Safely draining a 5kV 40uF cap? - Page 1
Forum section: electronics / projects

author: Pulseit
author_rank: Contributor
timestamp: March 21, 2014, 05:47:20 pm
content: Hi,I'd like to add a safe way of draining a 5kv 40uf cap that is integral to a power supply I built, It has a high voltage relay that I can use to switch the cap into the "drain" but am unsure what is best to use as the "drain". I'm picturing a resistor and light bulb but am unsure how to do this, and what values would be required, can anyone help?

author: PeteJM
author_rank: Contributor
timestamp: March 21, 2014, 06:14:41 pm
content: 4 megaohm resistor straight to ground?

author: SeanB
author_rank: Super Contributor
timestamp: March 21, 2014, 06:25:53 pm
content: Going to have to be rated for 5kV minimum. Likely will consist of 10 470k 1W metal film resistors in series, on a small PCB with generous clearance around each resistor.

author: Dave Turner
author_rank: Frequent Contributor
timestamp: March 21, 2014, 06:46:10 pm
content: The large caps in old valve TVs used to have a high value resistor directly over the terminals to discharge them relatively slowly when power was switched off.Nevertheless it was always sensible to assume that the resistor had failed and discharge them manually with a 10 Megohm resistor. Not as an acquaintance did with a screwdriver; which didn't do him or the screwdriver any favours.Would it make sense for you to take the same approach?

author: Marco
author_rank: Super Contributor
timestamp: March 21, 2014, 06:55:37 pm
content: A light bulb doesn't make much sense, a function as a current limiter would only work if you put a ton of them in series to take the full voltage. If you just want an indicator that there is current flowing use a LED.

author: krish2487
author_rank: Frequent Contributor
timestamp: March 21, 2014, 07:49:24 pm
content: Quote from: SeanB on March 21, 2014, 06:25:53 pmMight I make a suggestion...A metal oxide resistor is more likely to withstand the beating much better than a MFR.I have seen in my course of work ( the hard way), MFRs fail faster and more often a MOR of the same rating.Just my 2 cents...

author: SeanB
author_rank: Super Contributor
timestamp: March 22, 2014, 05:50:31 am
content: I would prefer carbon composition there, but they are getting rare these days as new. They at least have a good overload capacity and do not fail quietly open circuit that often.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 22, 2014, 09:09:45 am
content: For these sort of discharges, I am using a high quality automotive spark plug cable, made out of silicone with two layers and wire wound high voltage resistor in series with the cable. These wire wound high voltage resistors can be found in high quality ignition coil connectors and come as 1 kOhm, 5 kOhm, 10kOhm and 15kOhm. These resistors are wounded usually on a 5 mm diameter glass fiber rod, then get metal end caps and some kind of lacquer paint protection. There are only two companies in Germany, who build these resistors but both of them only sell to the OEM.

author: Riotpack
author_rank: Regular Contributor
timestamp: March 22, 2014, 09:57:23 am
content: Maybe use a bright LED/resistor combination to show the cap discharging, maybe across one of the resistors in the string. Protect the LED against over voltage by placing diodes across it.That way, when you discharge the capacitor you will see the voltage dropping. Once it has been discharged, it would be wise to apply a short circuit across the capacitor bank. This should be done in a manner where it is visible and poses no risks.You could also have a voltmeter across the bank as a third indication of the voltage across the capacitor.

author: Psi
author_rank: Super Contributor
timestamp: March 22, 2014, 10:46:51 am
content: Can you imagine putting your tongue on that, it would explode!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 22, 2014, 10:51:12 am
content: If you have to ask, you probably shouldn't be messing with anything that dangerous.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 22, 2014, 11:26:11 am
content: Quote from: mikeselectricstuff on March 22, 2014, 10:51:12 amVery good point.But then, someone has to start somewhere.

author: Psi
author_rank: Super Contributor
timestamp: March 22, 2014, 11:53:54 am
content: Quote from: HighVoltage on March 22, 2014, 11:26:11 amtrue, but starting out using 5kV 40uF caps is like learning to drive in a Formula 1 car with a coke bottle full of nitroglycerin taped to the front.

author: han
author_rank: Frequent Contributor
timestamp: March 22, 2014, 12:39:11 pm
content: 5KV is very dangerous, even if drained using high value resistor, the problem is how to connect the load, you need >5KV rating probe and cable..even when i probe 450V only (in 220Vac PS) live capacitor, i feel the fuke 1000V rating probe still give a little tickling sensation

author: amyk
author_rank: Super Contributor
timestamp: March 22, 2014, 01:17:50 pm
content: Quote from: han on March 22, 2014, 12:39:11 pmThat's because it's afuke...

author: robrenz
author_rank: Super Contributor
timestamp: March 22, 2014, 01:32:04 pm
content: Calling it a Fuke was a fluke

author: han
author_rank: Frequent Contributor
timestamp: March 22, 2014, 01:42:07 pm
content: Quote from: robrenz on March 22, 2014, 01:32:04 pmwew, sorry just typo,, fluke...im sure "Fuke" will make my hand shaking, not ticklether is real FUKE brand in internethttp://www.ekt2.com/products/productdetails?ProductId=4364170d-776b-42e8-bf0a-ad83f6d77c05

author: calexanian
author_rank: Super Contributor
timestamp: March 22, 2014, 02:31:23 pm
content: They make large cartridge resistors for this very purpose. We use then in out test sets mounted in a phenolic board with extra thick rubber insulated wires. The ground side has a welding ground clamp to bond to the equipment bonding point an the cap end has a brass hook on the end of a fiberglass pole about 2 feet long to discharge the caps. The resistors are in similar packaging as cartridge fuses.

author: chipwitch
author_rank: Frequent Contributor
timestamp: March 22, 2014, 07:36:37 pm
content: Quote from: AhhhChoo on March 22, 2014, 03:40:04 pmGood thing we don'tallhave that attitude or the pinnacle of technology today would be constrained to sticks and stones. Every important advancement in human history started with someone somewhere not knowing some thing.