Discharging a capacitor

[wristpin]

When working on something such as a single phase electric air compressor, what is the correct and safe way of ensuring that there’s no retained charge in the starting and run capacitors

 

[Seadog]

There should be no charge since the capacitors discharge through the windings.
If there's any doubt just short it. Wait for the bang and the sparks

 

[roofman]

When working on something such as a single phase electric air compressor, what is the correct and safe way of ensuring that there’s no retained charge in the starting and run capacitors

i put an old screwdriver across the 2 terminals until they spark and crack...make sure its insulated though

 

[minimutly]

I put my insulated pliers across a massive inverter dc bridge capacitor to check it was discharged once.
The bang deafened me, the flash caused me to be seeing a big green blob for quite a while afterwards, and my pants weren't too clever neither...

 

[skotl]

I put my insulated pliers across a massive inverter dc bridge capacitor to check it was discharged once.
The bang deafened me, the flash caused me to be seeing a big green blob for quite a while afterwards, and my pants weren't too clever neither...

That.
Time is the best way to discharge them. If you have a massive wirewound load then that would be a better option than pliers or a screwdriver.

All of that said, I must admit that if I'm working with high voltage caps then I'll leave them for 30 minutes and then put a screwdriver across them. Never seen so much as a fizz but I'd prefer a melted screwdriver and poor vision for a while over the alternative.

 

[AKN]

AVO Avometer 7 set to a high dc volt range, no arcs and sparks, and you can watch the volts go.

The AVO 7 is only 1000 ohms/volt so much quicker than an AVO 8 or dmm.

 

[sako243]

Just put a high value resistor across the terminals. Higher the resistance the longer it'll take to discharge but the less current it will pull so you don't have to worry so much about the current rating on the resistor.

 

[voipio]

Wires from a lightbulb holder with a bulb in. Any wattage bulb will do, preferaly old school tungsten filament, as LED bulbs may not be able to completely discharge the cap.

 

[skotl]

Just put a high value resistor across the terminals. Higher the resistance the longer it'll take to discharge but the less current it will pull so you don't have to worry so much about the current rating on the resistor.

Got to be careful that it can take the load (hence my suggestion of a huge wirewound resistor).

I like @voipio's suggestion of using a lightbulb (which is effectively a huge wirewound resistor!)

 

[OJCar]

When working on something such as a single phase electric air compressor, what is the correct and safe way of ensuring that there’s no retained charge in the starting and run capacitors

Get Norbert to grab the terminals.

 

[minimutly]

Get Norbert to grab the terminals.

When we were a roomful of apprentices in the mod, it became a habit to “test” the insulation of certain types of capacitors (with a megger), then leave them lying around on the bench for someone to idly pick up and grab the leads. Apprentice instructor picked up one I’d planted one day, he wasn’t amused. I had to apologise (the mod was big on formal apologies for everything). I had to confess though, what I was really sorry for was that I’d missed the actual moment - he didn’t laugh at that either...

 

[roofman]

Another way to discharge a cap is stab the end into your mates hand or arm...gets there attention like nothing else

 

[premmington]

AVO Avometer 7 set to a high dc volt range, no arcs and sparks, and you can watch the volts go.

The AVO 7 is only 1000 ohms/volt so much quicker than an AVO 8 or dmm.

+1 for the above.

I also put a meter on them and watch the Volts go...

Moving coil meter - will do it quicker - but any DMM will do it.

 

[MoreWellie]

As long as its not plugged in at the time....

I think @Seadog is right, it seems to me the other responses are likely talking about capacitors in different applications to motors

High value capacitors on DC circuits can store quite a charge or those in switched mode power supplies need to be safely discharged but I think low value capacitors (as in start and run capacitors) are on AC and store pretty much nothing. They are there to provide phase change voltage to the motor coils

 

[J1nx]

As per Fluke…

To safely discharge a capacitor: After power is removed, connect a 20,000 Ω, 5-watt resistor across the capacitor terminals for five seconds. Use your multimeter to confirm the capacitor is fully discharged.

https://www.fluke.com/en-gb/learn/blog/digital-multimeters/how-to-measure-capacitance

 

[MattF]

Another way to discharge a cap is stab the end into your mates hand or arm...gets there attention like nothing else

You're a bad man.

 

[brightspark]

i put a pair of leads on a cap i had . charge it up with the 500v megger and go prod someone with it it dont half make them jump

 

[MattF]

i put a pair of leads on a cap i had . charge it up with the 500v megger and go prod someone with it it dont half make them jump

You're bad too.

 

[Bornfree]

+1 for the above.

I also put a meter on them and watch the Volts go...

Moving coil meter - will do it quicker - but any DMM will do it.

But don't have it on current setting

 

[m_c]

@Seadog and @MoreWellie are correct for this application.
Motor start/run capacitors should be connected to windings, so should never retain any charge.

It could theoretically be possible for a bad connection (or faulty centrifugal switch if the motor happens to be a capacitor start/capacitor run type) to cause capacitors to charge and not drain, but the chances of that happening are pretty much negligible, especially if the motor hasn't been powered for a few minutes.
If you're really concerned, measure the DC voltage over the capacitor terminals before sticking your fingers on them.