Selecting a capacitor rating - Page 1
Forum section: electronics / beginners

author: jj5
author_rank: Unknown
timestamp: February 10, 2023, 10:07:18 pm
content: I'm planning to build this power-on indicator circuit:For C1 my choices seem to be 275VAC X2 rated capacitors, likethese ones, or 400V capacitors (not X2 rated), likethese ones.I can't seem to find X2 rated capacitors for voltages above 275VAC. So what's more important for my application, the voltage rating, or the X2 safety class?

author: Benta
author_rank: Super Contributor
timestamp: February 10, 2023, 10:16:16 pm
content: X2 certainly. 275 VAC are out of date, most are 305 VAC today.But honestly, wouldn't it be much easier with a resistor and a neon bulb?

author: BillyO
author_rank: Super Contributor
timestamp: February 10, 2023, 10:30:11 pm
content: A neon would be best, but if you are dead set on an LED, toss the bridge and just use a 1N4148 in parallel with the LED but in the opposite direction.

author: Benta
author_rank: Super Contributor
timestamp: February 10, 2023, 10:44:28 pm
content: ...which will make it flash at 50 Hz (extremely annoying) instead of 100 Hz (less annoying).

author: Kjelt
author_rank: Super Contributor
timestamp: February 10, 2023, 10:58:38 pm
content: Why not use a tiny ac/dc buck converter (Ali $1.50) , 3.3VDC + resistor , ideal for a led.

author: Benta
author_rank: Super Contributor
timestamp: February 10, 2023, 11:30:19 pm
content: Quote from: Kjelt on February 10, 2023, 10:58:38 pmGreat idea! Especially as a neon lamp costs less than 20 cents.

author: jj5
author_rank: Unknown
timestamp: February 10, 2023, 11:52:21 pm
content: Quote from: Benta on February 10, 2023, 10:16:16 pmBut are the old 275VAC ones safe to use in this circuit?Quote from: Benta on February 10, 2023, 10:16:16 pmYeah, but where's the fun in that?

author: Benta
author_rank: Super Contributor
timestamp: February 11, 2023, 12:06:03 am
content: Sure, you'll be OK with that one. And I'm certain that your construction leaves no chance of human contact, right?

author: james_s
author_rank: Super Contributor
timestamp: February 11, 2023, 12:28:04 am
content: I don't think I'd even bother with an X2 capacitor personally, it's not connected directly across the line, that 1k resistor will fuse if the capacitor becomes shorted. Even if it isn't a fusible resistor it will go pop and open the circuit when it ends up directly across 240V mains.

author: jj5
author_rank: Unknown
timestamp: February 11, 2023, 01:12:57 am
content: Quote from: Benta on February 11, 2023, 12:06:03 amGreat. Thanks.Quote from: Benta on February 11, 2023, 12:06:03 amYeah, I'm trying to do a good safe job of it. And anything would be safer than where I'm starting. Not sure if you've ever seen the insides of an OG Xbox, but the power board is exposed and just sits next to the motherboard. It's an accident just waiting to happen. I'm building an enclosure so there's no exposed mains wires.

author: jonpaul
author_rank: Super Contributor
timestamp: February 11, 2023, 01:23:43 am
content: line transients will eventually kill the cap and LED/diodes.Unsafe as well.Neon lamp and 100k 1/2 W R is about $0.30 and smaller, simpler and works for decades on a line.Use higher R for 240V line, and high brightness neon if desired.j

author: james_s
author_rank: Super Contributor
timestamp: February 11, 2023, 01:25:13 am
content: Either one is fine, I've seen LED indicators on mains voltage a handful of times, they hold up ok, at least the low efficiency LED types seem fine, the blue ones are more fragile.

author: floobydust
author_rank: Super Contributor
timestamp: February 11, 2023, 04:23:52 am
content: I would ditch the capacitive dropper and simply use a single resistor in place of the cap+1k. Modern LED's are very efficient so even 1mA is plenty bright.I use (120VAC) 66k ohm (mains voltage rated say 1/2W fusible) plus LED (with parallel reverse-diode so 60Hz flicker yuck) or a bridge rectifier (120Hz flicker) stuffed into the end of my extension cords, so I know I have power.The goal I design to is around 1/4W for the dropping resistor to dissipate, enough that a white or green LED is quite bright.Neon lamps today are tiny and cheap out of china and simply don't last, no way they make it to 25,000hrs bzzzt blink flicker blink bzzzt.Edit: added my schematics on it

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2023, 04:31:56 am
content: the number of dead mains LED indicators I have seen vs the # of neon failures I have seen is a elephant on a seesaw with a mouse.take something with a LED indicator apart and measure the capacitance of the capacitor, it will probobly be drifted under the nominal.Best place to see it, is in power strips. Very easy to find a powerstrip with the LED out. Or a UPS.

author: floobydust
author_rank: Super Contributor
timestamp: February 11, 2023, 05:58:11 am
content: The top left circuit, one diode 1/2 wave rectifier 1N4007 fails frequently, because it's common to see 1,000V spikes on mains which avalanches the diode and then the LED sees that reverse current and dies. Even on a 24VAC HVAC transformer with the same circuit (4k7) I shorted the 24VAC to test circuit breakers and it killed the LED. Put in an new LED, repeat the test and it's dead. A little blue spark seems to be over 1,000V rating of the 1N4007. So I use a diode shunt and live with double the resistor power dissipation (full wave).The use of a 50V bridge rectifier in the other circuit, it was my observation you don't need a 1,000PIV bridge there.Every power strip I have (chinese neon lamps) is going blink blink blink bzzzt. They push too much current through the lamp, the electrodes are too short in some parts or maybe the neon gas is low quality etc. I only use a few NOS American made lamps I have and they work but can't be found nowadays. LED's rule if done right.OP, the problem is dropping the high mains voltage down to what an LED can operate with. That can be done with a capacitor divider but the cap sees all mains transients and does age, even though X2 or X1 rated, the metallization fails.Or use resistor(s), which shed the excess voltage as heat. But you don't need huge 20mA for the LED so lowering that to say 1mA makes a resistor feasible.

author: james_s
author_rank: Super Contributor
timestamp: February 11, 2023, 07:08:25 am
content: You can still get good quality neon lamps, you just have to size the resistor appropriately, if you drive them too hard you get greatly shortened life for very little added brightness. I have nixie clocks I built with NE-2 lamps as colons and PM indicators that are working after >10 years, earlier some lamps had to be replaced because I used too small a resistor but they've been fine since then. These are even worse case since they're running on DC so all the wear is on one electrode.

author: CaptDon
author_rank: Super Contributor
timestamp: February 11, 2023, 01:46:18 pm
content: Use a 1N4007 and after it two resistors in series, the first is 100K @ 2w (it will radiate about .5 watts) and the second resistor is 5.6K 1/2 watt. This series string goes across the 240vac. Place a 47uf (anything close is good) at no less than 25vdc rating across the 5.6K and put your led right across the capacitor. Observe polarity for cap, diode and LED!! No flicker and will work forever. Provides a bit over 1ma to the Led which is plenty especially with a high efficiency LED. You can even place a 1K in series with the LED. If the LED would go open circuit the capacitor will still survive. I have had this identical circuit as a night light in an over the sink lamp fixture for 10 years and it still runs perfect. I used a high brightness white LED.The neon indicator is also fine and uses less power. The life of the neon bulb is dictated mainly by average current. Higher current-shorter life due to ion sputtering of the electrodes which blackens the glass. At 240vac ionization in the dark won't be a problem, neither will flicker. Cheers mate!!

author: mariush
author_rank: Super Contributor
timestamp: February 11, 2023, 01:59:44 pm
content: You could also just use a high voltage linear regulator like LR645 or LR8 from MicrochipLR645 :https://www.microchip.com/en-us/product/LR645LR745 :https://www.microchip.com/en-us/product/LR745LR8 :https://www.microchip.com/en-us/product/LR81n4007 for half-wave rectification, a tiny electrolytic or something that can handle 230v+ peaks, just enough to keep voltage above 12-35v (depending on regulator if you don't want flicker on led), regulator, resistor to limit current to led, led

author: TimFox
author_rank: Super Contributor
timestamp: February 11, 2023, 03:00:44 pm
content: I have used many a neon indicator over the last 50 years.Eventually, electrode material sputtered onto the glass causes them to go dark before they actually fail electrically.Unfortunately, NE51 replaceable bulbs are no longer popular, only NE2-style hardwired bulbs.They can also induce noise onto the AC line, but that may not be important for this application.

author: floobydust
author_rank: Super Contributor
timestamp: February 11, 2023, 09:59:53 pm
content: Quote from: james_s on February 11, 2023, 07:08:25 amThis is what I got from Digi-KeyVCC neon lampswhich I consider hilarious junk - electrodes 1/2 the glass envelope size, bent and trashy finish as if they came from recycled rebar. They are ~$1.50 ea. and obsolete now. Not nixie clock worthy at all. I despise companies ordering cheap chinese product and then marking up the price 20X. Better to hunt down made in USAsurplusneon lamps.The small power strip/rocker switch T-2 neons I see using a 15k resistor with 120VAC, so of course they have a very short life.OP is best using LED's I think if they can be safe (no path to the chassis or people) and the circuit is for long life.

author: Benta
author_rank: Super Contributor
timestamp: February 11, 2023, 11:02:55 pm
content: Yes, well, sad that your purchase strategy failed. That doesn't make neon lamps a bad choice.But I hope you feel better now after venting.

author: floobydust
author_rank: Super Contributor
timestamp: February 11, 2023, 11:44:51 pm
content: Do tell then where to purchase quality neon lamps- which people are advocating for OP, in 2023.They're soldered in place and a hassle to replace. Perhaps they are commonly overdriven but as I have mentioned, they have a short life and are no longer a good design choice.My point is in the race to the bottom for price, outsourcing has left the quality gone. Same for (technical) incandescent lamps. Very few manufacturers of decent components like that are surviving.JKL Componentsor Lumex but theirwebsitehas no data, so they are possibly discontinued. That leaves two manufacturers who get them out of china.LED's are the future.

author: Kjelt
author_rank: Super Contributor
timestamp: February 12, 2023, 12:04:49 am
content: I got neon lamps in some of my 3 phase wall powerboxes to indicate all the phases are active.Sometimes it takes tens of seconds before they give light, so not always the best for a quick power on indicator if tou ask me.

author: james_s
author_rank: Super Contributor
timestamp: February 12, 2023, 02:36:59 am
content: Quote from: floobydust on February 11, 2023, 11:44:51 pmMouser has several different JKL neon indicator lamps in stock. I don't think anyone is arguing that LEDs arent' the future, but when you need a line voltage pilot indicator, neon glow lamps are still a good option.

author: floobydust
author_rank: Super Contributor
timestamp: February 12, 2023, 09:40:49 pm
content: I presently have 4 power bars with flickering neons, 2 are out, and 4 are OK. These are in the illuminated rocker switch.A bushel of small neon lamps (w/resistor) fromAli$0.08 ea. shipping included. A replacement illuminated rocker switch is $1.00Once I swapped over to a 3mm LED inside the switch but it was gross because the leads are part of the switch and very cramped in there, but it works fine.I have NOS quality green neon panel indicators, UL/CSA approved snap-in IDI going for $13.50 at DK/Mouser now. Unbelievable.Next I got some LED mains indicator lamps on AliDIANQI Store$0.80 but these have no safety approvals for the insulation to the metal housing. I took one apart and there is a reasonable sleeve design but susceptible to sloppy manufacturing at the soldered wires. Inside it's a double-die 3mm red LED plus 68k 1/8W resistor (110VAC version).LED power indicators I did 17 years ago, today are fine at 150,000hrs (1/2 wave so 75,000hrs) InGaN, 25VAC, 1N4007, 4.7kΩ, 3.5mA RMS, 7mApk.