Directional LED Lighting

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electricity_bill

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Hello
Been playing with an old double ended OO scale diesel loco and fitting it with directional LED lighting. They work well and shine brightly but the problem is that when running forward, the back, light flickers and visa versa. They are 3V 10mA LEDs fed from the 12V track supply via the loco with a 1K resistor to reduce the volts. Any ideas how to stop it?

Bill
 
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Finding the cause and sorting it is obviously the best way. However fitting a 1N4148 or similar diode in reverse parallel with each of the offending LEDs may provide a bodge fix. I suggest trying on just one for starters.
 
GAP said:
Because when the loco is running forward the back light is bypassed ie shorted out by the parallel diode and vice versa?
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Oh, I was thinking that the back light ought to be on (red) - I wasn't thinking about it being off :rolleyes:
 
Rhinochugger said:
Oh, I was thinking that the back light ought to be on (red) - I wasn't thinking about it being off :rolleyes:
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I wasn't even thinking about colour just explaining the theory behind the suggestion, which is really electronics 101 diode orientation/biasing.
 
GAP said:
I wasn't even thinking about colour just explaining the theory behind the suggestion, which is really electronics 101 diode orientation/biasing.
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He he, and then the Japs do it in n gauge - quite phenominal :clap: :clap:
 
Gap, I get your mild insult in "electronics 101".

So, why does the additional diode stop the LED from lighting when reverse biased?, when the LED is ALREADY a diode?

All that diode does is draw an additional 10 ma when going in the opposite direction.

Please explain why you need that diode, I get the fact that reversed polarity will allow current through that diode, but WHY do you want to do that? How does this affect the LED?

Greg

p.s. there is a reason to use a diode in SERIES with an LED, but this is not what you stated.
 
I see this effect from time to time in some of my N gauge locos, but it's rare. I wonder if the slight flickering of the rear lights could be back-EMF from the motor, if you're using a pulsed controller or the track/pickups are dirty and there are brief moments of loss of power to the motor.
 
electricity_bill said:
They are 3V 10mA LEDs fed from the 12V track supply via the loco with a 1K resistor to reduce the volts.

Bill
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Actually the resistor is there to limit the current through the LED not to reduce the voltage.
 
ntpntpntp said:
I see this effect from time to time in some of my N gauge locos, but it's rare. I wonder if the slight flickering of the rear lights could be back-EMF from the motor, if you're using a pulsed controller or the track/pickups are dirty and there are brief moments of loss of power to the motor.
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See my post #6, i.e. remove the motor to make sure of this.... very possible I agree;

If this is the case, hard to rectify in this case since his LEDs are powered from the track, and so (apparently) is the motor.
 
His proposal is a diode in parallel with the LED, with reverse polarity... all the current still goes through the 1k resistor, with is 10 ma with 12v on the rails. Reah my post #13 I refer to what it does.

I am anxious to get my "electronics 101" certificate, so waiting on the answer from GAP ;)
 
Greg Elmassian said:
Gap, I get your mild insult in "electronics 101".

So, why does the additional diode stop the LED from lighting when reverse biased?, when the LED is ALREADY a diode?

All that diode does is draw an additional 10 ma when going in the opposite direction.

Please explain why you need that diode, I get the fact that reversed polarity will allow current through that diode, but WHY do you want to do that? How does this affect the LED?

Greg

p.s. there is a reason to use a diode in SERIES with an LED, but this is not what you stated.
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No insult intended sorry if you were.

"So, why does the additional diode stop the LED from lighting when reverse biased?, when the LED is ALREADY a diode?"
Typically, the forward voltage of an LED is between 1.8 and 3.3 volts while a 1N4148 typically has a forward voltage between 0.4 and 0.8volts.
While conducting it will maintain a voltage of around 0.7V across itself and therefore the LED will never receive its forward voltage and will not conduct.

Section 7.4 may help it is about protecting LEDs

And post #9 [SOLVED] - diode connected in parallel with an LED has a good explanation as well.

Sorry but I cannot award certificates I'm not a registered training organisation but I can sign your work experience journal if you like, using recognition of prior learning from your engineering degrees.;);)
That is the final word from me on this subject no further correspondence will be entered into.
 
GAP said:
No insult intended sorry if you were.

"So, why does the additional diode stop the LED from lighting when reverse biased?, when the LED is ALREADY a diode?"
Typically, the forward voltage of an LED is between 1.8 and 3.3 volts while a 1N4148 typically has a forward voltage between 0.4 and 0.8volts.
While conducting it will maintain a voltage of around 0.7V across itself and therefore the LED will never receive its forward voltage and will not conduct.

Section 7.4 may help it is about protecting LEDs

And post #9 [SOLVED] - diode connected in parallel with an LED has a good explanation as well.

Sorry but I cannot award certificates I'm not a registered training organisation but I can sign your work experience journal if you like, using recognition of prior learning from your engineering degrees.;);)
That is the final word from me on this subject no further correspondence will be entered into.
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View attachment 279806
 
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