Good morning,

I have an old microwave oven that I want to take apart.

I understand the danger involved is mainly in the energy stored in the capacitor, and in accidentally grinding the magnetron while taking it apart.

Are there other major safety concerns that I need to take care of?

Also, what kind of current should I expect when discharging the capacitor?

The schematic shows a discharge resistor but doesn't state any values.

By the time I take it apart, it would not have been operated for about 20 hours.

(just for the reference - I am an Electrical engineer, and I have taken lengthy safety training at school, so I will take the basic safety precautions.)

Thank you

 

20 hours should be enough, but put a shorting jjumper across the capo if your going to take it out of circuit, sometimes those things build up a charge after being discharged. dialectric abaorption and all that.
watch for sharp edges on the tin, they dont deburr those things very much these days.

 

The capacitor is not a DC filter cap. It is a voltage doubler. It should be dead empty in a minute or two. Just leave the installed resistor connected at all times and you'll be safe.

Do not connect the transformer to a power line unless you know what you're going to do with a thousand volts! Us nerds use them with a Variac to get a few hundred volts for vacuum tube circuits.

 

One time I had 3x 400 volts Microwave transformers around.

with a 9v adapter on the filament winding, I got 1cm sparks.

with a TRIAC regulator on the primary, I was able to melt the electrodes.

However, I disposed them- too dangerous, too much space, and not really good for any my purposes.

 

The capacitor is not a DC filter cap. It is a voltage doubler. It should be dead empty in a minute or two. Just leave the installed resistor connected at all times and you'll be safe.
...

Yikes!

Do you do much MWO servicing? Those caps are deadly, and can stay charged for quite a while!

Please BE VERY CAREFUL.

 

Did you just catch me being wrong? (It has happened before.) I'll go check my spares and see what the real values are.

10 meg attached in parallel with .85 uf. RC = 8.5 seconds.
(The schematic in post #1 shows the bleed resistor attached to the capacitor.)
10 time constants is 85 seconds.
What numbers are you working with?

The capacitor is rated at 2500 VAC which is 3535 volts peak. If the filament on the magnetron went cold at the instant the power was shut off (and it doesn't) and the capacitor was charged to 3535 volts at that instant, 85 seconds later, the capacitor would be discharged to 35.2 volts.

Reference: http://www.bowdenshobbycircuits.info/rc.htm

 

Picture is a little device that a lot of technicians likely have some version of..

Unfortunate part, is I made the body out of a brass tube, tho' the actual components were wrapped in a mica sheet before it was injected full of RTV silicone... Resistor is a ceramic wirewound.

Used to this day to discharge main capacitors in Photo-Strobe units, which in larger units could run 350 volt 3000 Mfd. Studio Strobe power packs are considerably larger, either capable of killing one outright if not very deliberate in hooking it up...

... Also used for MWO caps...

 

I made a wooden pole (maybe a foot long), with an old screwdriver (long shank) shoved in the end of it.
I then used another long screwdriver, with a clean heavily insulated plastic handle.
I touched the wooden pole / blade to one of the high cap prongs, and the other screwdriver to the other cap prong, and then crossed those across each other to short it out. In doing these repairs for several years, I've only heard a "snap' once.

YouTube probably has some demo videos (if you can get past all of the guys making welders out of MOT's.

Don't be afraid to sacrifice an old Harbor Freight Meter. In other words, always check for voltage across the two prongs after you've discharged it.

On other projects, I've fried more than one cheap meter.
I dropped one the other day, and now it's in a baggie (spare parts).

 

Do it safely, but there are great parts in there.
Sometimes you can use the whole control unit as a timer.
The magnetron has a couple of great magnets.
Nice 120 volt multi amp relays
Micro switches
Much more.

 

Good morning,

I have an old microwave oven that I want to take apart.

I understand the danger involved is mainly in the energy stored in the capacitor, and in accidentally grinding the magnetron while taking it apart.

Are there other major safety concerns that I need to take care of?

Also, what kind of current should I expect when discharging the capacitor?

The schematic shows a discharge resistor but doesn't state any values.

By the time I take it apart, it would not have been operated for about 20 hours.

(just for the reference - I am an Electrical engineer, and I have taken lengthy safety training at school, so I will take the basic safety precautions.)

Thank you

The manufacturers rarely bother to de-burr the sheet metal parts - the biggest single danger is injuries from sharp edges.

The voltage doubler capacitor is a low leakage type and can hold a charge for a long time - as long as the unit isn't powered the shock is more unpleasant than dangerous (YMMV). Better safe than sorry - short the lugs with the end of a screwdriver.

Before attacking the magnetron, I'd do a bit of research into whether there's any risk of encountering toxic beryllium, its often used in power electronics to enhance heat transfer. If you collect scrap metal to weigh in for a bit of extra cash - the magnetron cavity block is a weighty chunk of copper.

If all you do to the magnetron is strip off the steel frame that encloses the fins, there's a couple of pretty lively ceramic ring magnets.

The transformer is not something to be messed with lightly!!! Because of its current capability, its probably even more dangerous without the voltage doubler!!!

 

One time I had 3x 400 volts Microwave transformers around.

with a 9v adapter on the filament winding, I got 1cm sparks.

with a TRIAC regulator on the primary, I was able to melt the electrodes.

However, I disposed them- too dangerous, too much space, and not really good for any my purposes.

The MO transformer secondary is actually 2kV at enough current to be seriously lethal! - its fed through a diode/capacitor doubler to put 4kV on the magnetron.

 

In summary, leave the resistor on the capacitor. Under the worst possible combination of broken parts and chance probability, the capacitor will be down to 35 volts in 85 seconds.

 

Thanks everybody for the valuable advice.

I've disassembled it now. I had some difficulty with stubborn screws around sharp edges. Other than that, it contains a wealth of interesting parts.

I believe it's better to wear gloves/mask when handling the magnetron, since I can't find any information regarding what material it's made of.

 

its probably even more dangerous without the voltage doubler!!!

Can you please elaborate on this?

 

the magnetron is probably made with berilium oxide, a ceramic that conducts heat like silver, but is a good insulator. the stuff is EXTREMELY hazardous if crushed or abraded. when turned to dust and inhaled, it either gives you cancer, or kills you.

 

Can you please elaborate on this?

The capacitor/diode forms a voltage doubler - without making the capacitor *HUGE*, Xc is fairly significant at mains frequency, it does a reasonable approximation of a current limited source.

Without the doubler you get the raw current capability of the 2kV secondary. Either way - it could give Old Sparky a run for its money.

 

the magnetron is probably made with berilium oxide, a ceramic that conducts heat like silver, but is a good insulator. the stuff is EXTREMELY hazardous if crushed or abraded. when turned to dust and inhaled, it either gives you cancer, or kills you.

I'd go for possibly rather than probably, but it makes sense to research it before smashing the device with a hammer. There's plenty of pictures and diagrams online of magnetron insides from present day oven types back to WW2 radar tubes.

Personally I'd just strip off the steel frame and aluminium fins and chuck the remaining chunk of (mostly) copper in the scrap metal bag - let the smelting works figure it out.

 

Did you just catch me being wrong? (It has happened before.) I'll go check my spares and see what the real values are.

10 meg attached in parallel with .85 uf. RC = 8.5 seconds.
(The schematic in post #1 shows the bleed resistor attached to the capacitor.)
10 time constants is 85 seconds.
What numbers are you working with?
...

Decades of experience fixing MWOs, but I have to admit to it also being decades SINCE I last fixed a MWO.

I've seen them thow a huge HV spark across a screwdriver even 10 to 15 minutes after being turned off.

Either the older style caps didn't have an internal bleed resistor, OR (highly likely) the bleed resistors fail and go open circuit over time (like by the time a MWO gets to the age it needs servicing).

NEVER EVER rely on the cap to "discharge itself".

Even if caps are safer these days.

 

One of those, "Never bet your life" moments. Never bet your life that a 10 meg resistor will still be within spec after 5 or 10 years of suffering thousands of volts of stress. Or, in theory, there is no difference between theory and practice. In practice, there is.

 

The screwdriver method is a bad habit to get into.
Not saying the MWO cap is dangerous when shorted, just the habit!

It may be trading one injury for another.

I know from experience.

Posted one in another thread.

In yet another, I still wake up in a cold sweat thinking about it, almost shorted 100,000 amps. Luckily I had second thoughts and made another check with meter.