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ke5frf

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Hello there.

I used to post on this site several years ago by a different user name that I long since forgot, so... nice to visit again and thanks for the resource.

Quick introduction:
I am recently getting "back" into doing electronics projects as a hobby (amateur radio, building robots, tinkering etc), but I've been repairing electronics in industry and the military for a combined 13 years now. I have about 400 pieces of equipment that I service for my company, and generally speaking my repairs are more of a mechanical nature rather than electronic. That's because modern electronics are for the most part very reliable especially in the sophisticated, specialized equipment that I service! Moving parts will always wear over time so I still have job security :)

The consequence of such reliable electronics for a service technician is that troubleshooting opportunities come few and far between. About a month ago I had a 220 uF electrolytic that failed in a switching supply, which I repaired by putting two (on hand )470 uFs in series to get close enough to make it work. Before that it may have been 6 months since I'd warmed up an iron!!!

And just today, a particularly odd failure occured which required some troubleshooting (and begging of the manufacturer for schematics to guide me). Darn these modern manufacturers for being so stingy.

I had an I/O relay board with 15 SSRs in bus configuration (all DC in/out operating at 24 volts. I had never seen relays used in quite this manner. The logic side was a 5 volts bus with RED LEDs in parallel along the bus going to coresponding relay pins, pull up resistors going to another pin (all pulled up in parallel logic lines to the CPU), and the third pin being logic common. If I read it correctly the off logic state turned the LEDs on and the hi state was pulled up to the CPU.

The other side of the relays were 24 volts. But NOT ALL OF THESE WERE "COILs". Some were contact points to drive motors, so that the CPU sent a 5 volt signal to energize the relay. However, some WERE 24 volt "COILs" (I know they are SS relays but for lack of better word) and the switching side (contact points) was the logic/LED side sending feedback to the CPU. (Monitoring function)

The failure was a motor with two limits operated by microswitches. The drive relay would energize and the LED would indicate operation, but the "monitor" relay that indicated the motor had found its limit (and denergized the motor) seemed not to be working.

Mechanical parts are always suspect, so my first guess was a microswitch. So I checked NO and NC continuity on both. It WAS GOOD!! The CAM that operated the switches also seemed in working order, not getting caught or hung up.

Well, I swapped around some SSRs hoping to make the problem show up somewhere else, but no luck. Same failure.

After getting ahold of the schematic for the I/O buses, I wondered what the voltage at the LEDs was, and taking into account a drop across them, I read 4 volts at each LED EXCEPT the relay for MONITORING the suspect motor's position. It never told the CPU where it was.

I concluded it MUST be a bad diode because it was from 5 volt bus to ground bus across all 15 RED LEDs, and the suspect LED was the only one showing an odd voltage (3.1 volts instead of 4)

Knowing that CMOS logic requires a threshold voltage to change states, I figured the CPU wasn't happy with 3.1 volts.

I swapped the LED with one on an unused circuit and VOILA it worked.

...............................

My gripe is that I don't get to do fun troubleshooting like this often enough!!! LOL


Also, can anyone here, if you understand my description, offer me a REASON why a design engineer would put an LED in series with circuit, causing a failed indicator to interupt the circuit like a fuse? I've always tried to put indicator lamps in parallel with loads etc. This seems like a design flaw. Also, it is rare to see LEDS fail. This equipment has been in service for several years, so I doubt a defective LED from the manufacturer. Seems like some kind of catastrophic failure. Maybe Chinese LEDs flooding the market are too cheap?

Thanks for reading my little story and hopefully I'll learn something new.
I'll be starting some new projects at home in the future, perhaps some of you can offer help when I need it.
Thanks!!!
 

ke5frf

New Member
I apologize for the length. I guess it doesn't quite read like a technical manual ;)

Anyway, hello forum. Keep up the good work and go easy on the Indian engineering students.
 

Brevor

Member
why a design engineer would put an LED in series with circuit, causing a failed indicator to interupt the circuit like a fuse?
I have designed several circuits like that, like you said it's rare to see led's fail.
So it's not usually a problem. Doing it that way saves needing an extra resistor
for each led indicator. Sure resistors are cheap but it's available space on the
PC bd. that usually needs to be conserved.
 

ke5frf

New Member
HAHA!!!

I figured as much. Us repair types aren't too thrilled about all the space savings measures either. But I can't complain, at least I wasn't dealing with SM devices!!!

Give me equipment with room for a chevy small block and I'm a happy fellow :)

Yeah, I suppose LEDs are an afterthought when failures are considered into the design. The relays, however, are very service friendly, plug in modules, and generally they are the first to fail.

Thanks for the insight.
 

Berserk87

New Member
funny you bring this up, ive been planning on building some led lightning and the way id be wiring them i would not have to use a buck boost and i would keep my efficiency as high as possible if i wired them all in series, but then if one of them burns out i would have to test every one individually.

so i figured i would make use of a micro controller to tell me which one dies so if it ever happens it'll save me or another person a lot of time and testing in the end.
 
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