Why won't this power scaler circuit work?

[J M Fahey]

***somehow my earlier answer was eaten by the Forum dog, here it goes again.
It actually fits halfway between posts 29 and 30***

I don't have another zener on hand, so that will have to wait until my order arrives.

I can not believe you are a Service Tech and donīt have *one* Zener, any voltage, on hand.
You should have at least 15V zeners, the most used value out there.
If you need one and donīt have it itīs the most expensive Zener on Earth: 15 cents for the part and U$7 postage, plus 3 to 7 days wait.

Same for any other part.

With a 1N4002 in diode meter mode, I see 0.5v one direction, 0v the other direction.

Ok, you have a dead Multimeter.

With power supply + to 1K resistor, other end of 1K resistor to diode anode, diode cathode to power supply -, I get 0.7v across the diode.

Ok, what do you get if you reverse the diode?
+ to cathode through same 1k resistor, - to anode.

EDIT

Your last test shows things as they should be

The last test maybe.

The first one not at all:
again:

With a 1N4002 in diode meter mode, I see 0.5v one direction, 0v the other direction.

the very same diode that shows 0.7V forward biased and we are waiting fo the reverse bias test.

[Randall]

"For kicks, can you measure the resistance of the defective diode both ways and tell us what you get? "

500 ohms in one direction, 423 ohms in the other.

"I can not believe you are a Service Tech and donīt have *one* Zener, any voltage, on hand."

Well, that's how it is. I seldom need them. If I wanted to be berated for not meeting someone's expectations, I'd call my father.

"Ok, you have a dead Multimeter."

I do not believe this to be true.

"Ok, what do you get if you reverse the diode?
+ to cathode through same 1k resistor, - to anode."

With the zener in question I get 6.1v.

With the 1n4002 I get whatever voltage I send it from the supply up to 33 volts.

[J M Fahey]

Ok, canīt go on with impossible measurements so it ends here.

Good luck with your servicing work.

[Helmholtz]

With a 1N4002 in diode meter mode, I see 0.5v one direction, 0v the other direction.

Maybe you meant "OL" the other direction (makes a lot of sense) not "0V" (makes no sense).

[Randall]

"Maybe you meant "OL" the other direction (makes a lot of sense) not "0V" (makes no sense). "

Yes, this would be the case. I see my error now.

[Jazz P Bass]

"If I wanted to be berated for not meeting someone's expectations, I'd call my father."

Thanks for the coffee all over my keyboard.

[Randall]

In an interesting turn of events, Morgan finally did send me the parts he promised. The mosfet he sent is a STW15NK90Z, so now I am once again unsure which zener to use.

https://pdf1.alldatasheet.com/datash.../W15NK90Z.html

[nickb]

6.2V will be fine for that FET.

I have a saying -"If in doubt, throw it out". Just fit a new zener and rest easy.

PS: I you haven't got 6.2V 12V will also work just fine. The Vgs max is +/- 30V so plenty of margin. Use best anti-static practices to avoid damaging the FET.

[Randall]

Must be my lucky day, Morgan confirmed the 12v zener. So I look forward to receiving my order and getting this off my bench. I will confirm the measurements on the new one for those that are curious.

[Randall]

OK, the 12v zeners came in today, I orderred two so now I never have be embarrassed that don't have one on hand.

The test results are:

in diode mode: original 0.5v one way, 0.4v the other, new 0.68v one way, 0v the other

in resistance mode: original 500 ohms one way, 422 ohms the other, new OL one way, 395K ohms the other

in 32v power supply mode in series with a 1K resistor, with power supply + to zener cathode, zener anode to 1K resistor, other end of 1K resistor to power supply -, measuring across zener in voltage mode: original 6.1v one way, -6.1v the other, new 11.8v one way, -11.8v the other

I hope this is clear, and confirms that my original measurements were correct (aside from the 0V/OL resistance mistake). I dearly wish I hadn't posted the photos that got some riled up, but it made sense to me sitting front of it.

Now, to get it installed and see if it works.

[Helmholtz]

new 0.68v one way, 0v the other

Seems you are still/again mixing up OL and 0V. Any diode that reads 0V in one way with a good diode tester must be considered defective. OL means out-of-range (Over-Load) or very high value - just the opposite of zero.

[Randall]

Sigh, yes it seems I am. I stand corrected once again.

The good news is, it is now working.