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Discussion Starter · #1 · (Edited)
I've read I need to discharge the old capacitor completely before removing it to avoid getting zapped if it still has some charge. Seems simple enough however I'm pretty sure there are many here that know more about electrical stuff than I do so I figured I'd better ask what is the safest way to do this and are there are any other precautions I should take? (and yes the cord is unplugged! :yes:). BTW, it is a 125vac 200MFD cap if that makes a difference.
 

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I've never experienced a capacitor holding a charge that long. By the time you unplug it and get the cover off of it the charge should be gone. The wires are kinda hard to get off so if you use needle nose pliers to pull the wires you shouldn't get zapped anyway.
 

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Caps holding a dangerous charge is really only a factor in DC circuits.
 

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A 250 mfd electrolytic can will deliver a mighty painful whack if it is even partially charged.

Whilst some large capacitors come with an internal resistance, personally I never take chances with any capacitor, AC or DC.

Slapping a screwdriver across the terminals is an extreme sport, but the safest way to discharge a capacitor with an unknown charge is to discharge it by shorting it out through a high resistance which you can lash up yourself with a suitably insulated resistor and a couple of wires.
You need about 2 - 3 meg ohms, about 5 or 10 watts - the exact value is not important - just a high resistance.

It's an essential bit of kit if you are delving regularly into the innards of VFD speed controls and the like.


Hope this helps
 

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I've read I need to discharge the old capacitor completely before removing it to avoid getting zapped if it still has some charge. Seems simple enough however I'm pretty sure there are many here that know more about electrical stuff than I do so I figured I'd better ask what is the safest way to do this and are there are any other precautions I should take? (and yes the cord is unplugged! :yes:). BTW, it is a 125vac 200MFD cap if that makes a difference.
look's like only one time , i would just short both lugs with a screw driver , if cap is bad how do you know this ? end poped out ? now if you were doing this all the time and in no hurry than make up a unit with a resistor, with 5 or 10 watt , and some clip's and be a pro, but for a one time short the lug's and move on. now some of the big trinsmitter cap's will pick up a charge just settime, but that doesn't apply to you , i been around the dam stuff for 60 or so yrs, so you will be ok with the advice given,
 

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Discussion Starter · #6 ·
Thanks guys! Based on my description of the problem with my Delta 50-760 dust collector (motor was slow to start for several days and then just started making a loud humming but would not start) the tech said it is most likely the motor start capacitor. Since the part was only 10 bucks I figured I'd try it myself before taking it in however if anybody has any other ideas on what it may be I'm all ears!
 

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Discussion Starter · #8 ·
Thanks Dave! I actually read about that possibility during my research however I heard the switch "click" during the last shut down so hopefully that's not the problem!
 

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You need about 2 - 3 meg ohms, about 5 or 10 watts - the exact value is not important - just a high resistance.
A 250 mfd capacitor with a 200 volt charge shunted by a 2 megohm resistor would have an initial discharge current of 0.1 ma, or an initial power dissipation of 20 milliwatts, and it would drop from there. Also with a discharge resistor that large, the time constant is over 8 minutes--that's the time it would take the cap to lose about 2/3 of its charge.

We're not talking about much residual charge on an AC cap, if any, and the cap will probably self discharge relatively quickly anyway. If in doubt, short it with a screwdriver.
 

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Discussion Starter · #10 ·
Thanks again ed!
 

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I have done some A/C work in the past and have been shocked by a oil filled run capacitor. I have never had a problem with simply shorting the two terminals with a screw driver. I do this on an motor that I have to remove the capacitor.
Tom
 

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Discussion Starter · #12 ·
Thanks again for the guidance guys!
The new cap came in today so I started by touching the screwdriver across the terminals of the old one and no spark so I figured I was good to go. Once removed I saw that the new cap was a little larger in diameter than the old one so I ended up using electrical tape to hold the protective boot over the terminals (is this OK?).
Once installed I plugged the the dust collector back into my remote switch and gave it a try. Same buzzing sound but not quite as loud and it didn't start!?! My first thought was that maybe the cap needed to be charged first like a mobile audio capacitor so I plugged directly into the outlet and let it sit for a while. I hit the switch again and it came to life rather slowly but did get up to full speed! After running/stopping it several times all seems fine so I reconnected the remote and it now works through that as well.
One thing I did notice is it seems to start a little slower and the initial "hum" is much quieter however this may be because I don't really remember how it worked before?!
Thanks again and if I did something wrong or need to do something else please let me know!
 

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Marv,
No the capacitor does not to precharged. The capacitor make the electrical circuit seem like a two phase circuit for the motor. You need two phases to get the rotation. The capicator is in an AC circuit which is providing power to a set of start winding that a constantly changing form north to south poles. On motors that require high torque such as A/C compressors you have a large oil filled capacitor which is not only a start capacitor but stays in for a run capicator. I have no idea why your motor started after you thought you precharged it. In fact simply plugging the cord in would not put any power to the capicator.
Tom
 

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Discussion Starter · #14 ·
Thanks Tom! I heard the hum when I switched it on the first time and immediately shut it off because I thought it was going to pop the circuit breaker so I have a feeling I just didn't give it enough time to actually get going!
 

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Discussion Starter · #15 ·
Just wanted to follow up on this in case anyone else ever has a similar problem and finds this thread. My dust collector started acting up again occasionally so I did a little more research into the centrifugal switch Dave mentioned above. When I took it apart the contact was closed however while spinning the motor by hand I noticed the spring disc was warped and at one point you could see it was just barely losing contact. My guess is that the motor stopped occasionally at this exact spot making it hard to start the next time so to fix the problem I loosened the set screw on the shaft and adjusted it just enough to make sure the contact was closed when the warped spring disc was at it's highest point on that side. I also put the original start capacitor back in out of curiosity and that worked as well so I'm pretty sure the switch was my problem to start with.
 

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I've never experienced a capacitor holding a charge that long. By the time you unplug it and get the cover off of it the charge should be gone. The wires are kinda hard to get off so if you use needle nose pliers to pull the wires you shouldn't get zapped anyway.
You've never disassembled a disposable camera, have you? Yo the curious out there, capacitors hold a charge for a pretty good amount of time, provided there isn't anything to discharge into. In some applications, microwaves for example, the cap is grounded with a high impedance resistor in line to dissipate the charge when needed, but not all caps have this. A good way to be safe is to short the terminals to discharge the charge. On smaller caps you can usually get away with using a screwdriver, but a much safer option foot both smaller and larger caps is to get a 10 ohm or so 10 watt wirewound resistor, put some wire on both terminals and short the cap with that. Better safe than having a numb arm
 

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I assume you are referring to the flash circuit in the camera. It's quite a different thing, since it's charged to a pretty high voltage with DC. Microwaves also use pretty high voltages.

Motor caps see much lower AC voltages, and since the caps aren't required to hold a HV charge, they are relatively leaky.

Ed
 

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I assume you are referring to the flash circuit in the camera. It's quite a different thing, since it's charged to a pretty high voltage with DC. Microwaves also use pretty high voltages.

Motor caps see much lower AC voltages, and since the caps aren't required to hold a HV charge, they are relatively leaky.

Ed
High charge or not, better safe than being out of the shop for a day because you cant feel your hands
 

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Just wanted to follow up on this in case anyone else ever has a similar problem and finds this thread. My dust collector started acting up again occasionally so I did a little more research into the centrifugal switch Dave mentioned above. When I took it apart the contact was closed however while spinning the motor by hand I noticed the spring disc was warped and at one point you could see it was just barely losing contact. My guess is that the motor stopped occasionally at this exact spot making it hard to start the next time so to fix the problem I loosened the set screw on the shaft and adjusted it just enough to make sure the contact was closed when the warped spring disc was at it's highest point on that side. I also put the original start capacitor back in out of curiosity and that worked as well so I'm pretty sure the switch was my problem to start with.

Yeah, I was going to suggest that the centrifugal switch was bad because AC capacitors are pretty rugged devices. It has been my experience, especially with none totally enclosed motors, is that crud gets into the switch mechanism and a simple cleaning will fix things.

Glad you solved your problem.
 
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