Why don't 0.47uF caps across 240VAC explode????? - Page 1
Forum section: electronics / beginners

author: bzdata2
author_rank: Newbie
timestamp: March 22, 2016, 01:20:24 pm
content: What am I missing??The impedance formula for a cap says at 50Hz, at 240VAC RMS a 0.47uF cap will conduct (240 VAC / 6,773 Ohm) 35.4mA RMS, so at 240VAC RMS = 8.5W, which should blow up in a firery death when put straight across the A-N power lines - it does not - WHY, WHY, WHY?I see a lot of this in power supply input protection and EMI filters, all using 0.22 yo 0,47uF caps, which do not explode!

author: TimFox
author_rank: Super Contributor
timestamp: March 22, 2016, 01:31:49 pm
content: The current through a pure capacitor does not dissipate power: the current is 90 deg out-of-phase from the voltage.The power dissipated in a practical capacitor is that dissipated in the ESR due to the current through the device.That being said, 0.47 uF across 240 VAC seems a bit high. Where have you seen such a high value?

author: djococaud
author_rank: Regular Contributor
timestamp: March 22, 2016, 01:32:13 pm
content: Hi,It's because a capacitor induce a phase shift of 90° between voltage and current. So when the voltage is max, the current is min, and vice/versa. The average power is then always almost 0. That's why capacitive dropping is often used in low cost LED lamps ! (cheap current source).

author: Shock
author_rank: Super Contributor
timestamp: March 22, 2016, 01:41:40 pm
content: A non polarized cap working under it's rating, situation normal.But once they get old and start to fail...

author: danadak
author_rank: Super Contributor
timestamp: March 22, 2016, 02:06:22 pm
content: Power dissipation in a capacitor, see attached sim.of course this is instantaneous power, avg power in pure C circuit is 0.Regards, Dana,

author: wraper
author_rank: Unknown
timestamp: March 22, 2016, 02:17:44 pm
content: Quote from: bzdata2 on March 22, 2016, 01:20:24 pmBecause you do not calculate RMS power with reactive load that way.

author: coppice
author_rank: Super Contributor
timestamp: March 22, 2016, 03:08:17 pm
content: Quote from: Shock on March 22, 2016, 01:41:40 pmAge can degrade those caps, but a common cause for massive failure is running the device from a square wave output UPS. The cap may carry just tens of mA at the fundamental, but it can be carrying many amps at high harmonics, and the losses in the ESR can get pretty high.

author: wraper
author_rank: Unknown
timestamp: March 22, 2016, 03:12:21 pm
content: Quote from: coppice on March 22, 2016, 03:08:17 pmThose particular Rifa caps fail because of crappy construction. Shell cracks, moisture gets in and they go boom. Even if you get them NOS, they will be already covered with cracks.

author: Shock
author_rank: Super Contributor
timestamp: March 22, 2016, 05:22:00 pm
content: Quote from: wraper on March 22, 2016, 03:12:21 pmI smoked a Rifa, but I did not inhale.

author: pelud
author_rank: Regular Contributor
timestamp: March 22, 2016, 09:32:20 pm
content: The 35ma is the "displacement" current used to charge and discharge the electric field stored in the capacitor. The actual losses which would cause heating are caused by the ESR which is very low for this type of cap.

author: bzdata2
author_rank: Newbie
timestamp: March 23, 2016, 07:05:33 am
content: THANK YOU VERY MUCH TO ALL, the last 35mA charge/discharge answer makes intuitive sense to me, so extra thanks there)

author: bzdata2
author_rank: Newbie
timestamp: March 23, 2016, 07:37:31 am
content: Capacitive dropping in LED bulbs is what I am using to get a floating 12V/20mA supply straight from mains using two 1uF caps (1 for N and 1 for A). Won't go into details, here is the LTSpice model. I needed a 12V supply referenced to the 0V DC from a high power 240AC bridge, so that I have it for circuitry (that only needs about 10mA btw) doing some serious power control using PWM buck etc. controllers tat are not designed for high voltage use (xformer isolated MOSFET of course). anyway, ...not the subject here, so I'' won't go any further,....BUT, BUT, BUT... it is what led me to the 0.47F across mains the thing, when I was looking at commercial EMI filter designs! Most use 0.22uF, some 0.47uF, a few use 0.1uF.ONCE AGAIN THANKS ALL FOR SHOWING ME SOMETHING I SHOULD KNOW FROM BASICS)

author: SL4P
author_rank: Super Contributor
timestamp: March 23, 2016, 07:58:02 am
content: Quote from: wilfred on March 23, 2016, 07:14:15 amok, I'll take it one further...Rifa's are best experienced as a dry joint.

author: coppice
author_rank: Super Contributor
timestamp: March 23, 2016, 08:03:37 am
content: Quote from: bzdata2 on March 23, 2016, 07:37:31 amTry those cap drop LED lamps with a triac dimmer and the high current at the harmonics can produce some nice fires.

author: timb
author_rank: Super Contributor
timestamp: March 23, 2016, 01:41:45 pm
content: Quote from: SL4P on March 23, 2016, 07:58:02 amNo way man. There's nothing quite like the feel you get from a thermionic tube. Bypass it with 420nF and light that filament up."I get high voltage all the time, I smoke dem good caps. Gimme some 'dro Purple Vishays or Panasonics; a Sprague and an iron and I'll spark shit; yeah I stay high voltage all the time, I'm on that good shit." -50 Cent