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Need HELP with mod !!!

I need help powering a timer I salvaged from a microwave oven. Here's the schematic for the board. It needs a -5v power supply. I have a 9v battery and a 7805 regulator. Can anyone help me figure out how to make this thing work?

hi-res schematic


Picture of Need HELP with mod !!!
timer (clean).jpg
mrshow555 (author) 9 years ago
I used a 9v battery with a 5v regulator and hooked it up reversely to the -5v and gnd spot on the circuit board and the display just came up scrambled. I'm not looking to have the timer turn anything else on or off, just simple display countdowns. what's the easiest way to get this circuit to do that? (Note: I want to power it all off of battery - not 120 AC or anything)
gmoon9 years ago
I'll give this another shot, here goes...

You don't need a regulator, as the 7905 is already there.

The power transformer circuit (box #4 in schematic) is a full-wave rectifier on a center tapped transformer (12-0-12) or a 24V trannie (probably a 13-0-13, or 25 or 26V total) with a center tap. Unlike typical power supplies, the rectifier is filtering out the positive voltages. The neg voltage is relative to the center tap.

So:
V2 is unregulated -12V
V1 is regulated -5V
(the INT2 and RESET voltages should take care of themselves...)

The DOM switch in box #1(two blue wires in the pic) should be a closed connection--that's the 'door open' safety switch.

The supply circuit in box #5, which is half-wave rectified, looks like another neg power supply (the zener diode is wired backwards to GND compared to a typical pos regulator.) This is tapped off one half of the transformer secondary.

The relays are connected through the third pin on the white connector (AC in.) So AC is probably routed to the magnetron first, then to the board (and the chassis GND is probably connected to the plug ground pin there.) The relays obviously are closed while the timer is running. There's "truth table" on the PDF you posted before that shows the state of the relay control circuit.

The one part of the circuit that activates when the timer runs down is the the buzzer (BZ1.) You could replace that with a 12V relay if you want something to trigger at 0:00. A relay here will make any polarity weirdness moot (add a spike diode though, just like the other relays.)

OK, big finish....

To connect to a battery, you'll need 12 volts. And it will be wired backwards--the batteries positive terminal should be connected to the GND, and negative connected at the V2 symbol. Also connect the neg terminal inside box #5. The transformer should be removed, or the traces cut...

Or better yet, connect the negative terminal before the rectifying diodes (on the 'bottom' of the transformer secondary, where the circuit splits between the main supply and the box #5 supply)--that should prevent a any reverse polarity accidents (and you wouldn't need a separate connection to box #5, too.) But to do so, you might need a bit more than 12V, due to the diodes (I'm guessing not, as the board logic is probably run off the V2 supply--it's theoretically possible this could function with much less than 12V, as the higher voltage appears to be largely for the relays, etc. But the transistor drivers are tuned to relay current at 12V....)

I couldn't find any positive voltages just by looking at the schematic. The circuit just uses the 'bottom' of the AC waveform. Somebody else please double check my logic ;-) ......
guyfrom7up9 years ago
do you only need -5 volt? you could just use aw negative regulator and hook up the nine voolt accordingly
mrshow555 (author)  guyfrom7up9 years ago
but where in this circuit do I connect my negative regulator to?