Was drawn to AAC by Google treating me to a forum thread on experimental modifications of microwave ovens. My interest is related.

My wife and I tool around in a motorhome with continuous 12v D/C power. Then, when we park somewhere, we add 120v A/C, either through a "shore power" source, or by firing up an onboard generator. The big nuisance that I hate is that my microwave demands I set the clock before it will even turn on the light above the gas burner stove. [It's a GE PSA9120SF1SS]
Though the oven is powered by 120v, (shore or generator) I'm wondering if the clock, and associated control pad may operate on something more like 5 to 7 volts D/C. And if so, is there a safe way to substitute appropriate D/C from the coach to operate the switching circuit, leaving the switched circuit, which requires the A/C power, inactive? This way the clock wouldn't get "unset" every time we interrupt the 120v power.
Is the control circuit actually low voltage D/C? Is there likely to be an obvious step-down transformer, that would be easy to isolate? Anyone ever tried this kind of foolishness?

 

AFAIK, the control card of the microwave runs on a lower voltage, may 12Volts from an auxiliary transformer or a SMPS.
It should be possible to get this power from the van battery taking care to confirm to the polarities of the Van versus the Oven control card.

 

Not a bad idea, but a big grounding problem. Even if the control circuits are powered by an isolated power supply, there is nothing to guarantee that its GND is tied to the microwave chassis, and hence the vehicle GND.

And

I went to a couple of appliance parts sites and got exploded assembly drawing and the user manual. IF (big IF) the information actually is for your unit, it has a small switching power supply driving the control boards. The problem is that the supply has multiple output voltages. To know if only one of them needs to be backed up with uninterrupted power, we need the schematics for the control boards, and for that we need the service manual.

Depending on how much ***dangerous*** surgery you are comfortable with, one possibility is to power the internal switcher with an external UPS. No warranties expressed or implied.

ak

 

There are isolated DC-DC converter modules available in various input and output voltage configurations. One of appropriate current and voltage capacity could probably be hooked up alongside the normal DC power supply inside the oven using a dual diode (like a battery isolator but with Schottky rectifiers, which can sometimes be salvaged from junk switching power supplies).

 

Thanks for your encouragement! In my naivety, it seems you think this might be do-able.
I'm afraid my knowledge of circuits is so limited im befuddled by a "dual diode". Can you share a diagram of what you are suggesting?

 

Here you go:

https://www.ween-semi.com/sites/default/files/datasheet/BYV44-500.pdf

Without getting too deep, the images on the first page pretty much tell all. Two diodes in one package, with the two cathodes tied together. This is a common-cathode (CC) part, and is what you want; as opposed to a common anode (CA) part. I'm not saying to run out and buy this part; it is the first thing that came up in a datasheet search.

The power circuit arrangement you want is called a diode-OR. With two nearly equal DC power sources, one connected to each anode, the dual diode acts as an automatic switch. If only one source is present (either one), it runs things. If both are present, whichever one has the higher voltage provides most of the current. If you has two sources - A at 12.001 V and B at 12.002 V - and theoretically perfect diodes, then 100% of the load current would come from B. In the real world, the two sources will partially share the load current even if they are 0.25 V apart or more. This gets into how diodes work internally, their conduction curves, blah blah blah ... Get the idea?

ak

 

Would you even need a mains supply to the pcb? Just let the leisure battery run the controls side

 

I think this makes sense to me.
And courtesy of spending time diagnosing and replacing the "Automatic Transfer Switch" that governs (at least in part) whether the A/C power in the coach comes from "shore power" or from the onboard generator, it is clear to me both sources of A/C power are grounded to the vehicle chassis, as is the D/C power. ~~Someone had earlier noted a need to have a common ground for all power to the oven controls, and I now know we do.

So, it's a matter of finding the low voltage connection for the controls, and tieing in a double diode with appropriate voltage regulation.

I'll have to start probing with a meter!

 

Two things. First, the dual-diode thing is for DC *only*. Automatic power source selection for AC power is very different.

Second, a minor thing. The abbreviations for alternating and direct current are AC and DC. A/C is a common abbreviation for air conditioning. As your vehicle probably has air conditioning, this can be confusing.

ak

 

So, this microwave, according to the parts manual pages Control Parts and Interior Parts (2) here , has a number of circuit boards of interest. On the Control Parts page, part 1653 Microwave Control Board is the main CPU running at either 3.3 or, due to age, more likely 5v. Can't tell what MCU it is because of the label, and there are at least one, possibly two, regulators on board (IC4, IC5), an 8kbyte serial ROM 26C64 and an opamp KIA4558P which needs a minimum of 5v or 6v supply.

Also on this sheet is part 1614 Relay Board, which is a power supply and relay hub. It has a small transformer, a 4-diode bridge and an electrolytic capacitor, providing a supply to all the relays and their driver chips (also on this board, though unmarked). All of the relays are marked as 12v, so its a reasonable bet that transformer provides a basic 12v supply. However, I don't think this is as useful. More interesting though, is part 1624 Power Board, on the second sheet which is a line-driven, isolated switching supply apparently providing two DC outputs. The picture of the rear of this board helpfully shows the components in schematic form (well done GE!) though no voltages are identified. With that information and the diagrams it should be relatively easy to trace the harness from the Power Board to the Microwave Control Board, ascertain the required voltage(s) and formulate a plan!

 

Wow! Thanks for the detailed review. I had not managed to turn up that parts detail. SUPER!