On Regulated PSU - Which Direction Should LED Indicator Face? - Page 1
Forum section: electronics / beginners

author: penmarker
author_rank: Contributor
timestamp: January 05, 2023, 10:29:27 am
content: I'm building a basic regulated PSU and came across two different schematics.The first one has the LED wired the way I thought it should be with the anode pointed towards the ground or negative.The second one has the LED wired with the anode pointing towards the positive supply.Are both correct? Is one of them wrong?

author: 807
author_rank: Frequent Contributor
timestamp: January 05, 2023, 10:51:19 am
content: The first diagram is the correct one.Very sloppy of RS to have an incorrect diagram on their website.Although it makes no difference, the layout actually has the resistor at the ground end.OEP have made the same mistake on the PS4008 & PS4009 modules as well...https://docs.rs-online.com/c007/0900766b80550c73.pdfhttps://docs.rs-online.com/c8bb/0900766b80550c74.pdf

author: penmarker
author_rank: Contributor
timestamp: January 05, 2023, 11:46:49 am
content: Thanks. I had learned about the first diagram, but the ones on RS by OEP was consistently the same and it made me doubt.

author: Gyro
author_rank: Super Contributor
timestamp: January 05, 2023, 12:09:11 pm
content: The only diodes you are likely to come across (of any type) that have the arrow pointing towards the positive (Cathode +ve) are Zeners. That's because they work in reverse breakdown mode.

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2023, 01:31:43 pm
content: Quote from: Gyro on January 05, 2023, 12:09:11 pmAnd fly-back diodes across inductive loads: motors, relay, solenoid, electro-mechanical buzzer etc.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 05, 2023, 02:08:40 pm
content: Quote from: Zero999 on January 05, 2023, 01:31:43 pmDiodes are used in both ways depending the application but I don't think a LED ever used in reverse bias

author: Gyro
author_rank: Super Contributor
timestamp: January 05, 2023, 02:13:52 pm
content: Quote from: Zero999 on January 05, 2023, 01:31:43 pmDoh! That's the problem with blanket statements.

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2023, 04:14:37 pm
content: Quote from: BeBuLamar on January 05, 2023, 02:08:40 pmAn LED can be used as a photodiode in reverse bias mode. It will only respond to wavelengths shorter than the light it emits. I've done it before with a blue LED.

author: BillyO
author_rank: Super Contributor
timestamp: January 05, 2023, 06:25:36 pm
content: I've designed battery chargers with LEDs the "wrong" way on the output to give a "battery connected wrong" indicator. There are all sorts of ways diodes and LEDs can be utilized, but I think we can all agree that the RS schematic is in error.

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2023, 10:42:03 pm
content: Quote from: BillyO on January 05, 2023, 06:25:36 pmThat works, but beware, newer InGaN used to make pure green, blue, violet, near UV and white, as well as some other colours with a phosphor, isn't as tolerant to reverse voltage, as old GaAsP and GaP, used to make red and orange. I haven't done any tests, but have found older red LEDs can tolerate reverse breakdown, whilst the newer ones are ruined. Data sheets typically specify maximum reverse voltages of 5V, but higher voltages can normally be tolerated.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: January 05, 2023, 10:58:10 pm
content: Quote from: Gyro on January 05, 2023, 12:09:11 pmA cheap way of reverse voltage protection is to put a diode "backward" across VCC and GND. The idea is to blow a fuse in the upstream power supply and protect the downstream device.

author: BillyO
author_rank: Super Contributor
timestamp: January 05, 2023, 11:39:24 pm
content: Quote from: Zero999 on January 05, 2023, 10:42:03 pmYup. With newer LEDs I'd use a small diode like 1N60 in series too.

author: penmarker
author_rank: Contributor
timestamp: January 06, 2023, 01:36:24 am
content: Hey this turned out to be a bigger discussion than I had initially thought, thanks a lot everyone. I love the wrong way battery indicator the most.

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2023, 09:34:35 am
content: Quote from: penmarker on January 06, 2023, 01:36:24 amThere are even LEDs, with two different colours chips wired back-to-back in the same package, so it will light up a different colour if the polarity is reversed.http://www.kosmodrom.com.ua/pdf/OSRPM25A32A.pdf