Cautionary tale on microwave oven repair

[GnatGoSplat]

We all know it could happen, but until I found this article, I'd never read about it actually happening to someone.
http://worthcounty.walb.com/news/news/60220-update-deadly-microwave-was-not-plugged

[nixxon]

It cannot be stressed enough: Never fiddle with high voltage circuits on an empty stomach

[nicknails]

Maybe I'm just morbid, but I snickered a little at this:

"Family members say they are shocked by the tragic accident, and Roddy will be missed."

[PbFoot]

I don't know if this guy was qualified or not, but I will add - don't open equipment, especially high voltage equipment if you do not know exactly what you are doing.

-PbFoot

[tom66]

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.

Either way it's another reason I don't touch (work on) microwaves if I can avoid them. The cap is supposed to have a bleeder resistor but I don't put any trust in it, especially in cheaper units.

[N2IXK]

The bleeder resistor is often inside the capacitor, and it is often burned open.

Those are pretty big oil-filled caps, and can hold a charge for weeks if the bleeder is open. Microwaves are VERY dangerous to work on if you are unfamiliar with high voltage circuitry. 

[nitro2k01]

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.

[GnatGoSplat]

Either way it's another reason I don't touch (work on) microwaves if I can avoid them. The cap is supposed to have a bleeder resistor but I don't put any trust in it, especially in cheaper units.

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, then I'd check the cap for voltage using my multimeter, if volts were low but not zero, I'd short across with the screwdriver until zero and then place a short across it with alligator clips for the duration of my working on it.

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.

[M0BSW]

 For what they cost it's not worth it, when ours went pop, I put it in the bin for land fill, after cutting plug off.

[IanB]

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.

You can drop dead up to a day after receiving an electric shock. If you ever get an electric shock bad enough to make you black out of feel dizzy you need to go straight to hospital and get checked out. Death by electric shock can result from a disturbance to the heart rhythm that may have a delayed action.

[GnatGoSplat]

For what they cost it's not worth it, when ours went pop, I put it in the bin for land fill, after cutting plug off.

The standalone ones are quite inexpensive to the point of being disposable, but over-the-range models are still quite expensive.  They're typically about US$200-300 here.

[M0BSW]

For what they cost it's not worth it, when ours went pop, I put it in the bin for land fill, after cutting plug off.

The standalone ones are quite inexpensive to the point of being disposable, but over-the-range models are still quite expensive.  They're typically about US$200-300 here.

OK Shawn didn't realise it was that type , we have them built into the ovens over here in the  UK , I still wouldn't touch it, I'd get someone in , ours  was a stand alone one about £25.00 UK not sure what that is in dollars.Guessing about $35.00 just goes to show these things are always willing to bite you , or kill you in this poor mans case.
Paul

[G7PSK]

A piece of wire with a croc clip each end is the best thing to discharge these caps. Also leave the wire in place while working on the unit, I have had oil filled capacitors charge themselves up from from background emf, I had one from an old TV when a kid ad at school I would hold it by the brass studs at each ends to show it was safe then wave it near a florescent tube and produce a big fat spark from it.

[SeanB]

I am wary of that side, but most faults are in the door switches losing alignment or the magnetron or diode dying. Only ever had one cap go short circuit.

[c4757p]

I have had oil filled capacitors charge themselves up from from background emf, I had one from an old TV when a kid ad at school I would hold it by the brass studs at each ends to show it was safe then wave it near a florescent tube and produce a big fat spark from it.

Wow. I've had capacitors appear to recharge themselves - when you discharge them quickly, the internal resistance means they discharge more from the points directly near the electrodes, and then when you stop, the charges that were far from the electrodes redistribute. But I've never heard of one charging itself from background EMF. That must be one low-leakage capacitor.

[M0BSW]

I am wary of that side, but most faults are in the door switches losing alignment or the magnetron or diode dying. Only ever had one cap go short circuit.

,  I just looked at mine there's a big sticker on it , DO NOT OPEN NO servisble user parts,lethal voltage, good enough for me.

[GnatGoSplat]

OK Shawn didn't realise it was that type , we have them built into the ovens over here in the  UK , I still wouldn't touch it, I'd get someone in , ours  was a stand alone one about £25.00 UK not sure what that is in dollars.Guessing about $35.00 just goes to show these things are always willing to bite you , or kill you in this poor mans case.

Oh, I don't think that man's microwave was an over-the-range type since it's a big project just taking down one of those.  Mine were regular standalone types as well, though the wife does insist that our next one be a pricey over-the-range model.  I just mentioned that to point out that not all are disposable cheap throwaways.  £25.00 is about $40 US which I believe is not uncommon during holiday sales, but year-round normal price for the cheapest Chinese microwaves are about $50 or £31.00.  My current Panasonic inverter was about US$125 or £78.00, one of the pricier standalones.

Working on a microwave is safe if proper care is taken to discharge the big capacitor, but until it's discharged, it's definitely dangerous enough that I can understand the wisdom in just chucking it if it's a cheap disposable one.

[SeanB]

That would be a doorknob ceramic cap. In a dry climate they can hold charge for decades.

[M0BSW]

OK Shawn didn't realise it was that type , we have them built into the ovens over here in the  UK , I still wouldn't touch it, I'd get someone in , ours  was a stand alone one about £25.00 UK not sure what that is in dollars.Guessing about $35.00 just goes to show these things are always willing to bite you , or kill you in this poor mans case.

Oh, I don't think that man's microwave was an over-the-range type since it's a big project just taking down one of those.  Mine were regular standalone types as well, though the wife does insist that our next one be a pricey over-the-range model.  I just mentioned that to point out that not all are disposable cheap throwaways.  £25.00 is about $40 US which I believe is not uncommon during holiday sales, but year-round normal price for the cheapest Chinese microwaves are about $50 or £31.00.  My current Panasonic inverter was about US$125 or £78.00, one of the pricier standalones.

Working on a microwave is safe if proper care is taken to discharge the big capacitor, but until it's discharged, it's definitely dangerous enough that I can understand the wisdom in just chucking it if it's a cheap disposable one.

There quite cheap over here, supermarkets sell them discount sellers sell them the competition is fierce, we usually get two years out of them then it's bye bye  I just look in a electrical discount store are selling a chinese one for £19.00 UK, in all colours, it's not going to last but for £19.00 uk, if it lasted a year that's fine, only time I get near high voltage is when I'm doing something on one of my tube Ham radio transceivers, then I use 1 megaohm bleeder resistors on the cap left on there overnight, in fact I leave them on the caps any time one of my hands have to be inside, never put both in. otherwise its all low 9/12 volts.

[Zero999]

Wow. I've had capacitors appear to recharge themselves - when you discharge them quickly, the internal resistance means they discharge more from the points directly near the electrodes, and then when you stop, the charges that were far from the electrodes redistribute. But I've never heard of one charging itself from background EMF. That must be one low-leakage capacitor.

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.
http://en.wikipedia.org/wiki/Dielectric_absorption

I believe that oil filled capacitors used in microwaves have a fairly low dielectric absorption so any recharge shouldn't be bad enough to cause a hazard.

[tom66]

The typical microwave oven capacitor is sub 2µF - typically paper or oil capacitor? Is there a reason electrolytic caps aren't used - too high of a voltage?

[SeanB]

AC voltage across the capacitor. It is used in a voltage doubler directly across the 1000V secondary of the transformer. It has the full magnetron current flowing through it at all times, and has to withstand the full peak to peak voltage of the unloaded transformer during the magnetron warmup time. Thus 2800VDC rating minimum.

[Sionyn]

he obviously had no idea what he was doing let alone any chance of repairing it
shame

[HackedFridgeMagnet]

How to pretend you're linking your news story to real information.

<span style="text-decoration: underline;">Here is the full press release from the Sylvester Police Department</span>

[jucole]

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.

[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