Critique my capacitive dropper design - Page 1
Forum section: electronics / projects

author: arcfour
author_rank: Newbie
timestamp: January 07, 2021, 04:19:55 am
content: Hello everyone,I'm making a capacitive dropper to power an LED COB at ~10mA. I've built these supplies before (and am aware of the risks posed by the lack of galvanic isolation), but I decided I wanted to try and make a very robust and safe design this time, so I'd like to solicit feedback on the design and component selection. This isn't a commercial design or anything, I am just a hobbyist. Here's the schematic:And here's the BOM:ComponentDescriptionModel/DatasheetF1250V 200mA fast-blowBel Fuse MRF 200C1305V .33uF X2 capacitorTDK B32932A3334K000R1500V 68R 1W WirewoundTE EP1WS68RJR2, R3250V 470k 1/4WYageo MFR-25FTE52-470KDB1800V 2A rectifierVishay 2W08GD239V 600W TVSLittelfuse TP6KE39CAC263V 47uF ElectrolyticWurth 860020773013COB34.2V 80mA LED moduleLuminus CXM-3-30-90-36-AC40-F5-3C1 is specifically rated for series mains usage (also, I'm in the US with 120V). DB1 has a surge current rating of a whopping 60A. D2 is well within its limits under a crude IEC 61000-4-5 1kV surge test, shunting much of the pulse away from everything else (C1 sees about 850V of the spike). It also should prevent C2 from turning into confetti, should the COB go open-circuit. The COB itself is incredibly bright at just 10mA with a prototype of this circuit, but is rated for 80mA, so it should stay nice and cool - even during transients and fault conditions I don't think it ever sees more than 20mA. I know the TVS standoff voltage is pretty close to the circuit voltage, but I'm not too concerned if it's acting like a crude zener and wasting <1mA of current, I'm really just concerned with it properly blowing the line fuse in case of a serious fault.So, what do you guys think? Any room for improvement? I'm open to suggestions but from my simulations I'm confident this thing should be safe no matter what - I made sure there were no SPOFs and everything had a comfortable cushion to keep it within its design limitations. Heck, this thing seems to have better protection than any of the commercial designs I've seen...guess that's the benefit of not having to design it for mass market!Thanks in advance.Edit - changed the DB1 to the 800V version, not that it likely matters. I have both on my desk and mixed them up. Oops.

author: ledtester
author_rank: Super Contributor
timestamp: January 07, 2021, 07:19:38 am
content: Not criticizing, but I'm curious about your reasons for using a TVS over a Zener. Clearly zeners are cheaper which is one reason you see them in commercial products and in your case cost is not a concern.I found this little blurb about using a TVS for regulation:http://www.icchipword.com/article/What-is-A-TVS-Diode-Difference-between-Zener-Diode-and-TVS-Diode.htmlQuote

author: arcfour
author_rank: Newbie
timestamp: January 07, 2021, 07:27:30 am
content: I was having trouble finding a zener that I was sure could handle the peak pulse power of a fault/transient. Then I realized I don't really care about voltage regulation since it's a constant-current source driving the LEDs, and that the primary function of the zener was to act as a protective device instead - thus, TVS, which are much easier to source with the appropriate ratings.Edit: additionally, under normal conditions, there isn't really much extra current for the TVS to sink anyway (<1mA, if that), so I'm not too fussed.

author: jonpaul
author_rank: Super Contributor
timestamp: January 07, 2021, 11:01:45 am
content: R1 68 Ohm: Must be flameproofR2, R3 : use 1/2 W carbon compD2 Not neededC2 use only if LED must have pure DC, normally LED is fine for illumination with unfiltered DC.Jon

author: mikerj
author_rank: Super Contributor
timestamp: January 07, 2021, 11:36:20 am
content: Quote from: jonpaul on January 07, 2021, 11:01:45 amWhat do you think will happen if the COB goes open circuit?Quote from: jonpaul on January 07, 2021, 11:01:45 amOmit C2 if you enjoy flicker, otherwise keep it.

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2021, 01:37:35 pm
content: Quote from: mikerj on January 07, 2021, 11:36:20 amIn addition to smoothing out the flicker, it also limits the current surge through the LED. Even with R1, the peak current through the LED will be over 2A, if the power were to be applied, during the peak mains voltage, which could destroy it. C2 will bypass the LED, when the power is first applied, so no current will flow through the LED, until it charges up.

author: tunk
author_rank: Super Contributor
timestamp: January 07, 2021, 02:47:47 pm
content: I think usually there's a resistor in series with the LEDs and one in parallel with C2, e.g.:https://www.techliminals.com/electronics/5-5w-led-light-bulb-schematic.php

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2021, 03:02:11 pm
content: Quote from: tunk on January 07, 2021, 02:47:47 pmThe additional series resistor, reduces flicker slightly, at the expense of slightly higher losses. The parallel resistor ensures the LED doesn't glow dimly, due to the tiny current flowing through the stray capacitance in the wiring and should also help the capacitor to protect the LED from current surges, if the LED is turned on and off fairly quickly.

author: arcfour
author_rank: Newbie
timestamp: January 08, 2021, 11:25:11 pm
content: Great feedback guys, thanks!I've replaced R1 with a 120 Ohm 1W flameproof resistor. I found that it helped reduce anything nasty even better than the 68 ohm one.It's correct that C2 reduces current through the COB in case of a voltage spike or during inrush. However I had to add D2 as C2 + the COB alone was not enough current to blow the fuse should C1 fail short, but R1 would get *very* hot (as would the COB). It and C2 also take the brunt of anything nasty from the power lines that gets past C1 and R1, keeping the COB safe. Like I said, not strictly necessary but I wanted to make a safe and robust design that would last a long time, even if I could just throw something together from what I have on hand that would probably *work*.I've seen those parallel resistors with C2 before on bigclive teardowns to prevent stray capacitance from keeping the COB on dimly. I'll play around with that and see if I like it. A series resistor is not required since the design is essentially a constant-current source providing 10mA.