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Electronics => Beginners => Topic started by: Terry01 on March 25, 2018, 10:58:53 pm

Title: Is stripping microwaves really that dangerous?
Post by: Terry01 on March 25, 2018, 10:58:53 pm
I stripped my 1st microwave this weekend. Got a few good parts that I will use just tinkering around. I knew to be careful and didn't take any chances or anything careless.
This evening I watched a YouTube video of some nut using the HV caps to cause arcs between to points as the caps were discharging! HOLY SMOKES!! I'm so glad now I really took care!
Those things are mean!  :o
Title: Re: Is stripping microwaves really that dangerous?
Post by: Brumby on March 25, 2018, 11:22:23 pm
It would probably have been better if you had watched that video before tackling the microwave.  You would then have had a better appreciation as to why people advise extreme caution.

However, since you are still with us (with no stories of maim or fireworks), it would appear you maintained a basically safe approach.

This is good - and it is an attitude about which you should never become complacent.  The day you aren't paying attention is the day you will get reminded, possibly dramatically.
Title: Re: Is stripping microwaves really that dangerous?
Post by: Terry01 on March 25, 2018, 11:34:44 pm
Quote from: Brumby on March 25, 2018, 11:22:23 pm
It would probably have been better if you had watched that video before tackling the microwave.  You would then have had a better appreciation as to why people advise extreme caution.

However, since you are still with us (with no stories of maim or fireworks), it would appear you maintained a basically safe approach.

This is good - and it is an attitude about which you should never become complacent.  The day you aren't paying attention is the day you will get reminded, possibly dramatically.

For sure! I have had electric shocks twice before, just quick zaps but that's enough. It's no fun! I've seen HV electricity blowing the teeth of 360* excavator digging buckets too! I know electricity is not to be messed about with. I just found it a bit amazing that such a jolt can come from such a small package! Always good to learn :)
Title: Re: Is stripping microwaves really that dangerous?
Post by: Brumby on March 25, 2018, 11:41:52 pm
Everyone reading this will be glad you didn't find out the hard way.   :-+
Title: Re: Is stripping microwaves really that dangerous?
Post by: sokoloff on March 25, 2018, 11:44:31 pm
I address this issue by stockpiling crap unplugged for several years before beginning disassembly...  :-DD
Title: Re: Is stripping microwaves really that dangerous?
Post by: Brumby on March 25, 2018, 11:53:49 pm
Quote from: sokoloff on March 25, 2018, 11:44:31 pm
I address this issue by stockpiling crap unplugged for several years before beginning disassembly...  :-DD

A common technique for those well established in the collecting of crap donor equipment.
Title: Re: Is stripping microwaves really that dangerous?
Post by: Ian.M on March 26, 2018, 02:29:27 am
During operation, there's enough stored energy in the HV cap in a microwave oven to kill you quick, and if the integral bleeder resistor has failed open circuit, that cap can still have significant voltage on it a long time after the microwave was last switched on.

Working on or even simply scrapping microwaves without a good understanding of their theory of operation,  typical circuits and hazards is like playing Russian roulette.   Anyone who doesn't work on microwaves as a regular part of their job should read 'Silicon' Sam's Repair FAQ: Notes on the Troubleshooting and Repair of Microwave Ovens (https://www.repairfaq.org/sam/micfaq.htm), paying particular attention to its notes on safety, before removing the first case screw.
Title: Re: Is stripping microwaves really that dangerous?
Post by: Cerebus on March 26, 2018, 02:54:34 am
An old industry acronym/mnemonic that is worth learning by heart and practicing:

S.I.D.E.

Switch off
Isolate
Dump
Earth



Even if you're not set up to do this rigorously all the time, it's still a good idea to follow it in an ad hoc fashion when you're dealing with kit that your Spidey sense tells you might be dangerous. Disconnect all power, short circuit big and high voltage capacitors with insulated test leads, and clip some croc clip test leads across any big or high voltage caps and daisy chain those to another to the Earth connection on your bench PSU. Keep one hand in your back pocket while you're clipping those croc clips on.

Do not underestimate the ability of some types of capacitor to develop a significant charge again after they have been discharged. This phenomenon is called dielectric absorption, or soakage. Charge that has leaked into the dielectric of the capacitor can leak out again onto the plates after they have been discharged. It's quite possible for this mechanism to magically recharge a capacitor to lethal voltages.





Title: Re: Is stripping microwaves really that dangerous?
Post by: edpalmer42 on March 26, 2018, 04:18:47 am
Another point worth mentioning about stripping microwave ovens is:  Don't take the magnetron apart!  There's an insulator inside some magnetrons that's made of beryllium oxide.  It's relatively safe to handle if it's intact, but if it's broken or ground up, the dust is dangerous to inhale.

Ed
Title: Re: Is stripping microwaves really that dangerous?
Post by: james_s on March 26, 2018, 04:44:01 am
You can however salvage the magnets without cracking open the tube. They're nice strong donut magnets, I painted some to prevent chipping and use them to hold stuff on the fridge.
Title: Re: Is stripping microwaves really that dangerous?
Post by: AG6QR on March 26, 2018, 05:29:34 am
Quote from: Brumby on March 25, 2018, 11:22:23 pm
However, since you are still with us (with no stories of maim or fireworks), it would appear you maintained a basically safe approach.

Many people survive Russian Roulette.  That doesn't mean they were doing anything smarter or safer than the ones who didn't survive.

It could be that the OP avoided injury due to using sensible safety precautions, but it's also possible the injury avoidance had at least some element of luck to it.  Those of us who weren't there can't say for sure which component was more important.

But it does seem to me like step one of "sensible safety precautions" is "understanding the risk", and that was a problem here.

I don't mean this as a criticism:  I suspect many of us can think of a time when our lack of injury owed at least a little bit to luck.  I myself have been bitten by a capacitor in a small photographic flash unit, long ago when I was young and foolish.  I had removed the batteries, and then fired the flash, thinking that the flash firing would discharge the capacitor.  It obviously does discharge a large portion of the capacitor's energy (where else would the energy for all that light come from?), but I found out there was still plenty of energy left in the capacitor to give an unpleasant shock, perhaps from dielectric absorption, or perhaps because the flash just never completely discharged the capacitor.  Either way, the SIDE precautions posted by Cerebus would have saved me some unpleasantness.  I've always used those precautions around capacitors since then.
Title: Re: Is stripping microwaves really that dangerous?
Post by: chris_leyson on March 26, 2018, 06:21:01 am
Not really dangerous at all, just make sure the HV cap isischarged. More Berilium Oxide BS. Microwave oven magnetrons have alumina insulators NOT BeO, if they used BeO it would say so on the magnetron data sheet AND there would be a huge warning sticker on the magnetron.
Title: Re: Is stripping microwaves really that dangerous?
Post by: razberik on March 26, 2018, 08:32:35 am
I was once disassembling MW. I was about the section of HV caps and diodes.
Then suddenly lighting happened, I was totally freaked out, my heart was beating like crazy and then I realized what happened.

My dad was playing with his new camera and he thought it was a good idea to take a picture of me while I was disassembling MW. Of course he used the flash !
Title: Re: Is stripping microwaves really that dangerous?
Post by: IanMacdonald on March 26, 2018, 12:41:48 pm
Two main hazards are the HV cap remaining charged, and beryllium oxide washers on the magnetron waveguide. The latter are only a hazard if broken up, as the dust is a lung carcinogen. They are usually identifiable by a pink coloured ceramic.

Incidentally the HV circuit is rather interesting in that it's a voltage doubler rectifier, yet it only has one diode. How can that be? (The magnetron is the other diode.) No attempt is made to smooth the rectifier output, so there is a large 50Hz modulation on the RF.
Title: Re: Is stripping microwaves really that dangerous?
Post by: Terry01 on March 26, 2018, 01:02:48 pm
I have a cap discharger made up from a set of leads where the very point broke off one lead. Rather than throw them out I cut the banana jack ends off and soldered in a...I forget what size resistor and heat shrunk it all up.
It makes a great easy way to discharge caps. I also still short out any caps with my insulated pliers just to be sure before I touch anything. I leave the leads I made hanging on a hook above my desk so they are handy and it's amazing how often I use them since they are right there handy for when I need them.
Well worth doing for anyone else who has a broken set of leads. All you need is a couple 1k resistors (I checked) and a wee bit heat  shrink and 10 mins of your time  :)
Title: Re: Is stripping microwaves really that dangerous?
Post by: Gyro on March 26, 2018, 01:21:32 pm
Quote from: james_s on March 26, 2018, 04:44:01 am
You can however salvage the magnets without cracking open the tube. They're nice strong donut magnets, I painted some to prevent chipping and use them to hold stuff on the fridge.

 :o What, like cupboards?
Title: Re: Is stripping microwaves really that dangerous?
Post by: N2IXK on March 26, 2018, 02:43:49 pm
Pink/purple colored ceramic is NOT beryllium oxide.  It is aluminum oxide with a slight chromium impurity. In a single crystal form, commonly called ruby. As a ceramic, slightly stronger than standard white alumina.

Beryllium Oxide is an innocuous looking white ceramic.  Distinguishable from alumina by MUCH better thermal conductivity.  BeO is expensive and not used in consumer microwave magnetrons due to the toxicity hazard and the cost.
Title: Re: Is stripping microwaves really that dangerous?
Post by: mikerj on March 26, 2018, 02:51:29 pm
Quote from: james_s on March 26, 2018, 04:44:01 am
You can however salvage the magnets without cracking open the tube. They're nice strong donut magnets, I painted some to prevent chipping and use them to hold stuff on the fridge.

I'm surprised if you can pull them off the fridge without the use of a small crowbar!
Title: Re: Is stripping microwaves really that dangerous?
Post by: Terry01 on March 26, 2018, 04:44:07 pm
I just pulled the 2 ring magnets out the magnetron just now. They are quite strong! I didn't before as I was sure I had heard they weren't worth the bother. After reading this I went and got them out and am glad I did now! Thanks!  8)
Title: Re: Is stripping microwaves really that dangerous?
Post by: james_s on March 26, 2018, 05:47:57 pm
Quote from: mikerj on March 26, 2018, 02:51:29 pm


I'm surprised if you can pull them off the fridge without the use of a small crowbar!

They're not *that* strong. Nowhere near as potent as the rare earth magnets from hard drive actuators. I have a large one of those too, came from one of those full height 5.25" SCSI drives I scrapped about 20 years ago. It's so strong that you pretty much have to slide it off the edge. The magnetron magnets chip easily, but spraying with several coats of enamel seems to work well to protect them.
Title: Re: Is stripping microwaves really that dangerous?
Post by: Terry01 on March 26, 2018, 06:33:16 pm
Yes I agree they are no where near as strong as the rare earth ones. I have a 10 pack of the smaller rare earth ones which are very strong for their small size.
I'll keep the ones I got today just sitting on my desk. They're handy for even magnetising my screwdriver or sticking screws to or things like that. The weaker magnets are better for that I think. They all have their uses.
I'm well happy I saw this post and got the magnets off before I binned them
Title: Re: Is stripping microwaves really that dangerous?
Post by: Dominic on March 26, 2018, 08:05:36 pm
Quote from: Terry01 on March 26, 2018, 01:02:48 pm
I have a cap discharger made up from a set of leads ... resistor and heat shrunk it all up.
All you need is a couple 1k resistors (I checked) and a wee bit heat  shrink and 10 mins of your time  :)

Heat shrink tubing can be rated as low as 150V, and your 1k resistors (especially the carbon film types) may look like short circuits at higher voltages. It's a good idea to have a capacitor shorting lead, but just be careful about voltage ratings. I had to build some leads for bleeding charges from ESD testing and things get squirly above a couple of kV.
Title: Re: Is stripping microwaves really that dangerous?
Post by: Jwillis on March 26, 2018, 09:39:19 pm
The capacitor can carry quite a bit of energy.Most MO caps I've cracked open have a resister inside for discharging , but some don't.So on the side of caution always discharge the cap.I keep a piece of wire across the terminals to let me know that it has been discharged before I store it.The magnetron tube can have beryllium oxide insulators but not always .Again ,on the side of caution don't break them.The tube also contains a small amount of thorium .Although not really enough to be a hazard,take the side of caution and don't bother cutting  open the tube.Nothing very interesting in there anyway.
Aside from that there are sharp edges and the magnets are strong .Don't pinch your fingers.
WARNING .NEVER operate the magnetron outside of the shielding of a completely assembled microwave oven.It can burn and /or blind you.
Title: Re: Is stripping microwaves really that dangerous?
Post by: Terry01 on March 26, 2018, 10:37:42 pm
Cool thanks for the advice.
I leave the leads hanging up on their hook when in use and have a croc clip on one end of the leads too. I hook the croc clip onto one pin of the cap then use the other lead as normal to touch the other pin. I also short my pliers or screwdriver after i've used the leads just to be sure.
Most of the caps I discharge are smaller ones. Enough to give a jolt but that's it. The microwave was my 1st biggie. When you know it can kill you it makes you a little more careful!  :)
Title: Re: Is stripping microwaves really that dangerous?
Post by: Harb on March 26, 2018, 10:50:32 pm
Quote from: sokoloff on March 25, 2018, 11:44:31 pm
I address this issue by stockpiling crap unplugged for several years before beginning disassembly...  :-DD

This is what I have been telling my wife for years lol......"I have to stockpile this stuff because it takes so long for it to become safe enough to use".......400sqm shed later......
Title: Re: Is stripping microwaves really that dangerous?
Post by: james_s on March 27, 2018, 12:00:35 am
Quote from: Jwillis on March 26, 2018, 09:39:19 pm
WARNING .NEVER operate the magnetron outside of the shielding of a completely assembled microwave oven.It can burn and /or blind you.

Not that I would ever recommend this, but I know a guy who has been known to show off by warming his hands around the emitter of a bare magnetron. The inverse square law means that you have to be relatively close to get a burn, but you do have to be careful about hot spots. A propane torch can burn or blind you too if you get body parts in the flame, although being able to see the flame does help.
Title: Re: Is stripping microwaves really that dangerous?
Post by: Brumby on March 27, 2018, 02:56:53 am
Quote from: james_s on March 27, 2018, 12:00:35 am
.... although being able to see the flame does help.

Which is why I might play with a propane torch in this way - but I would never contemplate doing so with a magnetron!