistors and check each part's operation while running and use a dmm to see which component is not doing it's job when the (heater controls are changed). or snatch one out of a junk yard - you can then at your leasure compare the two and why they differ

also: i'm a little surprise but not completely that nowhere on parts sites, google youtube etc was a picture of the origional ? did you do much searching?


Joined Feb 11, 2016
hope that helped it was all general advice.

if FORGOT why i got back on!!!

my experience is: DO NOT OVERLY TRUST THE SCHEMATIC. they can be wrong, altered after printing - or have mistakes. and that's another general tip.

especially automotive diagrams - they're infamous for mistakes

hey - one mfgr recently release a car that had the radiator fan motors running backwards!! wrong motors you couldn't just change polarity. don't overly trust the auto-world

but recently i had a part i removed from a board that wasn't in a schematic - figuring the company was reputable and the part looked poorly soldered (and near things some one had broken). bam i fried something when i turned it on. i trusted "the engineers" and they're versioned schematics "per serial number" too much.