PS3 steam kit, how many LEDs off the headlamp circuit?

Measure the current draw of one MTH 6V bulb and multiply by 4. That will give you max current the circuit is rated for. Divide that number by 20 ma which is generally the max current rating of many common LEDs. If you are too close to max use a larger series resistor for each LED, Brightness will only be marginally affected.

Pete

The answer is probably more than you'd be likely to use.  Like Pete says, the factory puts up to four 60ma bulbs on the circuit, so that's 240ma minimum, and I suspect the FET that's rated at several amps would support a bit more if needed.  Running the LED's at a full 20ma each, that's a dozen of them.

FWIW, the lamp outputs are PWM operating between zero and around 18 VDC, and they come off the unregulated PV circuit.

Where did they put 4?  They may have on a model or two, but the normal recommendation was 3 which put you at 180ma.  Granted it is pulsed.  The current rating of the fet is high enough, but the wattage is only about .5W.

MTH rated those bulbs at 60ma.  G

George, I had one that had four several years ago, I was a bit surprised myself.  As I recall, it was the number boards, all four of them had a lamp, and they were all on one circuit.  As far as the lighting FETs, the wattage is more than sufficient for a lot more power, remember the FET doesn't dissipate power like a triac or bi-polar transistor, the internal voltage drop is minimal, hence the power dissipation is also minimal.  I use the same package FET with similar ratings to drive 400-500 milliamps, and they never break a sweat.

Makes sense, I think the current rating is about 1.7A.    G

If the circuit can handle 240ma than I'll be a bit under that at 180ma for 9 LEDs. What I want to do will be fine then. 

It won't even break a sweat Lou.   Besides, many times for stuff like markers and class lights, I drop the current in the LED to lower the light output to something reasonable.  I don't like them to be like headlights, just want to see them clearly lit.

gunrunnerjohn posted:

It won't even break a sweat Lou.   Besides, many times for stuff like markers and class lights, I drop the current in the LED to lower the light output to something reasonable.  I don't like them to be like headlights, just want to see them clearly lit.

Sweet. I've been using 560ohm resistors in all the LEDs but the headlamp, that gets around a 400ohm resistor to make it brighter. I could go lower on the resistor rating for the headlamp but then it's too bright, to me eyes.

I use 220 ohms for the headlights, and for steam I use an amber colored LED.  I get a nice bright focused headlight that looks like a real train headlight.  For the markers, my go-to resistor is typically a 1k resistor.

gunrunnerjohn posted:

I use 220 ohms for the headlights, and for steam I use an amber colored LED.  I get a nice bright focused headlight that looks like a real train headlight.  For the markers, my go-to resistor is typically a 1k resistor.

I've been using warm white for the headlight and cab light, and amber for number board lights. 560ohm for everything but the headlight, which gets a 390 ohm resistor.