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Old 11-07-2019, 01:33 PM   #1
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Novice on discharging capacitors....??


I'm a GC...able to fully wire up residential....but I am a COMPLETE NOVICE in regard to what I'll call electronic issues.

We never had A/C issues at7500 feet in the mountains (involving capacitors) and I am about to F--K with a broken microwave.

So to discharge a capacitor, I ordered and received two resistors... a 25W 100-Ohm and a 25W 2000-Ohm resistor.

I believe I understand the Ohm rateing...but correct or elaborate my understanding if I don't.:

The resistance rateing will just determine the speed at which I discharge a capacitor..(and lessen the "shock" to the capacitor relative to hitting it with a screwdriver.) Generally, the higher voltage incurred with a capacitor, the greater resistance of the resister is called for. For the micowave I got the 2000 -Ohm resistor.....for A/C capacitors I got the 100-Ohm resistor.

Is that appropriate. (Apart from the time involved to discharge, is there any reason to not use a high Ohm resistor to discharge any capacitor)

As to wattage rateing, I do not understand.

Is wattage rateing the maximum wattage a resistor can handle. (It seems with such high voltage of some capacitors, there would be alot of wattage involved..?)

Similarly, if wattage is a maximum rateing, does it matter if I use a 100 watt verse a 25 watt resistor to discharge a capacitor.

How does wattage effect my use of the resistor...??

Thanks for any help understanding...hopefully in relatively simple novice layman terms of electronics.

Best Peter
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Old 11-07-2019, 02:19 PM   #2
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Re: Novice on discharging capacitors....??


Hi Peter, sounds like over kill. Do you know what voltage and capacitor size you will be dealing with? If this is a home appliance and not a transmission tower, wattage will be low and discharge time will be in seconds.

What you working on?

Bud
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Old 11-07-2019, 02:46 PM   #3
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Re: Novice on discharging capacitors....??


Your microwave capacitor will most likely look something like this (example is GE)

You want to short the terminals in the one black collar to the terminals in the other. I do it with a pair of needle nose pliers with insulated handles, being careful to touch only the insulated handles.

I short them before attempting to remove the wires.

You could also do it with 2 screwdrivers. I don’t know anyone that bothers with resistors on these capacitors.

The needle nose pliers are also great for removing/reinstalling the wires by gripping the wire end terminals.
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Old 11-07-2019, 03:28 PM   #4
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Re: Novice on discharging capacitors....??


Just short across the terminals. You will most likely not know it. Some larger ones will give a little spark.
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Old 11-07-2019, 04:05 PM   #5
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Re: Novice on discharging capacitors....??


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Hi Peter, sounds like over kill. Do you know what voltage and capacitor size you will be dealing with? If this is a home appliance and not a transmission tower, wattage will be low and discharge time will be in seconds.

What you working on?

Bud
Bud... May very well be overkill.... part of my question is based just on understanding better the concepts involved.

Actually, what started my thoughts was reading on discharging and repairing my krap Samsung $500 micro-wave. The internet seemed to imply that a microwave capacitor was a lot more voltage with no bleed down capability.

Most articals advised against just dead shorting it.

In the past when running into a capacitor I have just dead shorted it....but while I was concerned about the micro, I thought I might as well learn something about the principles involved and future use on two A/C units I now have.

Most internet advises that dead shorting can be an element of harmful to a capisitor.... not immediately, but over time.

So... I thought I would learn.

Best
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Old 11-07-2019, 04:25 PM   #6
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Re: Novice on discharging capacitors....??


I admit I'm guilty of being too casual with voltages like this and have had my share of zaps. The times to be ultra cautious are when things are plugged in.

You asked about power and because the cap is being discharged any power involved quickly goes to zero.

Good to be cautious and the people who write those instructions will be glad someone read them.

Bud
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Old 11-07-2019, 04:31 PM   #7
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Re: Novice on discharging capacitors....??


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I admit I'm guilty of being too casual with voltages like this and have had my share of zaps. The times to be ultra cautious are when things are plugged in.

You asked about power and because the cap is being discharged any power involved quickly goes to zero.Yes...Very quickly

Good to be cautious and the people who write those instructions will be glad someone read them.

Bud

I agree... but I would still like to understand it...if only for curiosity.
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Old 11-07-2019, 04:58 PM   #8
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Re: Novice on discharging capacitors....??


A little more detail.

Search for "discharge time constant capacitor".

This is a quote that Google provides.

"To calculate the time constant of a capacitor, the formula is τ=RC. This value yields the time (in seconds) that it takes a capacitor to discharge to 63% of the voltage that is charging it up. After 5 time constants, the capacitor will discharge to almost 0% of all its voltage."

In their example R is in ohms and C is in Farads. So a 1,000 microfarad cap will be 0.001 Farads.

Bud
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Old 11-07-2019, 05:44 PM   #9
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Re: Novice on discharging capacitors....??


Not to hijack the thread, but it reminds me of when I was taking my Residential and Commercial Wiring courses. I had completed my syllabus, and was allowed to help the other students with their hands on projects. We had one smart a$$ who knew it all. While helping him with a project I told him to go get a certain size capacitor and bring it to the table. Mind you, they were top terminal caps for lab use. He walked to the cabinet and I heard it pop. He reached in and picked it up with his hand over the terminals. No harm, but it let the rest of the class know that they should always discharge their caps before storing them.
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Old 11-07-2019, 07:03 PM   #10
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Re: Novice on discharging capacitors....??


If the microwave has been off for a day or 2, ain't gonna be any sparks !
(short it to be sure, as it depends on the circuit it's connected to)

The Cap in the lab that Chandler mentioned, well t=rc , but a charged cap on a shelf, R is in the 10 (16th power) ohms (resistance of air). Going to take a lot of time to discharge.


MTN, get the screwdriver out and short that thing.

You did ask about the power rating of the resistor. You would have to figure out the voltage on the cap, and knowing the resistance, you would have the power rating required. The resistor will get hot if over it's power rating, but a capacitor won't power it long enough to get hot. A 1 watt would be fine, a screwdriver is rated at least 8000W thou
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Old 11-07-2019, 07:15 PM   #11
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Re: Novice on discharging capacitors....??


Quote:
Originally Posted by Bud9051 View Post
A little more detail.

Search for "discharge time constant capacitor".

This is a quote that Google provides.

"To calculate the time constant of a capacitor, the formula is τ=RC. This value yields the time (in seconds) that it takes a capacitor to discharge to 63% of the voltage that is charging it up. After 5 time constants, the capacitor will discharge to almost 0% of all its voltage."

In their example R is in ohms and C is in Farads. So a 1,000 microfarad cap will be 0.001 Farads.

Bud
Bud... Thanks but I was not interested in learning the intricacies of figuring the exact time necessary to discharge a capacitor.

I was just interested in better understanding some basics as:
"
"I believe I understand the Ohm rateing...but correct or elaborate my understanding if I don't.:

The resistance rateing will just determine the speed at which I discharge a capacitor..(and lessen the "shock" to the capacitor relative to hitting it with a screwdriver.) Generally, the higher voltage incurred with a capacitor, the greater resistance of the resister is called for. For the micowave I got the 2000 -Ohm resistor.....for A/C capacitors I got the 100-Ohm resistor.

Is that appropriate. (Apart from the time involved to discharge, is there any reason to not use a high Ohm resistor to discharge any capacitor)

As to wattage rateing, I do not understand.

Is wattage rateing the maximum wattage a resistor can handle. (It seems with such high voltage of some capacitors, there would be alot of wattage involved..?)

Similarly, if wattage is a maximum rateing, does it matter if I use a 100 watt verse a 25 watt resistor to discharge a capacitor.

How does wattage effect my use of the resistor...??


I know these are simple questions...but nobody seems to want to answer them.

But thank you...(I have no idea what a Farad is...Yes, I'm dumb....thought it was a mideastern prince or something in a haram)

Best
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Old 11-07-2019, 08:03 PM   #12
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Re: Novice on discharging capacitors....??


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Bud... Thanks but I was not interested in learning the intricacies of figuring the exact time necessary to discharge a capacitor.

I was just interested in better understanding some basics as:
"
"I believe I understand the Ohm rateing...but correct or elaborate my understanding if I don't.:

The resistance rateing will just determine the speed at which I discharge a capacitor..(and lessen the "shock" to the capacitor relative to hitting it with a screwdriver.) Generally, the higher voltage incurred with a capacitor, the greater resistance of the resister is called for. For the micowave I got the 2000 -Ohm resistor.....for A/C capacitors I got the 100-Ohm resistor.

Is that appropriate. (Apart from the time involved to discharge, is there any reason to not use a high Ohm resistor to discharge any capacitor)

As to wattage rateing, I do not understand.

Is wattage rateing the maximum wattage a resistor can handle. (It seems with such high voltage of some capacitors, there would be alot of wattage involved..?)

Similarly, if wattage is a maximum rateing, does it matter if I use a 100 watt verse a 25 watt resistor to discharge a capacitor.

How does wattage effect my use of the resistor...??


I know these are simple questions...but nobody seems to want to answer them.

But thank you...(I have no idea what a Farad is...Yes, I'm dumb....thought it was a mideastern prince or something in a haram)

Best

The wattage rating on a resistor is the maximum power it is designed to handle on a sustained basis. As a resistor passes current through it, it's going to heat up. As long as you keep within the resistor's power rating (and in some cases, have a heat sink on it), it will be fine. If you exceed the wattage rating, it's going to overheat and damage the resistor.

The formula for wattage is usually thought of as Volts x Amps. It can also be calculated as Volts x Volts / Resistance.

So, if you have a 10 microfarad A/C capacitor that is charged to 240V, and you discharge it through a 100 Ohm resistor, it will dissipate (240 x 240)/100 = 576 W. But that 576W will be just for an instant. As the capacitor discharges, the voltage will drop, and so will the wattage. After 1/2 a second, it will be pretty much down to zero. So although you greatly exceeded the 25W rating of the resistor, the time was so short that you're not likely to damage it. Think of turning on an electric oven element for about 3 seconds. How hot did it get in that time?

Now, say for the microwave, you have a typical 1 microfarad capacitor and the voltage on the capacitor is 2100V. The instantaneous wattage through the 2000 Ohm resistor will be (2100 x 2100)/2000 = 2205W. Even though you have a larger value resistor, because of the much higher voltage, you're way beyond the rating on the resistor. And because of the larger resistance value, it's going to take a bit longer to discharge, but still less than a second.

Will it damage the 25W resistor you have? You've exceeded the power rating by a factor of almost 100x. It might survive, but it's getting up there. A better choice would be something like a 100,000 or 200,000 Ohm resistor.
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Last edited by RAL238; 11-07-2019 at 08:21 PM. Reason: Changed cap and volatge to use more typical values
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Old 11-07-2019, 09:50 PM   #13
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Re: Novice on discharging capacitors....??


Hi Ral, in your first example of 10 microfarad and 100 ohms it would discharge to zero in 0.005 seconds. Basically a tiny spark and it's done.
10 uf = 0.00001 farads times 100 ohms = 0.001 seconds. Times 5 for 5 times constants and we get 0.005 seconds. been many years so welcome correction.

Bud

Peter, I realize we may not be hitting your question as needed so repeat each.
A farad is the size of a monster capacitor thus we use microfarad for most all caps. That is an extension of 10 to the -6 power.
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Old 11-07-2019, 10:15 PM   #14
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Re: Novice on discharging capacitors....??


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Hi Ral, in your first example of 10 microfarad and 100 ohms it would discharge to zero in 0.005 seconds. Basically a tiny spark and it's done.
10 uf = 0.00001 farads times 100 ohms = 0.001 seconds. Times 5 for 5 times constants and we get 0.005 seconds. been many years so welcome correction.

Bud

Peter, I realize we may not be hitting your question as needed so repeat each.
A farad is the size of a monster capacitor thus we use microfarad for most all caps. That is an extension of 10 to the -6 power.

Bud - You're math is absolutely correct. I don't disagree. I was trying to keep things simple and thought 1/2 second was a good enough description and easier to imagine than 0.005. I shouldn't have thrown precision out the window.
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Old 11-08-2019, 08:28 AM   #15
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Re: Novice on discharging capacitors....??


RAL and BUD....

Thanks..!!!..From a practical standpoint, which I was looking for,I am grasping the overall principle/understanding I was looking for. (I'm not going to designing any electronic circuits in the near future....if I do, I'll put a big warning sign on them)

Understanding the W rateing was use under a sustained basiswas very important for my practical understanding and curiosity.

(I was thinking that W was maybe a absolute maximum rateing, and with the very high voltage of a capacitor, even with small Amps, a resistors wattaage rateing would likely be exceeded in discharging a capacitor)

Thanks again guys
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