My microwave kept on tripping the fuse so i decided to open it. I naively took out some parts trying to figure out what was wrong when someone in my office told my that’s one of the most deadliest things in the house. He told me the HV capacitor can deliver 1000s of volts and 100s of amps to the body. So I decided not to touch and get an electrician
But seriously, how dangerous is it? If you were to touch the terminals accidentally would you feel it and would you have enough time to save yourself
Since watts = volts * amps, and since most microwaves use 600 watts of power, his numbers are suspect. While 600 watts is trivial to a 80kg person if spread evenly throughout the body, if may be disastrous if 600 watts are concentrated in one place (like the muscles of your heart).
.1 to .2 amps through the heart can put it into ventricular fibrillation, which means you’ll die unless someone gets there quick with a defibrillator. How much voltage is needed to get that .1 to .2 depends on a number of factors.
Electricity is a funny thing. People survive lightning strikes, but can die from relatively minor shocks.
Firstly, it was good advice. If you don’t know what you’re doing you should never dismantle mains electronics.
If you touched the terminals of a charged HV capacitor, you would certainly feel it, and you wouldn’t have time to save yourself from the shock. Assuming the machine was unplugged, though, the shock would be short lived (capacitors hold charge, and shorting the terminals will discharge them through you).
If you touched both terminals with the same hand, you’d get a jolt, but probably nothing more. If you managed to touch the terminals with different hands, the charge would discharge across your body, and your heart. This could potentially be fatal.
Not that HV caps may have insulated terminals that allow you to plug leads into them but that make it difficult to touch the terminal ends otherwise.
[QUOTE=devineathiest]
If you were to touch the terminals accidentally would you feel it and would you have enough time to save yourself
[/QUOTE]
Time? No, if if it stops (or screws up) your heart it’s game over.
I don’t know what the innards of microwaves are, but a high voltage capacitor may not store a lot of charge. If you got a shock off that it might hurt, but since it can only supply a small amount of energy you’ll live.
An electricians’ “trick” is to keep your left hand in a pocket. That way if you do get a shock it doesn’t go across your chest (read heart), but from your right hand to the floor. Odds are better you’ll live.
That is just a precaution to take when tinkering with unknown circuitry. It is not safe technique for day-to-day working with live hv electricity.
The cap in a microwave could bite you. You’d have to be unfortunate to be killed, but yes, the potential is there. Most likely some minor burns where you made contact.
How dry your skin is, and where specifically you make contact matter a lot. Also what part of the cycle your heartbeat is on when it happens.
Most people that have been killed messing around with microwave ovens were doing it “hot” probing around while it was plugged in. In that case it isn’t just one quick pop, but a sustained current, and the power supply in a microwave as plenty of umph to kill in that case.
Years ago I was trying to repair my broken film camera. As I was disassembling the case, the flash popped up and the circuit started charging the capacitor, with that classic increasing-pitch whine. I thought nothing of it and continued to dig into the camera, and at some point my finger bridged both terminals of the capacitor. Felt like a fucking bee sting, and when I looked at my finger there were two tiny burn marks where the terminals had made contact with my skin. According to this article, the voltage may have been as high as 380 volts.
A microwave oven includes a voltage-doubling circuit that takes a couple thousand volts from the main transformer and doubles it to give the magnetron what it needs for operation. So basically the cap could have dangerously high voltage stored on it. Unclear what the typical capacitance is, and therefore how much current it could deliver (or for how long). But if my little camera’s capacitor was able to give me a nice snakebite-burn, it seems like that’s the least you should fear from a cap in a circuit that handles 1000-1500 watts. Safe service would require bridging the terminals of the cap with a mega-ohm resistor to bleed off any remaining charge before laying hands on it.
I remember once visiting the test bay of a couple of co-workers who were designing a new power supply for an X-ray tube. This was a serious power supply - 120kV @ 400mA continuous. I asked if this was one of those “keep one hand in your pocket” deals. Their answer: “No, wouldn’t make a difference.”
(Os course there were tons of safety interlocks and procedures to make sure nobody was near the supply while high tension connections were exposed and the supply was on…)
Basically anything with a vacuum tube in it (CRT, magnetron, old television or stereo) has potentially lethal voltages running around inside it, and probably filter caps with that hold at least painful voltages long after the device has been turned off or unplugged. People who service such devices has the correct equipment to safely discharge the filter caps…
At the cellular level, everything typically works with membrane potentials on the order of a few tens of mV. That includes the nerves and muscles cells in your heart. If you had electrodes to the right (or wrong) part of the heart, a mere 100 mV would be bad news. Even 10 mV applied at the exact wrong time would seriously screw up the heart’s rhythm. On a larger scale, skin has a pretty high resistance but underneath we’re bags of salty conductive fluid arranged in all sorts of complicated channels. I don’t have a cite but my understanding was that a 12 V car battery can be plenty dangerous if the voltage is applied, ah, internally.
Sure, if you stab someone in the heart with electrodes wired with that AAA battery. Pacemakers use lil’ bitty 2-3 V batteries, around the same size as a AAA battery, that last for decades.
you need sufficient voltage to “push” the lethal amount of current through your heart. that could be as low as a couple of volts (like from your AAA batteries) if done through electrodes embedded in your chest, to hundreds of volts if applied hand-to-foot through dry skin. As with anything, the resistance of the circuit is key. there’s a reason it’s “Ohm’s law” and not “Ohm’s general observations.”
Interestingly, there is a “sweet spot” between 100-200mA which is enough current to produce ventricular fibrillation and certain death (unless you have a defibrillator, and someone to use it, handy). Over 200mA the heart muscle “clamps” and won’t go into fibrillation, but your breathing has likely stopped with predictable results in the absence of CPR.
Regarding the question of the resistance of the heart: I found a paper which has measurements of the resistance of a dogs heart (The Electrical Resistance of the Heart - PMC) which gives an average of 50 ohms. Assuming a human heart has a resistance on the order of 100 ohms, then a voltage on the order of 10 volts applied directly across the heart will produce a current on the order of 100 mA.
What saves you in real life is your skin resistance, which can range from around 1 kohm for wet skin to hundreds of kohm for dry (see the “Fatal Current” citation above).
