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nirav soni
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Join Date: Jan 2008
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Discharging Resistor

01/11/2008 6:48 AM

dear all,

i want to know about the purpose of discharge resistor connected on capacitors(for correcting pf). and the other quetion is how we can know that how much value of capacitance should be needed to correct the PF and then how much value of resistors to be needed.
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PWSlack
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#1

Re: discharging resistor

01/11/2008 7:09 AM

<....the purpose of discharge resistor connected on capacitors(for correcting pf)....>

It's there to ensure that any residual voltage on the capacitors will be discharged to zero a while after the equipment is switched off, so as not to surprise/alarm/shock/kill any individual that is called upon to work on it.

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TVP45
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#2
In reply to #1

Re: discharging resistor

01/11/2008 8:30 AM

When I was younger (and less kind), it was a common prank to discharge electrolytics in front of a newbie, then sit them aside and ask for them a couple hours later to see if the newbie touched the terminals. I, of course, had it done to me and I learned quickly about the phenomenum.

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user-deleted-1105
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#3
In reply to #2

Re: discharging resistor

01/11/2008 8:34 AM

I still like to occasionally clap my hands loudly behind someone who is switching on a new unit at work
...even tho' it's all low voltage...they still jump

Baaaad Kitty

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TVP45
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#4
In reply to #3

Re: discharging resistor

01/11/2008 9:34 AM

When smoking was still allowed here in workplaces, a prankster would often sneak behind a bench where a tech was working on a chasis and blow a lungful of smoke into the back. If you did it slowly, you could be gone before the tech saw it.

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Vulcan
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#7
In reply to #4

Re: discharging resistor

01/12/2008 8:30 AM

Yup, that's exactly how my mates initiated me.

I had my head buried into the back of a TV was checking. Next thing I knew, smoke was coming out of the whole thing! The fact that the smell was of cigarette smoke didn't register. I got my head out of there in a hurry! I thought I had just burned out one of our client's set. 'Thought I'd lose my job for sure!

Everyone was laughing and I just smiled but didn't like it much .

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Andy Germany
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#5

Re: Discharging Resistor

01/12/2008 5:42 AM

Del & TVP, you Guys have a baaaaad sense of humor!!! Me too!!

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CEngWannabe
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#6

Re: Discharging Resistor

01/12/2008 6:37 AM

The size of resistor can be calculated using ohms law, because once the power is off, the capacitor will discharge a dc current through the resistor.

To calculate the size of capacitor required is quite straight forward. The idea is to keep power factor at 1 or as close as possible. The formula required follows basic ac circuit analysis. A really good tutorial is found here:

http://en.wikipedia.org/wiki/Power_factor
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WANCLIK
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#8

Re: Discharging Resistor

01/13/2008 5:34 PM

triangle

S APPARENT POWER

Q REACTIVE POWER

P ACTIVE POWER
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Andy Germany (1); CEngWannabe (1); PWSlack (1); TVP45 (2); user-deleted-1105 (1); Vulcan (1); WANCLIK (1)

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