Directional LED Lighting

Greg Elmassian

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8 Mar 2014
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So, I don't think you quit follow.

First let's discard the easy stuff.... section 7.4 talks about damaging a LED from having an excessive reverse bias voltage. There are better ways to do this, and as I hinted in post #13... the accepted way to do this is putting the diode in series, and that utilizes the higher reverse voltage threshold of the extra diode, and does not conduct voltage when the input voltage is applied. That presentation on LEDs is not really a good example, and I suspect the author thought he was clever, but it's wasteful and not common practice now.

(also, if this was a case of exceeding the reverse bias voltage of the LED, the LED would ALREADY be destroyed)

The other link reinforces what I just said and also mentions destruction of LED if the PIV (peak inverse voltage) rating is exceeded... again old school thinking, 2013...

In any case, that has nothing to do with the issue the OP posted, the LED flickers in reverse, which implies that there is some pulses of voltage in a polarity opposite the track voltage.

Adding these diodes would have no effect on the LED flickering, because it can ONLY light when forward biased.

When reverse biased, it can do ONLY one of 2 things, either "resist" the reversed voltage, or turn into a dead short, in either case it would NOT light.


I do this stuff for a living, and believe me, I subscribe to protection of LEDs from exceeding PIV, and have had many arguments with people claiming they have never destroyed an LED from exceeding the PIV, but none of my engineers would have a job if they gave me that argument...

So, back the the OP's issue, if he would perform the test with the motor disconnected, we can test Nick's theory of BEMF from the motor providing the flickering, the stated issue in the first post.

(I would also recommend the addition of a diode in series with the LED to protect from PIV breakdown, any common diode will do, the faster 1N4148 is not necessary)

Greg
 

JimmyB

Semi-Retired; more time for trains.
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