Help identifying component across 5v supply on micro board - Page 1
Forum section: electronics / repair

author: shop now
author_rank: Newbie
timestamp: September 16, 2019, 01:53:27 pm
content: Hi, just looking for advice on identifying a failed component please.Am working on a large roadside led sign which has failed.Main 7.5 volt supply is ok … which feeds all the 16x16 led displays.The micro and memory pcb’s run off 5 volts via a TO3 LM78H05 regulator which has failed.I hooked up a bench supply and the sign initialises … but then starts to draw a lot of current and goes into overload … at 3 amps.I discovered a small diode which was leaky, across the 5 volts … cathode to + and anode to ground.This diode is labelled WD1, while other diodes on the board are labelled D3, D4 etc . It has no markings … and looks similar to a 1N4148 diode or a 500mW Zener.I ran the unit without it and it all runs ok … drawing 330mA in total.Would I be correct in assuming it is simply a small signal diode placed across the 5v supply as a crude reverse supply protection … or has anyone got a suggestion as to what else it could be.Thanks for your time.PS. I cant get over the price of a new TO3 5 volt regulator ... probably a long time since I bought one ... but $70.94 from element14 ... LM323K STEEL/NOPB

author: shakalnokturn
author_rank: Super Contributor
timestamp: September 16, 2019, 06:10:57 pm
content: Although it could be a simple diode, in your situation I'd be fitting a 5.6V zener just in case.You'll get more answers posting a decent photo of the diode.The diode may have saved the micro from a failed 5V regulator, the question I'd be asking is what could have happened on the power input in the first place to kill the regulator and what may be added to prevent it from happening again?As for the TO3, either you have a customer that likes to pay a lot of cash for little, or going by your current figures you could throw in something more TO220 like...

author: gamalot
author_rank: Super Contributor
timestamp: September 16, 2019, 06:26:19 pm
content: Some Chinese electronics engineers use WD as reference prefix for zener diodes.

author: shop now
author_rank: Newbie
timestamp: September 16, 2019, 11:48:27 pm
content: Thanks shakalnokturn, I appreciate the input.Quote from: shakalnokturn on September 16, 2019, 06:10:57 pmYes, that makes much more sense. It did cross my mind while having a coffee after I posted the question ... but in my mind I was questioning the use of such a tiny diode.Would I be better to use a 1 watt version perhaps?Quote from: shakalnokturn on September 16, 2019, 06:10:57 pmAgain, you're probably correct ... I had been thinking more along the lines that the diode failed ... and that overloaded the regulator and killed it.All I can say is I'm really thankful the diode held up and didn't allow excess voltage onto the micro boards as otherwise this may have been a write off.Another reason perhaps to use a 1 watt zener.I actually changed the main power supply about 18 months ago ... when it came in to replace 130 or so leds that had failed. It was already 10 years old ... so thought that was good insurance. Mainly because I was worried about all the electros being pretty aged ... and in a metal box out in the Aust sun for all those years. Should have changed the 5 volt reg as well obviously.Quote from: shakalnokturn on September 16, 2019, 06:10:57 pmI had considered a TO220 version already ... but when I started to think whether I needed to add a heatsink ... and what else could go wrong ... I figure I'll just pay the money. They had to hire a cherry picker to get the sign down ... and will need to repeat once it's fixed ... so not really a lot extra ... just a shock for me to see the price.Thanks for your input too gamalot ... that adds weight to shakalnokturn's comments.I've been working on my own too long ... need input from outside regularly.Thanks guys.