Capacitor discharge circuit

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bigkim100
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Capacitor discharge circuit

Post by bigkim100 » Wed Dec 11, 2002 4:50 pm

I work for a company that repairs industrial restaurant equipment, and the tech's constantly blow up our digital capacitance meters by not discharging capacitors before testing them. What Im looking for is a "safe" discharger that would give a visual reading of a capacitors discharge, then indicate when it was fully discharged. I would like to make one for each workers cubicle, and hopefully save some meters. They mostly read motor start/motor run capacitors.
Anyone have any schematics?
Thanks,
Kim :D
Kim..The man with the cute little girls name...and Frankensteins face and body.

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jollyrgr
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Re: Capacitor discharge circuit

Post by jollyrgr » Wed Dec 11, 2002 5:38 pm

The device you are looking for is called a RESISTOR. Simply take a one watt power resistor at about 10K ohms. Simply add long leads with insulated aligator clip leads or test probes for discharging the capacitor.<p>But you want some sort of indication as to the condition of the capacitor's charge. So do this. Connect (splice) the resistor to an ordinary set of test leads for a volt meter, close to the meter end. If you don't want to use your bench volt meter, use a cheap digital volt meter you can pick up a "Wal-Mart" (or other discount store) in the automotive or hardware section. I've seen digital multimeters with leads for under $5. (We're not talking a FLUKE bench meter, just something to read DC voltage.)<p>On a quick search, I came across this meter:<p>
http://shop.store.yahoo.com/cutratetool ... multi.html<p>While I'm not saying use this meter, I only present it as an example of what is out there. After all, this is not going to be used for anything but discharing a capacitor! Be sure that you get a meter with a high enough DC voltage range to cover the fully charged voltage of the capacitor!<p>Splice the resistor near the meter end of the leads (leave enough slack to connect the leads to the meter easily). Then simply place the leads on the meter and wait for a low voltage. <p>
Based on a 25uF capacitor you get:<p>T=RC
T=10000 Ohms * 0.000025 Farads
T=0.25 seconds.<p>Since a capacitor is considered discharged in five time constants, you get:<p>5T=1.25 seconds.<p>
As you can see you don't even need the meter as the capacitor would be discharged before the tech could take a reading! <p>As a final test place a short with a screw driver across the capacitor terminals. If you get a big spark and a melted screw driver, the tech has done something wrong. If you want to really stop this from happening you would have the techs PAY for the damaged capacitor meters. This would get them to discharge the capacitors in the future.<p>[ December 11, 2002: Message edited by: Jolly Roger ]</p>
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bodgy
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Re: Capacitor discharge circuit

Post by bodgy » Wed Dec 11, 2002 7:34 pm

:D Try shorting the cap terminals on a faulty SMPS with only one main cap - the crack and spark can be quite something.<p>On second thoughts best not try it.<p>colin
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greg123
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Re: Capacitor discharge circuit

Post by greg123 » Wed Dec 11, 2002 7:43 pm

I Always touch the cap off my tounge. It makes for a good friday night.

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Re: Capacitor discharge circuit

Post by greg123 » Wed Dec 11, 2002 8:04 pm

Another thought.<p>Simple series circuit. Bulb, Resistor. <p>Put the cap, the bulb and the resistor in series. Resistor is there for current limiting. That way your techies wount blow the bejepers out of the bulb with a big enough cap. (just in case)<p>If the cap is full of charge.....bulb will be on and then dim as per the time constant of the cap and the resistance of the current limiting resistor plus the resistance of the bulb (negligable). As jolly roger said.....it takes 5 time constants to fully discharge as per the "international time constant rule". Remeber with e to the power of -1.....<p>I think i am off topic....<p>good luck :-)<p>[ December 12, 2002: Message edited by: Greg ]</p>

ampedtech
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Re: Capacitor discharge circuit

Post by ampedtech » Wed Dec 11, 2002 11:23 pm

Make the techs buy their own meters. Trust me, they'll remember.

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jollyrgr
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Re: Capacitor discharge circuit

Post by jollyrgr » Thu Dec 19, 2002 10:41 pm

There is only one down side to using a light bulb in a discharge circuit. What if the filament is broken? Like Greg said, one major surge could have apparently discharged the cap or blown the lightbulb. You would never know for sure as no light seen, tech assumes capacitor is discharged. Since the filament is broken, the capacitor would remain fully charged. Even with a resistor, a lightbulb could get bumped and break the filament. OOPS!
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Crowbar
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Re: Capacitor discharge circuit

Post by Crowbar » Fri Dec 20, 2002 12:07 pm

I agree with digitech, make them buy their own meters and the problem will be solved shortly enough.
Keep Prying...

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