[OP]
Member
I have the plug-in flourescent light fixtures, and it seems almost a sure thing that whenever one is unplugged, I touch the plug and get hit by the capacitor. Is making a dead short from one side of the plug to a ground an acceptable way to discharge one of those? It really gets irritating.
Thanks,
Greg
Member
I'd get the wife to plug 'em in
[OP]
Member
Maybe that's why she left.
Member
Greg,
That’s the way we do it in the Navy only we use a shorting probe that has a long non conductive handle. Once you short the capacitors collector to ground it will discharge and you will be safe. I have never seen this done on lights though.
Dewey
"Everything is better with Inlay or Marquetry!"
Member
Don't touch the plug. Take a voltage meter and see if that is where the juice is coming from. I haven't seen one with a capacitor - are you sure it's not static?
Contributor
What?!
You should NOT be able to feel anything by touching an unplugged light fixture of any sort. While I've seen plenty of stupid designs, anyone smart enough to design a flourescent lamp balast would not do something like place a capacitor directly across the line. Was this fixture made in China, per chance?
I think there is something wrong with it, and you should replace it.
That being said, a screwdriver works. If there's enough stored energy, you might start to melt chunks out of your screwdriver and/or plug, though.
Contributor
I work with high voltage daily. We have a special tool used for discharging. It has a couple of resistors in parallel that are in series with a cable with an alligator clip on it and a "metal wand". By having the resistors there, it limits the current thus suppressing instant unlimited discharge currents. You attach the alligator clip to ground and then holding the insulated handle, you touch the "metal wand" to the device being discharged.
That being said....you should not being seeing the effect that you are. Something's not right.
Do all of the lights do this or just one. Somethings not right.
Ken
So much to learn, so little time.....
Moderator
Well, capacitance is just two conductive materials separated by a space. The space could be air or some dielectric material. This "device" can store energy and hence can also discharge the energy when the right situation strikes.
I think it could be static electricity you are experiencing but maybe not....
Member
+1.Originally Posted by Ken Fitzgerald
When I am discharging caps on my amplifiers I use a conductor with high value resistor in series to ground. May not be as exciting as shorting out with a screw driver, but I don't really care all that much for sparks.
If you're getting a jolt off the fixture, something is definitely not kosher and discharging any surface charge that may exist is the least of your problems.
Contributor
Very true, Chris. The parallel-conductor cable running to the lamp is a capacitor. However, if you've ever taken a "can" type capacitor apart, you'll find that they manage to stuff a LOT of foil (conductive surface area) in there. Also, the insulation separating the foils is VERY thin. C=e*area/insulation_thickness, so both of those factors together is what gets you a usable amount of capacitance. Even then, many capacitors use a wet electrolyte to further increase capacity by chemical means. Thus, if the OP can touch a screwdriver across the plug terminals and see a spark, there is a sizable capacitor inside that has somehow become connected in parallel with the cord.
However, I agree that static electricity may be the culprit.
Contributor
I remember learning about discharging a capasitor........
I still have a twitch.......
Member
Always liked to charge an ignition capacitor off a spark plug and leave it laying around on the bench with the wire folded back along the side. Some people never seemed to catch on or at least remember for more than a week or so.
gary
Member
How many ohms/kilohms are you talking? I have an old start capacitor I'd like to dispose of safely. I have the right screwdriver to do the job, but I'm not as fond of light shows as I used to be. And where do you go to buy stuff like that since Radio Shack isn't around anymore? Cheers, Ian
I'm guided by the beauty of our weapons -- Leonard Cohen
Member
If you know how much voltage is across the cap, you can apply Ohms law to get the instantaneous DC current.
V = I * R
You want the current to be pretty low, 10 micro amps is a nice number. Milliamps, properly applied is enough to stop your heart.
Worked example:
assume cap is at 100Vdc
100V = 10uA * R
R = 100V / 100uA
R = 1Meg Ohm
The larger the resistor, the longer it will take to bleed down the cap (drops by about 68% every 2*pi*R*C seconds, all things being equal).
If you want to do more math, the equation for current flowing from a capacitor is I = C * dV/dt. That is to say, the current is proportional to the change in voltage over time. Mathematically, the current is infinite if the voltage is instantaneously brought to zero. In the real world, this isn't quite possible but shorting with a screwdriver does produce a sufficiently fast discharge so you get nice, high currents. Enough to ionize the air at the gap and viola', a spark.
Oh, and if you want to size the resistor for power dissipation, the DC power is P = I * V or P = I^2 * R. Since you are dissipating and not conducting the current continuously, this isn't quite as critical, you can figure the peak power and just use that value.
A good source for small quantities of components would be www.digikey.com (minimum order $25), www.mouser.com, www.newark.com and there are lots of others.
By the way, there is an effect in capacitors where they will revert to a voltage after the short has been removed, sometimes called "bounce back". This isn't terribly critical unless you are working around ultra high voltage, oil filled jobs. It can be easily observed at the bench in smaller electrolytic and tantilum electrolyte caps but don't expect anything like 90% bounce back, think 1% or less. Still, when working with 10,000V, that leaves you with sufficient charge in a capacitor to push enough current to do damage.
As to the spark you are experiencing, if it isn't due to static electricity, there could actually be capactors across the AC line. If the balast is not using a transformer but instead was designed to run directly from the AC line, they sometimes include capacitors across the line in an attempt at power factor correction. If not done correctly, I suppose this could give you a pop each time. They should have included a tranzorb, SCR or some kind of back-to-back zener diode to snub out the caps when the power is removed but to save a penny, maybe not.
Last edited by Rob Young; 01-22-2009 at 12:21 AM.
Don't sweat the petty things and don't pet the sweaty things.
Member
Man, that was an answer and a half!! And, personally, I couldn't possibly do any more math than that :OIf you know how much voltage is across the cap, you can apply Ohms law to get the instantaneous DC current.
V = I * R
You want the current to be pretty low, 10 micro amps is a nice number. Milliamps, properly applied is enough to stop your heart.
Worked example:
assume cap is at 100Vdc
100V = 10uA * R
R = 100V / 100uA
R = 1Meg Ohm
The larger the resistor, the longer it will take to bleed down the cap (drops by about 68% every 2*pi*R*C seconds, all things being equal).
If you want to do more math [SNIP]
I'll have to look in the the surplus shop, or risk ionizing some air. $25 minimum plus S&H is enough to make me appreciate making sparks. Never thought I'd be nostalgic over Rad Shack and Heathkit.
Thanks for the info!
I'm guided by the beauty of our weapons -- Leonard Cohen
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