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StuartN

Where is resistor?

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Which version is best please?

One only uses 1 resistor but may not be as safe.

 

img002.jpg

Edited by StuartN
double image

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second is better, especially if LEDs are different colours as you will be able to change the values to get the same brightness

  • Agree 3

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I'm guessing this a point direction indicator wired to the track?

So it will only work if power is supplied to the track, and in one direction only (ie loco forward not reverse) ?

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9 minutes ago, stewartingram said:

I'm guessing this a point direction indicator wired to the track?

So it will only work if power is supplied to the track, and in one direction only (ie loco forward not reverse) ?

I'm guessing it's for a DCC set-up so there will always be power to the track - half the time it will be the correct polarity to light the LED.

 

Edited by sharris

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The top diagram with only the single resistor will blow the LEDs as you have full track voltage across them with no resistor

 

Andi

Edited by Dagworth

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Thanks for your replies.

You guessed right - it is for a DCC and I learnt something new. I'll use the bottom diagram

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9 minutes ago, Dagworth said:

The top diagram with only the single resistor will blow the LEDs as you have full track voltage across them with no resistor

 

Andi

It's not obvious from the original sketches, but the LEDs should be wired with both anodes or both cathodes pointing towards the frog.

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I understand both anodes pointing the same way, so does it matter if the resistor is wired to the anode or cathode?

Also which is the anode? I remember it being the shorter length.

Thanks again it is very useful and I apologise for being so ignorant.

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You're also reverse-biasing the LEDs while the DCC current is flowing the wrong way to illuminate it.  This could damage it.  You need a normal diode in parallel with each LED, but in the opposite polarity.  

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1 hour ago, Edwin_m said:

You're also reverse-biasing the LEDs while the DCC current is flowing the wrong way to illuminate it.  This could damage it.  You need a normal diode in parallel with each LED, but in the opposite polarity.  

 

True in theory.  In practise one can usually "get away" with the reverse bias current. 

A way to fix the reverse bias voltage is to use two identical LEDs on each side of the indicator, wires anode-to-cathode, with a common resistor.  Thus one LED protects the other from the reverse bias over-voltage in each phase of the DCC signal. 

 

- Nigel

 

  • Agree 1

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One resistor case: what if the connection to frog is interrupted (wire loose, or point motor's internal switch develops a fault or the switch is just too slow): then the LEDs will be in series across the supply with no resistors!

 

I too would put two wire bi-colour LEDs (those, that have two leads and which will have red or green LED inside the LED case on. depending on polarity, and yellow if fed with AC, like DCC), and two resistors into the schematic.

 

It appears that many consider reverse voltages in excess of LED manufacturers datasheet's advice safe enough, as LED will operate like a Zener, and -- provided the reverse current is low enough when the reverse break down voltage is passed there is no eminent damage.

 

If all LED manufacturers warn aginst reverse voltages, I belive one should not encourage or advice dismissing the warnings given by manufacturers, especially when the advice is given to a person apparently not yet quite familiar with the LEDs. I feel one should suggest to follow rules, and only cut corners when, due to experience, one is confident that one is knowing what one is doing, and one can estimate the risks.

 

A document evaluating the suitability of common automobile usage of LEDs in aircrafts is rather interesting (the PDF will automatically load) from: https://docs.broadcom.com/docs/5980-1504E

 

After reading this I went and crossed over my suggestion of using slow 1N4001 diode as forward protecting diode at at another thread here due to slow speed.

 

Just recalling: I have made a DIY DCC decoder (based on Probst decoder  https://taprk.org/eng/project/dcc/diary.html ) once that had the rectifier built from 1N400X type diodes that I happened to have laying around the desk. The diodes got quite warm with no load, as they were so slow to switch off at polarity change, and the bridge rectifier was shorting the track power for a short moment at each DCC polarity change. I should have learned from my mistakes...

 

pekka

Edited by PSi
Typos again

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Quote

One resistor case: what if the connection to frog is interrupted (wire loose, or point motor's internal switch develops a fault or the switch is just too slow): then the LEDs will be in series across the supply with no resistors!

As mentioned already up thread, in the one resistoor case the two LEDs are in series directly across the DCC supply with or without the resistor connected.

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