Titan A what's the best way to connect LED's for steplights?

[drudge]

Anyone used the QSI Titan A board to light the 4 steplights  with LED's for the step wells on diesels? I'm looking into the possibility of adding step lights to this Atlas 8-40BW using the QSI  aftermarket Titan A board with it's 10 lighting outputs. I was thinking that the 4 LED's would be connected to a single lighting funtion output.The Titan A lighting outputs which are 5v and has a max output of 100ma each,  according to my calculations and online info,using 3.4v 20 ma LED's would require the 4 LED's be wired in parallel with a 100 ohm resistor attached to each LED.

The part that's confusing me is the Titan board already has a single 110 ohm SMD resistor padding each of the lighting outputs (except the 12v and two 5v auxiliary ones) and I'm not sure how your supposed to wire the LED's with the required resistors in parallel if the board already has the output padded.

If the output's were 12v and you wired 3 LED's in series I could see using a single resistor.Do you wire 1 LED without a resistor and then the remaining 3 with their own 100 ohm resistor each in this case using a parallel connection?

[Mark R.]

The resistor on the board is part of the regulation circuit to bring the pads down to 5 volts. treat it as a constant 5 volt output by putting an appropriate resistor on each of the four LEDs.

You will probably want to go to a resistor value a LOT higher than 100 ohms though, otherwise, your LEDs will be as bright as headlights in the step-wells ! This is where some experimentation is going to be required. When I use white LEDs for class lights, I use 47,000 ohm resistors (on a 12 volt pad). This makes them glow nicely - class lights don't cast a beam of light, they are just an indicator.

For your step lights, I'm thinking you will probably end up in the 1000 ohm range for the 5 volt pad. Again, you are going to have to experiment to get the right brightness. Step lights aren't very bright.

Mark.

[drudge]

Thanks.

Ok, I was thinking that the pads were already outputting 5v prior to the 110 ohm SMD resistor connection and the 110 ohm SMD resistor was dropping the voltage to the appropriate level to hook white LEDs up directly without the need for an additional current limiting resistor. That's why I couldn't understand how you would hook-up more than 1 in parallel if it's already padded.

The Titan literature makes it seem that way: "Direct Connection of LEDs"  "10 Assignable Ports with resistors onboard for LEDs"

As far as the resistor value of 100 ohm being used.I was under the impression that you could just turn down the brightness using the decoder's 256 step PWM adjustable light intensity settings to compensate.I guess you could just do the same with a higher resistor value,but I was thinking I would have finer control with the PWM adjustment.

Would 1/8 watt resistors work fine without heat issues, due to space constraints, for either the 12v or 5v outputs?

[Mark R.]

I'd get close to what you want with the resistor, then fine tune it with the decoder settings. Eighth watt should be just fine for the five volt output, but even a 1/4 watt is just barely enough on the 12 volt pad. A 430 ohm resistor will need to disipate 240mW of power to power a 20ma LED from 12 volts.

You can connect LEDs directly to the pads as the onboard resistor works in conjunction with the voltage regulator. The 110 ohm resistor is going to run the LED at its maximum potential. That why I still use an external resistor on each LED. You can run LEDs at considerably less current and still be plenty bright.

If you have the PWM set too high to get the brightness down, the pulses sometimes become visible.

Mark.

[drudge]

Sorry for the late response.

I just realized that I can use the 12v pad and map it to the F3A or F4A function outputs,which are(not padded) unprotected.I can then wire 4 LED's in series-parallel to one function output and probably use a single 1K ohm 1/4 watt resistor for each series string. Would a 1k ohm resistor still be ideal for brightness level in this configuration?


Thanks for the helpful info

[Mark R.]

Nov 17, 2014 23:24:25 GMT -5 drudge said:

Sorry for the late response.

I just realized that I can use the 12v pad and map it to the F3A or F4A function outputs,which are(not padded) unprotected.I can then wire 4 LED's in series-parallel to one function output and probably use a single 1K ohm 1/4 watt resistor for each series string. Would a 1k ohm resistor still be ideal for brightness level in this configuration?


Thanks for the helpful info

If you insist on not placing a resistor on each LED, you should wire all four in parallel. Four white LEDs in parallel would require a 100 ohm resistor to work at their optimum brightness .... which you obviously don't want / need. The 1000 ohm resistor in that configuration "might" work. When wiring multiple LEDs to run at considerably less current than what they are rated, it's purely a trial and error kind of thing. 

Different LEDs will react differently to miniscule amounts of current. Some will light dimly as you want for step lights, while others may not at the same current setting. There is no formula for this. Mathematically, we know you cannot go lower than 100 ohms for four LEDs in parallel. The simple way of finding the desired value is to use a 5000 or 10,000 ohm linear potentiometer and connect it in where your resistor will go. Adjust the potentiometer until you have the desired brightness, then take a meter reading off the potentiometer and use the closest value resistor.

Start with the potentiometer set to the middle and turn it slowly - you don't want to go below 100 ohms and risk blowing your LEDs !


Mark. 

[drudge]

I'm a little confused on this still.From the calculations I used for a 12v connection,using 20 ma, 3.2v white LEDs. I came up with wiring 2 LEDs in series with a single 330 ohm resistor and then a second string of 2 LEDs wired in series with another 330 ohm resistor(for maximum brightness in this example) and then both of those strings wired in parallel to the single +12v and F3A or F4A function pad.

Are you saying that parallel connecting each single LED with it's own resistor will work better with 12 volts than wiring them in series-parallel in the above example?

Thanks

[Mark R.]

Your method will probably work ok - I just prefer not to wire LEDs in series for lower lighting levels. At full potential, there's no problem. Current is divided equally between LEDs in series. If one LED is slightly less efficient than the other, it may not light the same (or at all) when running them at a greatly reduced light level. There's a fine line between working and not working when an LED is dimmed to a great degree.

Wiring them all in parallel through a common resistor would be better as each LED would be receiving that regulated current. Obviously, having a resistor on each LED would be the best choice as each could be fine tuned to the proper light level.

If you were running them at full brightness, then no problems. As I said, there's a very fine line between dim and off when it comes to LEDs.

Mark.

[drudge]

I see what you mean. I'm kind of trying to balance between reducing the amount of wires and still getting the correct effect.Parallel wiring almost triples the amount of wires to run vs series. There is always a catch 22!

[Mark R.]

Not necessarily. Run two wires to each end and make your parallel connection of the two step lights on each end right behind the pilot. Same amount of wiring, you are just connecting in parallel instead of series.

Mark.