Wiring LEDs for DCC....?

Zerogee

Zerogee

Clencher's Bogleman
Country flag
This may seem a bit of a simple question, and I hope it has a simple answer!
When a model is fitted with LED lamps (standard LEDs with resistors soldered to them, not the tiny surface-mount things), and you want to install a DCC decoder, do you simply wire up each LED/resistor unit to the relevant lighting output of the decoder, just as you would an ordinary bulb? Obviously unlike a bulb they are polarity-sensitive, so I'll need to take account of whether the function output or the return is positive or negative (I'll be wiring the LEDs all through the four-pin micro-CT plug that handles the lighting on a Massoth LS decoder), but is there anything else I need to watch for?
The current (pun intended) resistors are for analogue operation - can I leave them as they are (and adjust the decoder lighting output voltage CV if necessary) or will I need to change the resistor value at all for DCC?

Thanks for any advice,
Jon.
 
You shouldn't need to change the resistor but you should definitely reduce the voltage to 5v or thereabouts, you don't want full track voltage applied. The manual for Massoth M L XL XXL starts off with the warning "The function outputs are set per default to full track voltage! Make sure the CVs of the function outputs are set to the appropriate value before hooking up any lights or other accessories."
 
Some decoders present a regulated 5v common, so you only need to drop a couple of volts instead of full track voltage.

That will greatly reduce the size of the resistor, usually you can use 1/4 watt.

But if you already have current limiting resistors that work from full track voltage, then just hook up to the decoder, no changes necessary, unless your DCC voltage is way higher than your DC analog voltage (should be 24v max in either case).

The only reason to change to 5v is to reduce the size of the resistor, I don't see the need in this case. I use QSI decoders and I have the option of the common being track voltage or 5v. If adding resistors for the first time, I go 5v.

Greg
 
Thanks for the replies so far.... when tested on analogue (LGB 1-amp 22volt power pack), the LEDs come on dimly at first and don't reach full brightness until the knob is at full throttle - so the resistors are obviously chosen with full DC voltage in mind. I don't necessarily want them on full blast (this is an old-style railcar and will look better with "period" lighting, I think) so I may set the decoder to turn the light outputs down just a little....

Jon.
 
I was wondering...... leds are polarity sensitive (as you stated in first post Jon.
Do all decoders put out DC voltages at their function terminals, or is it AC (whether 5v or full track voltage?). I have always presumed that it could be AC.
To this end I .......
In all my coach lighting and some loco installs I always put a rectifier at the function output (single or strip). I know that this drops the voltage a tad but I like the belt and braces system..... especially in case I make a mistake in complex loco install ...... at least I know that the led(s) are protected either way.
It also makes it easier when doing a multi-led install no need to worry about polarity when soldering up.
 
I had wondered that, Mike, but I think the function outputs are DC - they are usually marked as +24v and 0v.
No harm in being certain by putting in a rectifier as you do, though.

Does reversed polarity destroy an LED then, or does it simply not conduct as per an ordinary diode?

Jon.
 
Yes the function outputs are DC, but they are the PWM version of DC. i.e you get full track voltage for a time and then nothing for a time and the average works out at the DC voltage. This works just fine for LEDs and bulbs.
 
Zerogee said:
Does reversed polarity destroy an LED then, or does it simply not conduct as per an ordinary diode?

Jon.
Click to expand...

Short answer, yes reverse polarity will destroy an LED.
Longer answer, typical ordinary rectifier diodes have a small forward current voltage drop and can withstand a relatively high reverse voltage. LEDs also have a small forward voltage drop but can only withstand a small reverse voltage.
This means that LEDs may be connected in inverse parallel provided a suitable resistor or other current limiter is in circuit. With this arrangement one led will light for one polarity and the other for the other polarity as the conducting LED's voltage drop is less than the maximum tolerable reverse voltage for the other. This may be used to give different colours for opposite polarities.
If there is a possibility of reversing polarity of a difficult to replace LED it may be protected by a diode, either a rectifier or maybe a signal diode (e.g. 1N4148) in inverse parallel. led inv parra.jpg
 
Last edited:
Mike LEDs will work on AC in any case. If you fit a current limiting resistor on one leg of an LED and a reverse voltage protection diode ( which can be another LED) to the other leg you can connect it directly to the track/ bus of a DCC system without any smoke :-)
 
Personally I'd always fit a resistor suitable for the full voltage output from the decoder. I'm not a fan of using the decoder CVs to reduce the voltage, reason being if the decoder is reset or replaced (perhaps with a different model) and you forget to set the CVs again then you may blow the LED if only wired for (say) 5V.
 
I may be crazy, but this is not the circuit most people use to protect an LED.

I see the logic, that the other diode is conducting, thus the reverse polarity voltage should be equal to the forward voltage drop of the protecting diode. But it does "eat current" at reverse polarity. Worse than that, if the resistor is selected for 20 ma through the LED, then a lot more than that will flow through the protecting diode, which might require a much larger resistor, wattage wise, and just unnecessarily "eat" power.

The inverse parallel diode seems to be used more for reverse spike protection than just compensating for the low reverse voltage protection of the LED (which is usually 4 to 5 volts).

Most of what I have seen, to protect a diode from reverse voltage, you put the "protecting" diode in SERIES with the LED, thus using the higher reverse breakdown voltage of the diode to protect the LED.

I have seen both, including a cap across the LED...

Greg


Neil Robinson said:
Short answer, yes reverse polarity will destroy an LED.
Longer answer, typical ordinary rectifier diodes have a small forward current voltage drop and can withstand a relatively high reverse voltage. LEDs also have a small forward voltage drop but can only withstand a small reverse voltage.
This means that LEDs may be connected in inverse parallel provided a suitable resistor or other current limiter is in circuit. With this arrangement one led will light for one polarity and the other for the other polarity as the conducting LED's voltage drop is less than the maximum tolerable reverse voltage for the other. This may be used to give different colours for opposite polarities.
If there is a possibility of reversing polarity of a difficult to replace LED it may be protected by a diode, either a rectifier or maybe a signal diode (e.g. 1N4148) in inverse parallel. View attachment 210749
Click to expand...
 
Last edited:
I'm completely confused now, I'll sit back and let you guys discuss it and see what comes out at the end.... ;)

In the meantime, another LED-related question to muddy the waters still further...... you all know the strips/tapes of LEDs that are everywhere now, from kitchens to cars, which are neatly divided up into 3-light segments with all the necessary resistors etc to run off 12volts? Multiplying up the segments (by simply not cutting the strip up) connects each 3-light strip in parallel with the others, so if (for example) you leave 3 sections connected (9 LEDs) they will still run from 12v, but at three times the current draw.
So here's the question: is there any way you can take two separated 3-LED segments and wire them in SERIES so that the resultant set of 6 LEDs run from 24volts? Or is there something about the way each 3-LED segment of the strip is internally wired that makes this impossible?

Discuss..... :)

Jon.
 
Keep it simple - No.
3 or 8 or a dozen sets in parallel - Ok
 
Mmmmmmm...... I have read the posts and can see the logic in the circuits but it is dead simple and cheap as chips to put a rectifier at the front end of a led circuit (that could be multiple LEDs) which is then just hooked up to the decoder function output) so as to provide the correct polarity for all the LEDs in that circuit no matter what polarity they are supplied with.

Of course if using the higher voltage function output of a decoder, a limiting resistor, with individual leds, is pretty important if you do not want a very bright but short light show!

As to the voltage with led strips..... Well I have run 12v LEDs (with integrated resistors as in the strips that Zerogee mentions) at up to 20v without any probs so far.
The only problem with the led strips (the supposedly waterproof ones) is that they do not like being left outside as the damp seems to slowly get in and cause intermittent failures along a strip.....not all of the LEDs but just some batches of three in a strip.
 
Last edited:
beavercreek said:
Mmmmmmm...... I have read the posts and can see the logic in the circuits but it is dead simple and cheap as chips to put a rectifier at the front end of a led circuit (that could be multiple LEDs) which is then just hooked up to the decoder function output) so as to provide the correct polarity for all the LEDs in that circuit no matter what polarity they are supplied with.

Of course if using the higher voltage function output of a decoder, a limiting resistor is pretty important if you do not want a very bright but short light show!

As to the voltage with led strips..... Well I have run 12v LEDs (with integrated resistors as in the strips that Zerogee mentions) at up to 20v without any probs so far.
The only problem with the led strips (the supposedly waterproof ones) is that they do not like being left outside as the damp seems to slowly get in and cause intermittent failures along a strip.....not all of the LEDs but just some batches of three in a strip.
Click to expand...


Mike - thanks for the answer, very useful to know that the strips are OK at slightly higher voltages than the stipulated 12v.... if you've used 20v without blowing anything, then the sort of voltage I will probably be using (which will be around 14-15v) should be fine. They are for interior lighting of a railcar (my newly-arrived German-built "Baby T1"), not for permanent outdoor use.

Jon.
 
You must log in or register to reply here.
Back
Top