Cautionary tale on microwave oven repair

[c4757p]

You're right but your explanation is completely wrong. The internal resistance of a decent capacitor is low enough to ensure it'll discharge completely. The reason why some capacitors appear to recharge is down to dielectric absorption.

Thank you! Seemed simple enough to me without looking it up - guess I should research a bit before spouting off...

[GnatGoSplat]

I don't understand though how you could walk into a room after being electrocuted, then collapse? Perhaps he had a pre-existing condition, like a heart problem.

Yeah, most likely heart problem. After a cardiac arrest, you probably have 1-2 seconds before you black out.

Some very unfortunate people are actually born with heart defects that don't get detected until their later years or before; but by then it's sometimes too late.

I have also read that, "Medical examiners support the theory that electrical shock can cause a delayed ventricular fibrillation."  However, it is uncommon.
http://www.azcentral.com/specials/special43/articles/0304taserissue1-ON.html

[aluck]

Yeah, I've always felt uncomfortable working on microwaves, even though they should be perfectly safe if unplugged and capacitor discharged.  I've only had to do it a couple times in the past, and I recall going to the trouble to wear welding gloves just to discharge the cap, first with 2 flathead screwdrivers and a resistor connected between them

I believe that it is not just a resistor, but some 2W barrel, right?

Also, I was always wondering if it is safe even for a low-voltage capacitor to be discharged by shorting it's outputs... Could it actually just heat up and explode?

[GnatGoSplat]

I believe that it is not just a resistor, but some 2W barrel, right?

Also, I was always wondering if it is safe even for a low-voltage capacitor to be discharged by shorting it's outputs... Could it actually just heat up and explode?

Oh yeah, I always use one of the big power resistors in my junk bin.  I think smallest one is 5W.

I generally don't like discharging large capacitors by shorting their outputs.  Even if it doesn't hurt the cap, the pop and spark startles me and isn't good for the screwdriver!

[aluck]

Actually, I would've been worried more about my eyes, not a screwdriver.

[NiHaoMike]

I had to get into my Panasonic inverter microwave to repair some burnt wiring at the door switch, and was happy to see that inverter microwaves don't have that big capacitor.  There are a couple of non-polarized poly-looking capacitors on the inverter board which I checked for stored charge and shorted across just for safety's sake, but they didn't hold any charge with the unit off.  The entire board was very small and light-weight with what seemed like a much lower component count than conventional microwaves, it seems like a pretty nice improvement in technology.

Switching supplies have a large mains voltage (200V or 400/450V depending on design) cap or two. Those should self discharge but don't count on it especially on a power supply that is not working properly. On the plus side, it is much safer than a cap charged to well over 1kV.

[tom66]

Actually, the Panasonic design uses a very low capacitance primary cap due to the fact that the output is 100/120 Hz modulated, naturally giving it PFC as well. There is a small cap for the standby, but that's small enough to only give a small nip - probably 22uF or so.

[GnatGoSplat]

I had to get into my Panasonic inverter microwave to repair some burnt wiring at the door switch, and was happy to see that inverter microwaves don't have that big capacitor.  There are a couple of non-polarized poly-looking capacitors on the inverter board which I checked for stored charge and shorted across just for safety's sake, but they didn't hold any charge with the unit off.  The entire board was very small and light-weight with what seemed like a much lower component count than conventional microwaves, it seems like a pretty nice improvement in technology.

Switching supplies have a large mains voltage (200V or 400/450V depending on design) cap or two. Those should self discharge but don't count on it especially on a power supply that is not working properly. On the plus side, it is much safer than a cap charged to well over 1kV.

That's what I expected too, but the mains filter cap on the Panasonic inverter microwaves is only about 4-4.5uF.
I believe the image at this auction is the actual inverter board in my microwave:
http://www.ebay.com/itm/Panasonic-INVERTER-microwave-part-F606Y5X00CP-circuit-/120759498472?nma=true&si=dfuI70FoU8jcDaA%2FNWlO2i8SHsI%3D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557

Here is a PDF describing the technology:
http://www.eletrodomesticosforum.com/es/cursos/horno_microondas/Inverter.pdf

Also someone's interesting analysis of the inverter board:
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CEIQFjAE&url=http%3A%2F%2Fwww.fusor.net%2Fboard%2Fgetfile.php%3Fbn%3Dfusor_files%26att_id%3D3870&ei=RpqKUMW9Fc-DyAG3i4GwCw&usg=AFQjCNEy5DhKwBBLnPoWrt1rd6lfieiRfQ&cad=rja