Microwave Timer project....

I was wondering if anyone could help me figure out the proper way to connect a 5V battery power source to this microwave timer (pictured) I read this post- specifically the comments, but am worried I might fry the board if I do it wrong and with a 50/50 shot I'd like to ask the experts first. I've provided pictures below. Thanks for any help you can offer.


Picture of Microwave Timer project....
timer (clean).jpg
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mrshow555 (author) 9 years ago
Here's the schematics for this circuit:
Microwave Timer Schematics (repair manual RAR file)

Can I just make use this circuit (or one with a 7905 regulator) to power the board?
5v Regulator Circuit

gmoon9 years ago
I'm no expert, and I don't know much about microwave oven 'guts', but here's my guess...

Assuming the AC to the magnetron runs through this board, and is switched on/off by the relay, and there isn't an external 5V powersupply:

The on-board 5V supply is in the upper-right hand corner--the step-down transformer, some diodes for a bridge, a large filtering cap, and what looks like a 7805 regulator. See if it's labeled 7805...

If you remove those components and connect your external supply where the 7805 regulator was, it could work. Lookup the datasheet for the 7805 and the pinouts are there. At least remove the regulator, and the transformer if you can (without the transformer, the diodes won't do anything, and the filter cap will probably help, not hurt...)

Actually, remove the components and connect the external supply just about anywhere you're currently reading 5V and GND should work.

Again, this is guess-work.
mrshow555 (author)  gmoon9 years ago
the regulator says 7905....
gmoon mrshow5559 years ago
That's interesting--it's a negative 5V regulator (I thought the (-) in -5V on your drawing was a dash..) Question is, negative relative to what? There doesn't seem to be an other positive regulator on the board. Are there any other circuit boards in the device?
gmoon gmoon9 years ago
A couple more observations (based on your hi-res photo I just looked at):

--Looks like the + power tap from the transformer is routed through D6, then eventually to ZD1 and ZD2.

ZD1 is labeled 5.1V
ZD2 is labeled 3.1V

--Each of these is a Zener diode, and together they're used to regulate the positive part of the bipolar power supply. A discrete-component power regulator is often used in place of a more expensive 78xx regulator. Not surprisingly, they are close to the controller.

Beats me why they need the negative powersupply, too. I know nothing about the type of controller used. I can tell you that it's common for LCD screens to require a negative biasing voltage, however. And who knows what the magnetron, etc. need? (not me, that's way outside my knowledge.)

--It's possible the 'transformer' is somewhat more sophisticated than a simple stepdown inductor. In the hi-res image it looks a little fancier than a normal trannie. but it's certainly multi-tap, anyway, as there's also a -12V label near the 7905. But I don't know if -12V is used in the circuit--it might, there are both a relay (V?) and a buzzer (V?) on the board.

--Your best bet of reverse-engineering this is to power it back up and check voltages on the transformer secondary, relay, etc. But there are some serious safely issues to deal with here....especially if the magnetron is powered, too. Be very careful, or don't do it at all...