LED array "backfeeding" with half-wave rectified inputs - Page 1
Forum section: electronics / beginners

author: iroc86
author_rank: Regular Contributor
timestamp: August 24, 2021, 02:11:29 pm
content: I built an LED indicator panel and I'm experiencing some unusual behavior when the inputs are switched. I was hoping that someone might explain what's going on in my circuit.Each LED is connected to a 24 VAC input through a half-wave rectifier. When an input turns on, the related LED lights up. Very simple. The issue is this: when a voltage is applied to one of the five channels, theotherLEDs blink for a maybe 100 ms. If the same input is pulsed repeatedly, the other LEDs do not blink--it only happens when a different channel is activated.I figured that this behavior was related to the filter capacitors discharging through the circuit. I removed the caps and the issue went away. The capacitors are not really necessary, since the LEDs behave just fine with a 60 Hz half-wave-rectified input. I can omit the caps, but I'm curious why this is happening and how it could be prevented.I tried modeling the circuit in LTspice and didn't notice anything strange. In the waveforms below, LED1 is turned on at 1 second, and then LED2 is turned on at 2 seconds. When LED2 is activated, I "should" see a quick pulse across LED3, LED4, and LED5 according to real-world observations. The closest connection is a few pulses into the corresponding filter caps, but at only a few pA, it wouldn't be enough to flash the LED. I am thinking there are parasitic losses that I'm not accounting for in the simulation. (Note that I have modeled this with a 24 VDC input for simplicity. I've observed the same "backfeeding" behavior on the bench with a DC lab supply instead of the 24 VAC inputs, so it's not only occurring with an AC input.)Any insight would be appreciated--1) why are the LEDs backfeeding with the caps, and 2) how could this be modeled more accurately in LTspice?

author: Benta
author_rank: Super Contributor
timestamp: August 24, 2021, 03:57:17 pm
content: The reverse breakdown voltage of LEDs is low (<10 V). Either remove the caps or mount additional diodes for the LEDs.

author: Ian.M
author_rank: Super Contributor
timestamp: August 24, 2021, 04:12:11 pm
content: @Benta,Please explain. Surely, in the O.P's schematic above, the 1N4148 diodes prevent reverse current through the LEDs (neglecting any reverse leakage current through the 1N4148 diodes)?@iroc86,Is there any possibility there is excessive resistance or inductance in your AC 'hot' feed to the switches or ground return from the capacitors and LEDs?

author: Benta
author_rank: Super Contributor
timestamp: August 24, 2021, 04:58:46 pm
content: Quote from: Ian.M on August 24, 2021, 04:12:11 pmYeah, but the caps discharge during the cycle and the low reverse breakdown enable backfeeding. The 1N4148s are OK, the caps are the problem.

author: mariush
author_rank: Super Contributor
timestamp: August 24, 2021, 05:07:30 pm
content: Wouldn't it make more sense to change your circuit so that you do full wave rectification to reduce flicker and use a single bigger capacitor if needed? Why so many extra components?

author: Ian.M
author_rank: Super Contributor
timestamp: August 24, 2021, 05:17:49 pm
content: Quote from: Benta on August 24, 2021, 04:58:46 pmHow does a simple loop of a capacitor, resistor and LED ever reverse bias the LED assuming that at some point in the past the capacitor had enough positive charge to forward bias the LED? (... and don't say 'loop inductance' as the resistor can be assumed to be large enough that its critically or over-damped.)

author: Benta
author_rank: Super Contributor
timestamp: August 24, 2021, 06:15:51 pm
content: Strike my inputs. Something else odd is going on here.

author: iroc86
author_rank: Regular Contributor
timestamp: August 24, 2021, 06:37:59 pm
content: Thanks for the ideas. I can try a lower input voltage to see if there's a breakdown issue. Up to this point, I had only tested at 24 VAC rectified to ~29 V DC-ish and 24 VDC from my bench supply.FWIW, this circuit is on a PCB with soldered joints; it's not a breadboard. The components are SMD, but I don't think that'd matter. Diodes are MELF versions of the 1N4148 and the caps are ceramics. LEDs have a Vf of 1.9 V and Vrev of 5 V[PDF datasheet here].If I take out one of the caps, say on the D1/R1 node, the corresponding LED does not pulse when the other inputs are activated. The backfeeding only affects loops with a capacitor. I haven't hooked up a scope yet to see the response, but I can try that.Quote from: mariush on August 24, 2021, 05:07:30 pmI'd do that if I could, but the LTspice schematic is only an approximation of the actual circuit--these 24 VAC inputs are discrete signals from a piece of controls equipment, so I need to treat them as separate channels. They are likely fed from a single 24 VAC source within the controller, but I don't have access there, so I need to rectify each output individually. I thought about using an optocoupler on each channel and a dedicated DC supply to power the LEDs, but that'd be more complicated than a couple half-wave rectifiers. Didn't expect the backfeeding issue, though!

author: Ian.M
author_rank: Super Contributor
timestamp: August 24, 2021, 06:57:06 pm
content: Why not ditch the caps and wrap a bridge rectifier round each LED? You can use two small signal dual series diode packages to implement each bridge rectifier so the component count wont increase any and the cost is similar. Keep the resistor outside the bridge rectifier to minimize the reverse voltage rating needed, and to avoid shorting the signal if any of the diodes fail.

author: Manul
author_rank: Super Contributor
timestamp: August 24, 2021, 07:27:10 pm
content: The answer must be: the circuit you've built is not how it is drawn. Either because of miswiring, switches misbehaving or weird parasitic effects. There should not be any backfeeding of any sort in such circuit.My guess: capacitors going to ground are not actually connected to ground, just to each other.

author: iroc86
author_rank: Regular Contributor
timestamp: August 24, 2021, 08:47:28 pm
content: That would be a slick idea, Ian.M. I may have to try that. Any potential issues with losing the filter caps?Here is a copy of the schematic from KiCad. As I mentioned, the LTspice screenshot above is a variation for simulation purposes, but the connections are otherwise identical. Ignore connector P1 and jumper JP1; those are for an external LED board and are not being used in my current test setup. The only components on the PCB are the diodes, resistors, and capacitors. Any chance that solder flux residue is causing this? I haven't cleaned the board yet.Manul, I guess there's technically not a "ground" here since the inputs are AC. The caps are indeed connected to each other through the neutral N wire.EDIT:Nevermind. I may have figured it out. Manul was right, I think. I had neutral tied to JP1 pin 1 and the caps are connected to JP1 pin 2 (no jumper yet). Trying to be clever...Sorry guys! I will check this out later today and see if this is the issue.

author: Benta
author_rank: Super Contributor
timestamp: August 24, 2021, 09:03:40 pm
content: Where the f... did that jumper suddenly come from? Thanks for your imprecise/incomplete question, we have nothing else to do anyway.

author: iroc86
author_rank: Regular Contributor
timestamp: August 24, 2021, 09:06:23 pm
content: Quote from: Benta on August 24, 2021, 09:03:40 pmWell, the design is sound I guess; it's just the idiot (me) who's testing it that's goofed up...

author: Benta
author_rank: Super Contributor
timestamp: August 24, 2021, 10:02:25 pm
content: It's OK, all forgiven