Stanley oil cooker clock problem...R23 value please

mmack287

Would anyone have parts details or schematics for a stanley oil cooker two zone control clock card. One resistor, R23, and 2 zener diodes are burnt out - I need the value of the resistor to repair. The resistor is large - used in cap dropper power circuit, I think.
TiA,
Mike.

DublinDilbert

Is the old resistor completely black? Is the zener ok?

You should be able to test the circuit off a 12V PSU/battery, just put 1k resistor from 12V to the top of the zener and connect ground. Much safer to do this than initially messing with the mains.

If the circuit is still functional, I would start with a large value resistor (eg 200K) then reduce its value till it works, but don't go below 10K or so.

Sounds like the original resistor wasn't able to handle the power, so once you get a value that works make sure the power ratting is great enough, you can always parallel / series up a few resistors to get better power handling.

mmack287

I want someone who knows, to tell me what R23 on the control board should be...please.
Must be someone out there with a good controller. I have been asked for 175 smackarooons for a new one:mad:

DublinDilbert

You'll be lucky to get some who knows exactly which value resistor is on it. I doubt anyone repairs them or schematics are avaiable.

If you post the value of the cap and zeener, I'll help you calculate a range of resistor values that will work.

In this type of dropper circuit the exact resistor value isn't critical.

mmack287

The zeners are BZT03C47's and the cap is a 0.82uF.

DublinDilbert

mmack287 wrote: »

The zeners are BZT03C47's and the cap is a 0.82uF.

I've a similar circuit here on my shelf, its using a 0.55uF cap + 2K resistor (2W) & a zener.

So your cap has a reactance of approx 3.882k Ohms @ 50Hz, say your time clock is taking 20mA and you want a little bit of current for the zener too, so say 30mA.

Rtot = 220 / 0.03 = 7.333K
The cap gives you 3.88K
So the resistor should in theory be approx 3.45K

Pres = 0.03 x 0.03 x 3.45k = 3W

I would pick a resistor > 3.45K, 5K should work, but to be on the safe side i would choose a 10K 2W reistor and see how that works. If there's not enough power getting to the circuit, put 2 10K's in parallel ;-)

You know your self, but let it run for 10 mins then check nothing is getting too hot. I would pop say 4.5V onto the main circuit first and check that it's still working ok before going to the mains.

mmack287

Thanks DD. I will try your suggestions.
I am curious about the zeners, they are 47V, I thought they would have been a lot lower.

DublinDilbert

That is funny about the zeners, are they using them as a rectifier as well? Or do they have a rectifier else where on the board?

If you sketch up the input circuit in paint I'll have a look...

mmack287

I notice they use 48V DC relays for switching on power, hence the 47V zeners
I tried a 220 ohm for R23 and it works and doesnt get hot. I am currently trying it out on the cooker.