Faulty Led repair

Post all your model railway electronic problems, solutions and discoverys here.
James109
Posts: 40
Joined: Fri Feb 15, 2019 7:03 am

Faulty Led repair

Postby James109 » Sun Oct 13, 2019 12:28 pm


abenn
Posts: 237
Joined: Thu Jun 05, 2008 11:03 am

Re: Faulty Led repair

Postby abenn » Mon Oct 14, 2019 8:37 pm

I don't understand: He demonstrated that the faulty LED was getting 6.5v and, I think, mentioned that the voltage-regulator had failed open-circuit, which would have eventually caused the LED to fail. So why didn't he change the voltage regulator as well -- or did I miss it?

User avatar
Ironduke
Posts: 953
Joined: Mon Sep 26, 2005 12:04 am
Location: Ballarat Victoria Australia
Contact:

Re: Faulty Led repair

Postby Ironduke » Tue Oct 15, 2019 12:07 am

abenn wrote:I don't understand: He demonstrated that the faulty LED was getting 6.5v and, I think, mentioned that the voltage-regulator had failed open-circuit, which would have eventually caused the LED to fail. So why didn't he change the voltage regulator as well -- or did I miss it?


What I got was that the LED had failed open circuit. I don't think a voltage regulator was mentioned. It should have measured ~2V but was measuring 6.5V. That is not a cause of the faulty LED but a symptom, it's the LED that makes it 2V.
The LED does shine very brightly when DCC is connected. I wonder if the LED is being driven directly from the DCC input and what the circuit is.
Regards
Rob

User avatar
Ironduke
Posts: 953
Joined: Mon Sep 26, 2005 12:04 am
Location: Ballarat Victoria Australia
Contact:

Re: Faulty Led repair

Postby Ironduke » Tue Oct 15, 2019 12:40 am

Ironduke wrote:I wonder if the LED is being driven directly from the DCC input and what the circuit is.


Hm, yes it is. According to their circuit (see page 4) the indicator LED is reverse parallel across the LED input of an opto-coupler and has in series 2x2K resistors in parallel (so 1K basically). 1K might be a bit low if the DCC p-p voltage is very high. I wonder why they thought they needed 2 resistors in parallel? Lucky the opto-coupler didn't blow really.

So the reason it measures 6.5V is probably that the multimeter is actually measuring an alternating voltage , ~2v when the opto LED is on and -ve full DCC voltage when it's reversed. When both LEDs are working you'd get alternating +/- 2V.

I'd try removing R8 and see if it still works.
Regards
Rob

User avatar
Ironduke
Posts: 953
Joined: Mon Sep 26, 2005 12:04 am
Location: Ballarat Victoria Australia
Contact:

Re: Faulty Led repair

Postby Ironduke » Tue Oct 15, 2019 4:07 am

Ironduke wrote:1K might be a bit low if the DCC p-p voltage is very high.

Actually belay that. Even if DCC peak voltage was 18V and the forward voltage drop on the opto-coupler input was 1.7 (which is the max for a 6N137 apparently) you'd only get 16.3mA coming through the resistors ((18V-1.7)/1000). Maybe the indicator LED was just not a very good one?
Regards
Rob

James109
Posts: 40
Joined: Fri Feb 15, 2019 7:03 am

Re: Faulty Led repair

Postby James109 » Tue Oct 15, 2019 5:55 pm

Been working now for 3 days none stop, led is no where near as bright so thinking just a faulty led, thanks for the comments. Still think for a hobby kit it's amazing.

Jim


Return to “Electronics”

Who is online

Users browsing this forum: No registered users and 1 guest