LED ring light from hell, filter or dump - Page 1
Forum section: electronics / projects

author: Martinn
author_rank: Frequent Contributor
timestamp: December 07, 2023, 05:43:13 pm
content: Hi all,I have a microscope ring light (like this onehttps://www.aliexpress.com/item/1005004073906084.html) which I found is the dominating noise source when I want to do low noise measurements on the bench.Apart from that it works great, a bit on the bulky side but near 40 klx in the center. Great for SMT work.PCB:and noise, scope probe held against light:Electronics looks quite decent. Isolated (would not have expected that, given that the housing is plastic), decent spacing on PCB (even slots), optoisolated feedback. Judging from the 2.4 ohms shunt it is current regulated. Keeps 0.2 A (near 12 V) from 90 to 250 V AC input.Solder quality is OK, I have seen better, but also much worse. Pretty much standard flyback, I'd say.But: With the exception of one choke at the secondary absolutely zero filtering. No X or Y caps, no CM chokes at input or output. At least they have the decency not placing a CE sticker onto it.So, what should I do? The LEDs are OK, 12 V 0.2 A. The driver is also OK, just noisy as hell. Dump it and find a (hopefully better filtered) new one? Choices are pretty thin for the given voltage/current. Dimmable would be nice, but not essential.Or try to add filtering? Maybe start with earth connection, proper input filter with X capacitors, secondary ground connected to earth, CM choke at output? Needs a different housing of course.What do you think?

author: madires
author_rank: Super Contributor
timestamp: December 07, 2023, 05:57:19 pm
content: You could disable/remove the SMPS part and power the light directly from a 12V source.

author: ajb
author_rank: Super Contributor
timestamp: December 07, 2023, 06:18:33 pm
content: You'd want a bit of headroom for regulation, so 12V would be iffy with 4 LEDs in series. There are some small DC-DC LED driver modules that should cover the voltage/current needs here, I think Cincon, Recom, and Meanwell all have options, along with others. Mostly board-mount bricks, but some are available with wire leads, and many have analog and/or digital dimming options (current adjust and/or PWM). May or may not be enough of an improvement in terms of noise, but at least it wouldn't be switching mains right under your nose.Personally I would probably just replace it with one that takes a standard 12V wall-wart.

author: Martinn
author_rank: Frequent Contributor
timestamp: December 07, 2023, 06:29:58 pm
content: Quote from: madires on December 07, 2023, 05:57:19 pmSure, that's what I meant with "find a new one". I'd like to keep it current regulated - many LED strips or ring lights aren't, but given the negative tempco of LEDs constant voltage drive is kind of brutal.So, anyone has an idea where to find a 12 V, 0.2 A CC LED driver? No luck with Recom, Meanwell so far. 12 V and 0.2 A (CC, not CV!) is pretty rare.

author: jfiresto
author_rank: Frequent Contributor
timestamp: December 07, 2023, 06:43:14 pm
content: What about anLDDS-250HW?

author: TimFox
author_rank: Super Contributor
timestamp: December 07, 2023, 06:49:45 pm
content: That series has 150 to 350 mV pk-pk output noise/ripple (typical of switch-mode supplies): is that good enough for the application?

author: Martinn
author_rank: Frequent Contributor
timestamp: December 07, 2023, 06:50:40 pm
content: Quote from: jfiresto on December 07, 2023, 06:43:14 pmDid not look for DC/DC. But with wall wart, why not?Not sure how they mean "PWM frequency" w.r.t. dimming. Do they PWM the output for dimming (probably) or do they accept a PWM signal as dimming input? Probably both. DC current regulated would be nicer of course.

author: Martinn
author_rank: Frequent Contributor
timestamp: December 07, 2023, 06:53:04 pm
content: Quote from: TimFox on December 07, 2023, 06:49:45 pmI have no idea. Being an offline switcher, reducing the noise from 400 V to 400 mV probably is an improvement already.

author: madires
author_rank: Super Contributor
timestamp: December 07, 2023, 06:53:41 pm
content: U3 could be a TL431, i.e. the SMPS would be voltage regulated with an overcurrent protection (Q1).

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2023, 06:59:27 pm
content: You could of course return it.Failing that, adding a filter is possible. A common mode choke on the mains input, with a couple of X capacitors and a Y1 capacitor between the primary and secondary. If the mains plug is polarised, connect the Y1 capacitor to neutral, otherwise, the negative side of the bridge rectifier. It may also be necessary to add filtering to the secondary side.

author: Martinn
author_rank: Frequent Contributor
timestamp: December 07, 2023, 07:09:37 pm
content: Quote from: madires on December 07, 2023, 06:53:41 pmYou mean they added the expensive 2R4 R7 shunt just for OC protection?! Hard to believe.Still, I tested the supply with LED ring connected and it kept the current constant from 90 to 250 V input. But maybe it just regulated the voltage and not the current, I would have to vary the load to verify the CC claim.0.2 A over the 2.4 Ohms shunt makes 0.48 V only - probably not enough to bring a transistor into regulation.Maybe you are right and it is indeed CV with OCP. That would be amazingly overengineered, hard to believe they cared that much.

author: ajb
author_rank: Super Contributor
timestamp: December 07, 2023, 07:13:48 pm
content: Quote from: Martinn on December 07, 2023, 06:50:40 pmThat's the general style I was thinking of, and running it from a wallwart. Many, including that one, have an optional multifunction control input that will accept a resistance, analog voltage, and/or digital PWM signal for dimming control. This is shown on page 3 of the datasheet, along with the variant that only has an on/off input.Some drivers may have a PWM input that directly PWMs the output, but in some cases the PWM input may be used to adjust the current, and that isn't always clear from the datasheets unfortunately. In some cases you can do analog and PWM dimming simultaneously, by adjusting the 'on' voltage as well as the duty cycle.Digikey has many options inon-boardandoff-boardformats.

author: Martinn
author_rank: Frequent Contributor
timestamp: December 07, 2023, 07:15:44 pm
content: Quote from: Zero999 on December 07, 2023, 06:59:27 pmIn Switzerland we have indeed polarized mains connectors (not sure how reliably the polarity is correct). But only if you use three pin mains plugs - and then, it's probably more effective to use the PE connection to get rid of the noise. Not sure about the optimal filtering setup with PE present. Connect secondary GND to PE? It's fully isolated after all.Maybe one should take this as a filter-Fu workout.

author: Martinn
author_rank: Frequent Contributor
timestamp: December 07, 2023, 07:19:22 pm
content: Quote from: ajb on December 07, 2023, 07:13:48 pmYes, analog dimming would be ideal. The linked datasheet does not state this (in an obvious way).Thanks for the board mount link. I already went through Digikey filtering for off-board ones.

author: jfiresto
author_rank: Frequent Contributor
timestamp: December 07, 2023, 07:25:59 pm
content: Quote from: Martinn on December 07, 2023, 06:50:40 pmI only quickly scanned the datasheet, but I believe it mentioned it supports three way dimming. At Mean Well that means you can vary that output from minimum to 100% by applying: a 0-10V DC signal; a typically 300~3000hz, PWM signal time averaging to 0-10VDC; or a varying resistance (0~100K? x 100µA?) = 0-10V across the resistor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2023, 07:41:12 pm
content: Yeeeeeah... that piece of shit is lacking... everything. I see no reason to mince words, it's... literally the bare minimum application circuit.They had the politeness to put in the fusible resistor? I guess? Or it's an actual fuse, dunno, not familiar with the glass-body part. Hopefully it's rated for braking fault amps...The trash on the turn-off peak (leakage inductance into RCD clamp snubber) is peculiar. I wonder if a ferrite bead in series with snubber diode would help? Maybe it's secondary side as well, could be any of the diodes. That's whole volts, at frequencies that "don't like to stay in wires" (10s MHz+), and right into the isolation barrier that has no Y-cap across it, so your LEDs are just absolutelysingingthere.No CMC either, so anything unbalanced at the top end is emissions up the cable as well, and plenty of differential noise.Doesn't look like there's enough room in there to add chokes, unless maybe you can find small enough ones, but will they still have enough value to be effective? Y caps easy enough. Can even put a choke on the LED cable and a small Y-cap from mains to LEDs, to get more attenuation in there.Tim

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 07, 2023, 09:03:21 pm
content: Quote from: Martinn on December 07, 2023, 07:09:37 pmThe OCP is just a transistor Vbe across shunt resistor. That's 2 cent approach, but I agree it's probably overengineered by their standard.I mean, they don't even bother at least putting some ballast resistors for each LED strings.

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2023, 09:22:32 pm
content: Quote from: T3sl4co1l on December 07, 2023, 07:41:12 pmI've seen worse. The PCB has sufficient creepage and clearances for mains. It's difficult to tell whether the transformer is built to an adequate standard, just from a photograph, but it's probably all right. The LEDs all appear to be enclosed, so I wouldn't worry about it safety wise, even if it's not quite up to scratch.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2023, 09:31:45 pm
content: Quote from: Zero999 on December 07, 2023, 09:22:32 pmYeah, isolation -- at least by looks, a dismantling of the transformer would really be needed to know for sure -- is one of the things it seems is done well. It kinda doesn't matter, oddly enough -- assuming it is enclosed properly (so, I didn't mention it).The pot probably doesn't have much isolation (500VAC is typical) so that would be one good reason for SELV on the secondary side.Tim

author: Martinn
author_rank: Frequent Contributor
timestamp: December 07, 2023, 09:42:23 pm
content: Quote from: ArdWar on December 07, 2023, 09:03:21 pmI find this a strange combination. Partly reduced to near death (filtering wise), on the other hand OC limiting, even isolation slots on the PCB (and isolation at all in the first place) and input fusible resistor.Yes, the pot is the weak spot in isolation. They put a plastic knob on it, adding a few mm of additional isolation (again overengineered?).What I find strangest of all is the output choke between C6 and C7. Why bother? This must be expensive - why here? It's an additional LC filter at the output.All said I think the primary cause of noise is simply capacitive coupling from the switch node. It's 400 V swing after all. So probably the circuit would be fine if I'd put the driver some distance away, possibly in an shielded (PE connected) enclosure. Still one would probably add a handful of CM chokes and X caps.