DC from AC without transformer

[grant fair]

My toaster oven does not work (toaster only) and the controller PCB which obviously needs 15 volts DC or less (IC is cmos 4060) only gets 2 volts at the supply pin.<p>The controller gets 120 volts AC. The controller includes 4 1N4001 diodes and a large mylar cap (maybe 1 uF). I asssume the fault may be here.<p>What is the usual way of deriving voltages around 15 VDC from 120 AC?<p>Grant

[Joseph]

Sounds like it uses capacitive impedance to drop the voltage down for the low voltage circuit. Look for a zener diode being fed by the capacitor and small rectifiers which may have become leaky, loading down the voltage. The same could apply to a small electrolytic filter capacitor.

[George Herzog]

Without a transformer, this is known notoriously as unisolated DC. That means that if you happen to touch a wire, it may serious harm or kill you as your body may be grounded by whatever you are touching or standing upon.<p>again and again you will find Nuts and Volts recommending using a transformer not only to step down the voltage, but to assure that you live long and prosper.<p>Everyone knows transformers ad cost and are a bit heavy, but would you rather be cheap and dead or poor and alive?

[Robert Reed]

Unfortunately George, the manufacturers don't see it that way. The power supply as installed is quite safe as long as it remains "untouchable". Here in the States virtually all AC outlets are polarized by now, so that circuits like these referenced to the white neutral line remaiun that way.
Grant--Your supply may be anywhere from an intended 5vdc to 15 vdc and as far as "4000" series chips go, they will perform satisfactorily all thru this range, so with only this info its hard to say whats proper . These types of circuits are usually associated with one resistor ,lytic and zener. The zener part number should give you the supply voltage. Barring that being unreadable, the 'lytic voltage rating may give you a clue. When I encounter this kind of problem ( assuming solid stae components are unremovable, such as nonsocketed) I always break the supply line at the feedpoint and check for proper voltage resumption. I've found this saves a lot of time as now you know which direction to proceed with troubleshooting. Also ,the capacitor mentioned sounds a little hi (1 mfd),as they are usually on the order of 0.05 mfd to 0.22 mfd. This may have a low wattage resistor in series with it also. For starters, give the circuit a good visual before digging into it--just to eliminate the obvious.
I believe there was a similar topic posted a month or so ago.Try scrolling back for more info.<p>[ August 09, 2005: Message edited by: ROBERT REED ]</p>

[Dean Huster]

Same subject, same toaster oven. Grant was the OP on the earlier thread also.<p>http://206.131.241.58/ubb/ultimatebb.ph ... 1&t=003003<p>Grant, last time, you ended up somewhat resolved to the fact that you were going to have to reverse-engineer the schematic of the board for us. How are you doing on that task?<p>Mention was made of finding the part number of any zener diodes in the supply. Knowing appliance manufacturers, you'll be lucky if there's any number at all on the solid state parts, let alone numbers that aren't non-cross-referenceable house numbers.<p>There have been some good suggestions so far on this thread. But at least a partial schematic from the power cord on back about 15 components sure would be nice.<p>Dean