Microwave H.V. capacitor discharge

Yarome · October 11, 2016

For a microwave with a discharge diode on the capacitor.. how long does it typically take to fully discharge once power has been cut? Not the mag charge.. just the high voltage cap. I know it's NOT 2 days... and it REALLY REALLY stings! :lol:

Just for future reference (it's certainly discharged NOW) can you manually short them like low voltage capacitors without doing any damage?

TIA

Kirk W · October 11, 2016

I can't speak to microwave issues as I have not fooled with them in such situation, but I do know that electrolytic capacitors have been known to store a significant charge lever for several years. Of course the size, design, and capacitance rating plays a part but I don't believe that it is ever safe to just assume that one has discharged. Large power-supply capacitors can be dangerous!

oldjohnt · October 11, 2016

Yarome, I'm with Kirk on this one. A quality capacitor which has extremely HIGH insulation value and an extremely LOW self discharge rate can hold a charge a longggggggggg time. Its just (oversimplified) two big plates sitting there across from each other with an accumulation of electrons, its like a big huge electron bank and absent discharge current flow, it just sits there charged up ready to toss to your buddy and yell CATCH!!! (Id never do that would you?)

Now if you place a conductor (screwdriver lol) across it the discharge time is extremely fast and a diode is near a short circuit at the correct polarity with a small voltage drop, but to limit current, reduce heat and not overcurrent and fry the diode, Id "guess" it has an inline resistor HOWEVER the discharge time is still extremely fast SUBJECT TO to the resistor size and resultant current flow.

BOTTOM LINE subject to (if it has a discharge circuit????) how the diode and any discharge circuit and resistor are all designed, if its in the circuit, DISCHARGE WOULD STILL BE FAIRLY RAPID (depends on resistance) and you shouldn't get stung...........

YES many of us "dudes" have stuck a screwdriver across a cap to discharge it and usually there's no harm THE RISK is the intense heat that might be generated and chance of warping or burning up something inside.

Nuff said, that's as sweet n short n simple as an engineer attorney can possibly make it lol Its in our DNA

PS given the capacitor specs and discharge circuit its a fairly easy formula to plot out the discharge current and time (did it 50 years ago all the time in a test or homework at Purdue) but I'm too old n lazy n rusty to do that, please DONT ASK lol

John T

Kirk W · October 11, 2016

Not to beat a dead horse(electrocuted?), many years ago I worked with a power-supply that used a large bank of capacitors to fire a xenon lamp & a tech in the company was killed by taking one waiting for return to the factory & attempting to repair it himself. The one in a microwave is probably not large enough to kill, but it might be capable of burning you and sure could ruin your day.

docj · October 11, 2016

To look at the OP's question slightly differently, it's worth noting that "super-capacitors" are now being introduced as power sources in situations where a battery might otherwise have been used. Implicit in this is the fact that a well designed capacitor can hold a charge for a long time.

I recently purchased a Honeywell in-wall timer that uses a super-capacitor in lieu of a backup battery to retain its settings during power outages. My understanding is that the change away from batteries in this case was motivated by the fact that the capacitor will, essentially, never go bad whereas a backup battery will age and eventually die (often without the user being aware of it).

Yarome · October 11, 2016

The one in a microwave is probably not large enough to kill, but it might be capable of burning you and sure could ruin your day.

This one reads 2600vac / 5060hz.. so yeah.. no wo w wond wonder it ss st sss tung a little. Haven't been put back on my rear like that in awhile. Lesson learned. I wasn't actually anywhere near the cap, but should have taken more precautions.

The diode reads 50k ohms. If I'm reading the schematic correctly.. could be wrong.. but the diode runs from a 3rd connector on the capacitor and grounds out. My impression was that it is intended to slowly drain the capacitor, but I don't have the math skills to calculate how long it should take to do that and won't ask John to. I just wondered if there was some type of rule of thumb to follow. I mean... "someone" has to be able to work on these things with a wide range of specifications, right. Unless they manually short them or have some specialized tool or procedure.

The discharge diode makes sense to me though for a consumer product of this type. It's probably not the best idea having a loaded cap sitting around in a recycling center once they're retired.

When it says H.V. I'm sure it can't be too good for the cap to manually short one. Maybe.

docj · October 11, 2016

When it says H.V. I'm sure it can't be too good for the cap to manually short one. Maybe.

Actually, there are quite a few applications where discharging a high voltage capacitor by essentially shorting it is pretty routine. It shouldn't bother the capacitor. My dissertation research used as a tool a pair of lasers that worked by discharging capacitors charged to ~10kV using large hydrogen thyratons. Spectacular to watch and noisy as all get out.

Yarome · October 12, 2016

Actually, there are quite a few applications where discharging a high voltage capacitor by essentially shorting it is pretty routine. It shouldn't bother the capacitor.

Thanks. I appreciate all of the good info and will remember it next time. I guess it does make sense... ohms and micro ferrad's tested out just fine on the meter after "manually" discharging it the first time.

Kirk W · October 12, 2016

Joel is correct. Most of us who work with them have just made it a habit with smaller capacitors. I used to do it with a screwdriver just as a matter of course. For larger electrolytic capacitors, we had a device that was designed for the purpose of discharging them which had a built-in current limiting circuit. You don't often see the gigantic version or banks of them except in commercial or military power supplies. That is good also because those are very dangerous.

oldjohnt · October 12, 2016

Of course many of us over many years have discharged certain capacitors using a low resistance screwdriver lol HOWEVER ITS NOT ROCKET SCIENCE to understand when you start talking high voltage and high capacitance, its safer for the components to LIMIT CURRENT when you do so which is accomplished simply by placing resistance in the discharge circuit. Higher energy, higher voltage and higher capacitance can produce significant short term current which could produce excessive heat and warp or burn or damage components. SO IF YOURE USING HIGH VOLTAGE HIGH VALUE CAPACITORS I HAVE TO SUGGEST THE USE OF SOME DEGREE OF CURRENT LIMITING RESISTANCE unlike a screwdriver as in lower energy lower voltage conditions.

That's my story n Ima stickin to it, HOWEVER feel free to do as yall please.

Yarome, I may have to dust off my old EE books and find the formulas for discharge time and current relative to capacitance and voltage and the discharge circuit lol

John T

Lou Schneider · October 12, 2016

When I was in college, the electronics lab received a bunch of surplus oil filled high voltage capacitors from a nearby air force base. Each one was about a foot wide, a foot and a half tall and about 6 inches deep.

The instructor decided it would be a good idea to hammer home what a charged capacitor can do.

He set them up in a string that stretched about 20 ft. down the hall and used copper buss bars to connect them in parallel, then had us figure out the total capacitance. It came to something like 10 farads. Then he proceeded to charge them using a bench power supply. At the end of the hour the string had been charged to about 200 volts.

Then we had to figure out the amount of stored energy and he proceeded to demonstrate that force by dropping a wrench across the buss bars. The resulting blast disintegrated the wrench and was loud enough to bring the local police and fire department looking for a bomb explosion.

After the excitement died down, he proceeded to demonstrate the memory effect by shorting the string again, drawing several additional loud arcs from the charge remaining in the capacitors.

Yarome · October 15, 2016

I may have to dust off my old EE books and find the formulas for discharge time and current relative to capacitance and voltage and the discharge circuit lol

No worries, John.

It's a done deal now and I have NO plans of dinking around in my nuker again. The stutter seems to be resolving itself. LOL

Appreciate the offer though.

oldjohnt · October 16, 2016

Yarome, you done got me too curious, an old engineer thinks about such trivial things lol. Fifty years ago I knew the formula by heart, wonder what happened???

http://learningaboutelectronics.com/Articles/Capacitor-discharge-calculator.php

John T