Resistor in parallel with LED? why? - Page 1
Forum section: electronics / projects

author: metertech58761
author_rank: Frequent Contributor
timestamp: December 21, 2022, 07:45:16 am
content: So I recently got my hands on an I/O card for the sake of reverse engineering its schematic, and I cannot understand why part of the circuit is arranged as it is.Each LED has a 330-ohm resistor in parallel with it, as well as a diode / resistor pullup. Is it just to protect against accidental overvoltage situations?

author: wasedadoc
author_rank: Super Contributor
timestamp: December 21, 2022, 08:51:24 am
content: Are you sure you have drawn that correctly? I cannot see how any of the LEDs can light up.

author: ppTRN
author_rank: Regular Contributor
timestamp: December 21, 2022, 08:27:44 pm
content: I guess the weirdest thing is that all the LED and their series resistor are all shorted, than of course since all 1N4148s have their cathode pulled up to 5V, they basically behave as an open circuit, unless OUT_A# rises above 5V. But than what? I think that the schematic might be wrong.

author: metertech58761
author_rank: Frequent Contributor
timestamp: December 22, 2022, 02:03:30 am
content: Turns out I goofed on the schematic after all. The right-hand pull-up should be 12V, not 5V.

author: Ian.M
author_rank: Super Contributor
timestamp: December 22, 2022, 02:35:05 am
content: Its still crazy wrong. In your schematic, there is no way that there can be any forward current through the LED for any Out_A voltage between 0V AND 12V. LTspice sim attached.I/O card from what?Post the model and/or part number of the I/O card, tell us what its used for, and if you want suggestions on what the actual schematic could be, post well lit, sharp, high-rez photos of both sides of the card, taken looking straight down on it, with the camera lens directly above the center of the board to minimise distortion. Do not use on-camera flash!

author: fourfathom
author_rank: Super Contributor
timestamp: December 22, 2022, 02:39:28 am
content: You still have (probably schematic) errors. The LED D5 and series resistor R1 are shorted by that net between D6 anode and Out_A1, as are seven other connections. And it looks like there may be other problems.

author: metertech58761
author_rank: Frequent Contributor
timestamp: December 22, 2022, 04:03:03 am
content: The rest of the schematic makes sense -I'm every bit as confused as you are by this section, but I'll go back and check the traces for the LED banks one more time.Those 330 ohm resistors are DEFINITELY across the LEDs as they divert just enough current to prevent them lighting when doing a diode check with a multimeter.This was part of an acid processor system used by chip foundries back in the '80s. P/N is 290017-400. Pics from the auction attached, I'll see about getting a close-up of the LED banks.I used to service the electronics from that system and when I stumbled across a few cards from that system online recently, I decided to pick this one up for old time's sake.

author: timenutgoblin
author_rank: Regular Contributor
timestamp: December 22, 2022, 05:05:57 am
content: Quote from: metertech58761 on December 22, 2022, 04:03:03 amIronically, that is probably the reason that the resistors are connected in parallel with the LEDs to prevent them from turning on when leakage current is flowing in the circuit.When the LEDs turn on, there would need to be sufficient current for VF. Also, VRwould need to be equal to VFto ensure the LED turns on.VR= VFwhen the LED in turned on.VR<< VFwhen leakage current flows.

author: WattsThat
author_rank: Frequent Contributor
timestamp: December 22, 2022, 05:22:39 am
content: A hand taped layout with no bodge wires, well done. Given the pads appear the same on both sides, it was probably a red/blue tape-up on one sheet. I always found the colored mylar tape harder to curve than the black crepe.Ah, the memories.

author: metertech58761
author_rank: Frequent Contributor
timestamp: December 22, 2022, 06:30:37 am
content: I looked again and found that I had the diodes / LEDs backwards and updated my schematic.The part about leakage current makes sense, as there was probably a fair bit of electrical noise in the actual installation - between the unfiltered bulk 12V supply, yards of ribbon cable throughout the system, multiple control points on some of the outputs, and pneumatic solenoids throughout.

author: Ian.M
author_rank: Super Contributor
timestamp: December 22, 2022, 06:39:15 am
content: You left out the information in your original query that the diodes are actually TND903 eight diode array, and its being driven from a ULN2803, which has open collector outputs.It makes more sense with the LED polarity the other way round as then they could turn on when the ULN2803 outputs are low. There's still something screwy with the diode arrays, as reversed they would only provide pullup current below about 4.3V on the Out_A lines, but the resistors associated with the LEDs pull them up to 12V, so their only effect is to increase the current the ULN2803 outputs sink when low. One possibility is there is a logic level signal going elsewhere tapped off between the diode anodes and the pullups to +5V.

author: metertech58761
author_rank: Frequent Contributor
timestamp: December 22, 2022, 04:09:36 pm
content: While the TND903 was the preferred option, I do remember handling boards that were assembled with 8x 1N4148 when the diode packs were temporarily unavailable.As there was no symbol for the diode pack in KiCAD, I went with individual diodes in the schematic as the two are apparently functionally equivalent for that application.