Lighting voltage for PS2 engine?

MTH PS-2  lights are 6V rating controlled by PWM.  Older PS-1 diesel headlights where 1.5V. G

So maybe 100ohm resistor, then?  I'm just guessing now.

Do they tell you what the output voltage of the decoder light circuit is?  Based on it, you can determine what resistor.  The bulbs are 60ma, so you can pick a resister based on the current, or use a lower current like 40ma.  G

i think it is 50 mA

Use Ohm's Law to calculate the resistor required.  Ohm's law is I (Current) = E
(Delta Volts) / R (Resistance).  In this example we will use a bulb
current rating of  (50 ma or 0.050 amps) and we can get the Delta
Voltage by subtracting the rated bulb voltage from the decoder output
voltage.  If you have your DCC track voltage set to HO, then the
locomotive decoder function output voltage will be roughly 10 - 12
volts DC.  If the bulb is rated for 6 volts, then the delta voltage
between the decoder function output voltage and the rated bulb voltage
is 6 volts.  Therefore, we do a little algebra and solve for R
(Resistance) and we get R = E / I.  In our example R = 6 volts / 0.050
amps or R = 120 ohms.  Next, we need to calculate the power rating of
the resistor and we use P (Power) = E (delta volts) * I (current) = 6 *
0.050  = 0.3 watts; therefore use the next larger resistor power
rating, which would be a 1/2 (0.5) watt resistor.  So our resistor
selection for this bulb example will be a 120 ohm 1/2 watt resistor.

If you don't know the rated bulb voltage, then you can find it by
stacking up normal AA, A, C, or D cell batteries in series until you
get to the bulb brightness that suits your needs since each battery has
a constant voltage output of around 1.5 - 1.6 volts when brand new.  In the
case above, it would take 4 batteries placed in series to get to
~6-volts.  Maybe have a helper hold the batteries stacked together
while you touch the bulbs leads to the opposite ends of the battery
stack.  You'll know you have reached the rated voltage when the bulb is
nice and bright, so don't go any further or the bulb will turn into a
one-time use flash bulb.  Also remember that these little incondescent
bulbs last A LOT longer if you run them at a lower voltage then they
are rated for so if you want to run them at a lower voltage, just input
a greater voltage delta value for E in the formula above, say 7 volts,
in the above example, and the bulb will have a lower net voltage coming
to it and last significantly longer, but it will burn slightly dimmer too.  If you
have a multimeter, then you can then measure the bulb's current by placing
the meter in series with one of the bulb's leads when the bulb is
running at the rated voltage in your battery test above.

Scott K.
Austin, TX

OK so I put a 100 ohm one quarter wattresistor in there.  The bulb is now a dull yellow instead of bright white like it was before.  Any ideas?  Are we sure this isn't a 1.5v bulb?

It is a 6V bulb.  In your first post you said headlamps (plural).  So if that's true that would be a factor of 2 since it appears the calculations above were for 1 bubl.

 

Also, most Digitrax decoders have an extensive set-up selection for lights includes ability to set the output for LED vs. bulb, Rule 17 dimming, and so on.  When you say "bright white like it was before" I assume you mean when driven by PS2 electronics.  How do you know the Digitrax decoder is set up to deliver 10-12V DC to the headlight as assumed above?

No, I mean compared to when it is attached to the decoder without a resister.  In that case, it is bright white, but hot and smoking !!

So go to a 50ohm an see what it does.  Or put 2 6V bulbs in series as a test.

After trial and error, it seems 30 Ohms is the answer. At 10 ohms the bulb was still almost burning up the holder it was in.  30 Ohms  preserved most of the brightness without getting too hot.

 

Thanks again to those who responded.

Sounds like the bulbs are 9 Volt bulbs.  The math would work out if each bulb was a 9 Volt 50 ma bulb and you had one 30 ohm resistor in series with both bulbs.  R = (12 volts - 9 volts) / (0.050 amps x 2 bulbs)  or R = 3 volt delta / 0.100 amps or R = 30 ohms.  Just make sure the decoder function output current rating is not exceeded.  If this is a large scale decoder, then nothing to worry about since the function outputs on those can handle loads in the order of amps, not milliamps, but if it is a small scale decoder retrofit, like an HO decoder, then I would double check the maximum function output current on the aftermarket decoder specs as many smaller scale decoders have a 100 ma max current capability so that little mosfet transistor will be running right at the limit, which may shorten its life to the point of becoming a slo-blow fuse .

 

Would be curious to know (maybe it is too late and the bulbs have been reinstalled) if you touched the leads of the bulbs to just a simple 9 Volt battery terminals if they would burn at the same brilliance as what you have using your resistors to confirm the bulb's rated voltage.

 

Scott Kay

Austin, TX

The issue is not the bulb rating, we know that, the issue is what voltage does the decoder use at output and what current capacity do they have.

 

MTH bulbs rated at 6V and can be tested on 9V battery, they even stay lite at higher voltages like when a fet shorts, but only for a period of time.  G