Calling the electrical experts

[lpstealth]

Hey guys,

I have this decent sized 3 phase rectifier and want to strip some parts out of it (4 transformers, 4 decent sized caps and 4 smaller caps along with some other bits) My question is what are the odds that the caps are discharged and what would a good idea to discharge them be?

I want this thing out of my room but to be honest I am wary of touching it.

 

[jander6442]

Hey guys,

I have this decent sized 3 phase rectifier and want to strip some parts out of it (4 transformers, 4 decent sized caps and 4 smaller caps along with some other bits) My question is what are the odds that the caps are discharged and what would a good idea to discharge them be?

I want this thing out of my room but to be honest I am wary of touching it.

you could drop it in the bathtub... no seriously Google it.

 

[Pilgor]

Are there bleeder resistors?? If there are, your okay.

 

[lpstealth]

you could drop it in the bathtub... no seriously Google it.

not a bad idea, but I don't think anyone would appreciate me dropping a giant metal suitcase into the newly renovated bathroom/tub

I was thinking if I could get to the back of the circuit bards then maybe throwing steel wool at it and poking it with a stick :thinking:

 

[lpstealth]

Are there bleeder resistors?? If there are, your okay.

Well thats a good question, if you are asking if there is a resistor across the 2 terminals of the caps then no, but there is so much electronics packed in there its hard to say whether there is a bleeder circuit or not.

Here are 2 quick phone pics.

 

[Things]

If it has been sitting for more than a week, you should be fine, but always short them out manually with something before you touch them, just to be safe, but I'd be very surprised if a regular electrolytic could hold any charge after even a day or 2.

 

[Cyparagon]

I killed a laser once because I forgot to discharge an external input filter cap. It sat for about two weeks, and the voltage only dropped from ~17Vto ~12V. It was 0.15F though.

Put a high value resistor across the terminals to discharge them. 100k or so for a few seconds should do it. I use a screwdriver sometimes because I'm reckless and impatient, but it's not the best idea.

 

[Things]

I killed a laser once because I forgot to discharge an external input filter cap. It sat for about two weeks, and the voltage only dropped from ~17Vto ~12V. It was 0.15F though.

Put a high value resistor across the terminals to discharge them. 100k or so for a few seconds should do it. I use a screwdriver sometimes because I'm reckless and impatient, but it's not the best idea.

0.15F is a pretty serious cap I meant more like the ~470uF heaps of SMPS's use.

 

[Meatball]

If it has been sitting for more than a week, you should be fine, but always short them out manually with something before you touch them, just to be safe, but I'd be very surprised if a regular electrolytic could hold any charge after even a day or 2.

The problem with large caps is that they can build a small potential over time even after being discharged. The dielectric can sometimes take time to "heal" from the electric field, and when it doesn't heal very fast, charge separation occurs as a response to the small residual electric field.

Its either that or the quantum tunneling effect. Some say both are responsible.

Either way, this is why you ALWAYS store capacitors shorted out. If they do not appear to be shorted, it is best to assume they have a charge.

I would just disassemble the unit. One screw at a time, using only one hand with a winter glove on. The other hand would be behind my back. Not everyone has the patience for this however.

 

[lpstealth]

The problem with large caps is that they can build a small potential over time even after being discharged. The dielectric can sometimes take time to "heal" from the electric field, and when it doesn't heal very fast, charge separation occurs as a response to the small residual electric field.

Its either that or the quantum tunneling effect. Some say both are responsible.

Either way, this is why you ALWAYS store capacitors shorted out. If they do not appear to be shorted, it is best to assume they have a charge.

I would just disassemble the unit. One screw at a time, using only one hand with a winter glove on. The other hand would be behind my back. Not everyone has the patience for this however.

Thanks I agree with everything you said, I think I ill do just that.

 

[Cyparagon]

0.15F is a pretty serious cap I meant more like the ~470uF heaps of SMPS's use.

Yeah, lower voltage tends to drain slower, too. I charged a 470µF cap up to 340V and put it on a shelf. I'll let you know what it is tomorrow.

 

[Seoul_lasers]

Looks like a great project.
Lots of useful parts in there I see!!
Do be careful with the caps. They do hold a charge for quite some time. I have had Electrolytic caps around 600V 10,000uF stay charged for nearly a week. 600V 10,000uF was from an industrial plasma cutter. Enough current stored in a cap like that to blow your hands off when fully charged. There may not be a full charge in a cap sitting for a week, (as in my case) but a charge of +50V can deliver a nasty shock and in some cases prove fatal especially if there is high currents involved.
Indeed there is several reasons to store caps with their terminals shorted. Be safe rather than sorry.

 

[Flaminpyro]

I like the stick and steel wool idea :eg:

How about split a pop sickle stick a little bit just enough to stuff some steel wool into the end and go after it.

It should be fun don't forget the safety glasses.

Hell I would plug it in first just to charge em up a bit so you know when your on a good cap :crackup:

 

[Cyparagon]

Just checked the cap - still at 86V. I know photo-flash caps are known for storing residual charge for a few days if not a few weeks.

 

[Things]

86V is a pretty significant drop from 340 though, but the rate of discharge would decrease as the Voltage dies anyway. 50V is what I'd consider pretty safe on dry skin.

 

[lpstealth]

50V is what I'd consider pretty safe on dry skin.

yeah, thats what I was taught. anything under 50 volts and you are safe from the resistance of your body.