Author Topic: Microwave capacitor  (Read 1274 times)

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Offline Maxwell-X

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Microwave capacitor
« on: January 13, 2019, 06:09:11 pm »
I have an old microwave that has been unplug for over a year now and stored in a rubbish pill out the back of my house. I wanted to salvage the transformer from the microwave but would this high voltage capacitor have any charge left in it even if it been 1 year since plugged in ?
 

Offline German_EE

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Re: Microwave capacitor
« Reply #1 on: January 13, 2019, 08:15:02 pm »
Probably not.

HOWEVER...................................

If you need to ask questions like this you should not be messing around with microwave oven transformers. Get it wrong just once and you will be dead before you hit the ground.
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

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Offline Ian.M

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Re: Microwave capacitor
« Reply #2 on: January 13, 2019, 11:34:47 pm »
Before you do *anything*, read *all* the safety notes in 'Silicon' Sam's Microwave Overn repair FAQ: http://www.repairfaq.org/sam/micfaq.htm

Even if the capacitor was correctly discharged, if its bleeder resistor has failed open circuit it can recover to a significant charge by dielectric relaxation while in storage.  As with any other large HV capacitor, if you keep it, store it with its termnal shorted.

Don't attempt to use the  transformer with its original HV secondary as a step-up transformer.  Once you've cut the HV secondary off *without* damaging the primary insulation, its no more dangerous than any other cheaply constructed high power mains transformer.
 

Offline soldar

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Re: Microwave capacitor
« Reply #3 on: January 14, 2019, 12:23:21 am »
Don't attempt to use the  transformer with its original HV secondary as a step-up transformer. 
He, he... many years ago i went to a guy's place and he was using the HV of a microwave transformer as an animal fence.  He just inserted several very high value resistors in series so that if you touched the wire the current would be limited.  I didn't think it was a good idea but it seemed to work... if you touched it you would get shocked.
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline TheUnnamedNewbie

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Re: Microwave capacitor
« Reply #4 on: January 16, 2019, 06:12:09 pm »
Don't attempt to use the  transformer with its original HV secondary as a step-up transformer. 
He, he... many years ago i went to a guy's place and he was using the HV of a microwave transformer as an animal fence.  He just inserted several very high value resistors in series so that if you touched the wire the current would be limited.  I didn't think it was a good idea but it seemed to work... if you touched it you would get shocked.

These kind of things seem to work, until one day they don't and people die.
The best part about magic is when it stops being magic and becomes science instead
 

Offline Wolfgang

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Re: Microwave capacitor
« Reply #5 on: January 18, 2019, 09:17:58 am »
Microwave caps usually have a bleeder resistor built into them. Check their datasheet.
 

Online coppercone2

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Re: Microwave capacitor
« Reply #6 on: January 22, 2019, 04:31:22 am »
Hv resiztors can short out. They can also accumulate dust and stuff possibly.
 

Offline GeoffreyF

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Re: Microwave capacitor
« Reply #7 on: January 22, 2019, 04:55:44 am »
Maybe not but are you willing to bet your life on it?   Discharge it properly.  Some capacitors can retain a considerable charge for quite some time.
 

Offline Wolfgang

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Re: Microwave capacitor
« Reply #8 on: January 22, 2019, 05:03:11 am »
... the bleeder resistors in microwave caps are *inside* the cap. No dust there.
If the short out, the cap cannot hold any charge anymore.
Why not measure your caps with an Ohmmeter ?

Still: Play safe !!
 

Offline Yansi

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Re: Microwave capacitor
« Reply #9 on: January 22, 2019, 05:19:38 am »
The cap from microwave oven can store a couple of joules at most.  There are WAY MORE dangerous appliances and components present in labs if handled improperly.

Note I don't say the cap isn't dangerous, just saying to not make an elephant out of a mouse.

I have had couple times my head near around dismantled large power supply (10kW) under test, where the caps on the DCbus were holding about 1kJ of energy and the damn thing did blow up! Hell of a bang! 

But it all comes to knowing what one can or can't do and how to handle these things, in a SAFE manner. (That takes experience.)
 

Offline james_s

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Re: Microwave capacitor
« Reply #10 on: January 22, 2019, 05:23:26 am »
The capacitors used in microwaves almost always have an internal bleeder, they will be discharged within a few seconds of turning off the oven. Whenever I work on a microwave oven though I clip a jumper lead across the capacitor just to be safe.

The transformer and capacitor are both potentially dangerous, but ignore the hysteria and instead use common sense. I was playing with this sort of thing when I was 12 years old and I'm still here. HV should be treated with respect, it can kill you but so can many other things, crossing the street for example, but it does not need to be feared.
 

Offline Ian.M

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Re: Microwave capacitor
« Reply #11 on: January 22, 2019, 06:11:15 am »
Bleeder resistors can fail open circuit, so don't let that bullet have your name on it.   The risk there's any dangerous voltage on the HV cap after a year disused and outside is pretty minimal - you are probably more likely to win the lottery with a ticket you found on the street -  but if you are keeping any of the parts, there's no excuse for developing poor safety habits.
 

Offline james_s

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Re: Microwave capacitor
« Reply #12 on: January 22, 2019, 11:19:10 am »
Well that's why I say I clip a jumper lead across the capacitor. It's also not a bad idea to discharge before handling although unless the bleeder has failed it will normally already be dead. I use a plastic stick with a screw stuck in the end connected to a wire I can clip to the grounded frame. With a few basic safety techniques you can work with this stuff with little risk of injury. A lot of people fret over HV but don't stop to consider that even 120V mains voltage has killed people.
 

Online coppercone2

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Re: Microwave capacitor
« Reply #13 on: January 22, 2019, 11:35:53 am »
... the bleeder resistors in microwave caps are *inside* the cap. No dust there.
If the short out, the cap cannot hold any charge anymore.
Why not measure your caps with an Ohmmeter ?

Still: Play safe !!

i meant a ghetto circuit using a MOT for a electric fence.

I have a feeling someone who thinks to do this in the first place did not build it right.
 

Offline German_EE

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Re: Microwave capacitor
« Reply #14 on: January 24, 2019, 05:52:42 am »
OK, a quick safety tip that you can all (including the original poster) remember before touching anything high voltage

SIDE

Switch off the power to the device.
Isolate the device from mains power by removing the power plug
Dump all HV lines to chassis using a discharge tool***
Earth all HV lines to chassis that you might be working on.

Remember, stay safe, and if in doubt STOP and ask for help because sizzling in your own juice is not a nice way to die.


***https://img2.stewmac.com/product/images/42849/SnufferStick_High_Voltage_Discharge_Stick.jpg
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

Warren Buffett
 

Offline Yansi

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Re: Microwave capacitor
« Reply #15 on: January 24, 2019, 06:04:38 am »
Such suggestion may only work with microwave ovens. Not all HV operated equipment is ground (chassis) referenced.

Last one of my test contraptions with a CRT tube comprised of a completely floating 2.7kV supply.  Discharging against chassis ground would do close to nothing.

So the equipment servicing personal should be familiar with the internal workings of the device beforehand!

And also beware of short circuiting larger capacitors.  The microwave oven one may make a loud pop, others may even vaporize your discharge stick.  (again it falls out to knowing what is or is not necessary to do  to make a specific device to be safe for servicing).
« Last Edit: January 24, 2019, 06:09:45 am by Yansi »
 

Offline james_s

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Re: Microwave capacitor
« Reply #16 on: January 24, 2019, 01:05:48 pm »
There aren't many consumer devices with a bigger HV capacitor than a microwave oven. A resistor in line with the discharge stick is not a bad idea, although usually I don't bother and it still works. For discharging bulk filter capacitors on the input side of switching power supplies I typically use a line voltage incandescent lamp.
 

Online coppercone2

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Re: Microwave capacitor
« Reply #17 on: January 25, 2019, 12:50:11 am »
thats actually a good idea because you can tell if its its open circuit visually and maybe with a tap in 99% of cases.
 

Offline Wolfgang

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Re: Microwave capacitor
« Reply #18 on: January 25, 2019, 03:05:39 am »
There aren't many consumer devices with a bigger HV capacitor than a microwave oven. A resistor in line with the discharge stick is not a bad idea, although usually I don't bother and it still works. For discharging bulk filter capacitors on the input side of switching power supplies I typically use a line voltage incandescent lamp.

Another part that does contain a nice high voltage cap are medical defibrillator units. They normally have a several kV Maxwell in them.
 

Offline james_s

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Re: Microwave capacitor
« Reply #19 on: January 25, 2019, 03:38:54 am »
Yes although I wouldn't really consider a defibrillator to be a consumer device. I worked at a place that had one in the kitchen area but I don't know anyone who has one at home and it's not the sort of thing an unqualified person would generally be working on.
 

Offline Ian.M

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Re: Microwave capacitor
« Reply #20 on: January 25, 2019, 05:52:28 am »
Large photoflash capacitors can be pretty nasty.  A 2500uF cap charged to 350V DC has over 150 joules to ruin your day.
 

Offline Wolfgang

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Re: Microwave capacitor
« Reply #21 on: January 25, 2019, 07:03:08 am »
The worst caps I have seen were 100uF at 10kV salvaged from some particle accelerator project. These could zap you to hell in a microsecond. Total energy is 5kJ, enough to shrink some coins or crush some cans.
 

Offline james_s

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Re: Microwave capacitor
« Reply #22 on: January 25, 2019, 11:43:13 am »
Those sound like fun. I've seen a couple of coin shrinking machines, impressive amount of energy involved.
 

Offline Yansi

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Re: Microwave capacitor
« Reply #23 on: January 25, 2019, 09:12:58 pm »
The danger of high energy is multiplied by the high voltage present, that can deliver extreme amounts of instantaneous power to the load, such as your body.

1kJ cap at 15kV is way more dangerous than 1kJ at 300V.
 

Offline Wolfgang

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Re: Microwave capacitor
« Reply #24 on: January 25, 2019, 09:24:38 pm »
... I would say 5kJ from a 10kV cap is a one-time experience.  >:D Play safe !
 


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