Microwave capacitor  - Page 1
Forum section: electronics / rf-microwave

author: Maxwell-X
author_rank: Newbie
timestamp: January 13, 2019, 07:09:11 am
content: 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 ?

author: German_EE
author_rank: Super Contributor
timestamp: January 13, 2019, 09:15:02 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: January 13, 2019, 12:34:47 pm
content: Before you do *anything*, read *all* the safety notes in 'Silicon' Sam's Microwave Overn repair FAQ:http://www.repairfaq.org/sam/micfaq.htmEven 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.

author: soldar
author_rank: Super Contributor
timestamp: January 13, 2019, 01:23:21 pm
content: Quote from: Ian.M on January 13, 2019, 12:34:47 pmHe, 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.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 16, 2019, 07:12:09 am
content: Quote from: soldar on January 13, 2019, 01:23:21 pmThese kind of things seem to work, until one day they don't and people die.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 17, 2019, 10:17:58 pm
content: Microwave caps usually have a bleeder resistor built into them. Check their datasheet.

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2019, 05:31:22 pm
content: Hv resiztors can short out. They can also accumulate dust and stuff possibly.

author: GeoffreyF
author_rank: Regular Contributor
timestamp: January 21, 2019, 05:55:44 pm
content: 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.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 21, 2019, 06:03:11 pm
content: ... 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 !!

author: Yansi
author_rank: Super Contributor
timestamp: January 21, 2019, 06:19:38 pm
content: 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.)

author: james_s
author_rank: Super Contributor
timestamp: January 21, 2019, 06:23:26 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2019, 07:11:15 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: January 22, 2019, 12:19:10 am
content: 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.

author: coppercone2
author_rank: Super Contributor
timestamp: January 22, 2019, 12:35:53 am
content: Quote from: Wolfgang on January 21, 2019, 06:03:11 pmi 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.

author: German_EE
author_rank: Super Contributor
timestamp: January 23, 2019, 06:52:42 pm
content: OK, a quick safety tip that you can all (including the original poster) remember before touching anything high voltageSIDESwitch off the power to the device.Isolate the device from mains power by removing the power plugDump 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

author: Yansi
author_rank: Super Contributor
timestamp: January 23, 2019, 07:04:38 pm
content: 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).

author: james_s
author_rank: Super Contributor
timestamp: January 24, 2019, 02:05:48 am
content: 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.

author: coppercone2
author_rank: Super Contributor
timestamp: January 24, 2019, 01:50:11 pm
content: thats actually a good idea because you can tell if its its open circuit visually and maybe with a tap in 99% of cases.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 24, 2019, 04:05:39 pm
content: Quote from: james_s on January 24, 2019, 02:05:48 amAnother part that does contain a nice high voltage cap are medical defibrillator units. They normally have a several kV Maxwell in them.

author: james_s
author_rank: Super Contributor
timestamp: January 24, 2019, 04:38:54 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: January 24, 2019, 06:52:28 pm
content: Large photoflash capacitors can be pretty nasty. A 2500uF cap charged to 350V DC has over 150 joules to ruin your day.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 24, 2019, 08:03:08 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: January 25, 2019, 12:43:13 am
content: Those sound like fun. I've seen a couple of coin shrinking machines, impressive amount of energy involved.

author: Yansi
author_rank: Super Contributor
timestamp: January 25, 2019, 10:12:58 am
content: 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.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 10:24:38 am
content: ... I would say 5kJ from a 10kV cap is a one-time experience.Play safe !

author: coppercone2
author_rank: Super Contributor
timestamp: January 28, 2019, 05:54:03 pm
content: I think the biggest threat is using these devices in clutter conditions that are normal in most labs and workshops.Like big cap half an inch away from edge of table with a giant pile of shit pushed behind it.That and exposed contacts.