Help w/ Low Voltage Bulbs and DCC

[Thomas1911]

I am in need of some assistance with trying to wire some Details West ditch lights on one of my locomotives.  I will be wiring one bulb to F5 and F6 so they will operate like the prototype.  The bulbs that came it the kit are rated at 3.0V, 26mA.  The instructions stated to use a 270 ohm resistor if wired in series or 200 ohm if wired in parallel (I assume these values to be generic).  I measured the function output of my decoder (Soundtraxx TSU-AT1000) to be ~12.7V as powered by my NCE Powercab.  By using the formula E=IxR, and providing my math is correct, I concluded that the resistance needed was ~370 ohms, so I will have to use a minimum of a 390 ohm resistor.  And with the formula P=(IxI)xR, gives the wattage to be right at 0.25W.  The nearest value resistors I had on hand are 330 ohm 1/4W, and after testing, they result with the lights being a bit on the bright side.  One is also brighter than the other for some reason.

While shopping for some 390 ohm resistors, I came across a diagram on the Tony's Train Exchange website that have the bulbs in parallel with a pair 1N4001 diodes in series claiming to provide voltage regulation/over-current protection and each diode would drop the voltage by 0.7V.  I have a few of these diodes so I tried wiring one diode in parallel with each bulb in hopes of not having to order more resistors and wait for them to arrive.  Adding the diodes has now made the bulbs too dim.  Getting this result, I tried several resistors of decreasing resistance up to about 100 ohm, with only slight improvement in brightness.  My internet searches came up practically nothing on dealing with the 3V bulbs, seems the majority are only 1.5V.  Seems I'm in unexplored territory.  Though I would think the mechanics behind all of it would be the same. 

What I'm wondering:
- Should I forget about the diodes and just run the 390 ohm resistors?
- If not, what value resistors do I need or how can I determine the value accurately to work with the diodes?
- Should I jump up to 1/2W resistors?  (Probably so, the 1/4W do get warm to the touch.)
- Replace the 3.0V bulbs with 1.5V bulbs?
- Any different recommendations?

Link to the diagram on TTE website: (Section 2)
http://www.tonystrains.com/technews/install-lamps-decoders.htm

Thanks.

[Jim Banner]

Your understanding and use of Ohm's law are good.  Let's see if we can bring you up to speed on diodes.

One property of diodes is that they conduct electricity only in one direction.  Another property of diodes is that when they conduct electricity, there is a voltage drop across the diode, known as the forward voltage drop.  This voltage drop is relatively constant, that is, is changes very little with changes in the current through the diode.  For your 1N4001 diodes, the forward voltage drop is typically .70 volts at .01 amps flowing through the diode.  This rises to about .78 volts at .1 amp, .92 volts at 1 amp and 1.38 volts at 10 amps.  Compare this to a 70 ohm resistor which would also drop .70 volts at .01 amp flowing through it.  At .1 amp, the resisitor would have 7.0 volts across it, at 1 amp, 70 volts and at 10 amps, a whopping 700 volts across it.  The relatively constant forward voltage drop of a diode can be used to regulate voltages.

In Tony's circuit, the forward voltage across the two diodes is about .7 volts each, or 1.4 volts for the pair.  If you use a smaller resistor, the current through the diodes will increase but the voltage across them will change very little.  Since the bulb(s) see only the voltage across the diodes, their current and brightness will remain virtually constant.

In your case, you tell use that you put one diode across the bulb.  This then limited the voltage to the bulb to about .7 volts maximum.  The result, as you observed, was a very dim bulb.  Lowering the resistance with the diode still in place made little difference to the bulb as its voltage changed very little.  But the diode current increased and the diode probably got a little warmer.  What your 3 volt bulb needed was a voltage closer to 3 volts.  You could try three diodes in series across the bulb to run it at about 2.1 volts or 4 diodes to give it about 2.8 volts.  If 2.8 volts is too bright but 2.1 volts is too dim, we can go into that for the next lesson.

As far as the resistor is concerned, just calculate it as before but go to the next LOWER standard value.  For example, if you calculated 388 ohms, instead of going up to 390 ohms, go down to 330 ohms.  This would pass 29 milliamps, 25 for the bulb and 4 for the diodes.  At 4 milliamps, the diodes will probably give a voltage just over .6 volts each.

Jim   

[Thomas1911]

Jim, thank you very much.  I didn't realize the diode was actually limiting the voltage, figuring that's what the resistor itself was doing.

[richg]

Here is a useful link for DCC with some links for DCC lighting which will prove helpful at times. Info for LED's, 1,5 volt bulbs and 12 volt bulbs.
I use 560 ohm, 750 ohm and 1,000 0hm resistors. With 560 ohm, I have 15ma current and most LED's we use are rated at 20 ma max. My experiences tell me to go no lower that 560 ohms.
With two or three LED's I never worry about voltage when the supply voltage is 12 vdc..
With five or six LED's and using 12 volts DC I would be concerned but have never done that. The max I use in series are two LED's.

Others reading this message, save the link to Favorites and take some time to read though the site. There is a lot info.

http://www.members.optusnet.com.au/nswmn/index.htm

There are two DCC links to the right of the page.

Rich