# Constant DC Lighting With 1.5 Volt Bulbs



## Mark R. (Jan 26, 2013)

I've been playing with this all day trying to find the answer ....

I have a dummy engine with two 1.5 volt / 40ma bulbs in it. I tried the usual bridge rectifier with a 12 volt bulb as the load, wiring the 1.5 volt bulbs across the jumpered AC terminals on the bridge rectifier. Trouble was, it took nearly 10 volts to get the 1.5 volt bulbs to light up !

Is the problem the fact the bulbs are rated at 40ma ? (appears to be).

What should I create so the 1.5 volt bulbs light up at 1.5 volts and remain there as DC track voltage increases ? Due to their installation - changing the bulbs themselves really isn't an option on the table at this point.

Mark.


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## gunrunnerjohn (Nov 10, 2010)

It would help to know what scale this is, and what kind of track voltage is available.

If this is HO, I'd wire a bridge rectifier (to deal with polarity changes) and then an LM317T regulator configured for 1.5V output. This should light them easily for any track voltage over 4-5 volts.


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## Mark R. (Jan 26, 2013)

Thanks John,

This is a dummy slug engine in HO scale. Track voltage is typical 0 to 12 volts. I've been playing with an LM317 regulator as well, but can seem to get it to hold at 1.5 volts.

I've followed a few of the circuits I've found online, but no success so far.

DCC is SO much simpler in cases like lighting .... hwell:

Mark.


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## NIMT (Jan 6, 2011)

If your running DCC why not just put a decoder in it?
If it's DC, Here's a good example of a regulator circuit.








Also put a super cap on the output to get real constant lighting.


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## Mark R. (Jan 26, 2013)

You can see in my opening post, it's for DC. (I do have DCC, but the engine isn't for me)

Regardless - I found my problem. I WAS working with the circuit shown above, but I had my resistor calculations messed up somehow. I ended up using a 100 ohm for R2 and a 1000 ohm for R1 which gives me a steady 1.45 volts.

I also used a bridge rectifier on the variable DC input as I wanted the light to be on in both directions. A single diode on the input provides nice directional lighting if desired.

Now, the big question .... if for some reason this dummy engine ever gets set on live tracks with DCC, will it damage it ? - or will it work just the same ?

Mark.


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## gunrunnerjohn (Nov 10, 2010)

The only addition I see there Sean is he'll need a bridge rectifier to make sure the voltage is always positive going into the LM317. Also, for 1.5 volts out, the best common resistor value for R1 is 47 ohms.


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## NIMT (Jan 6, 2011)

Mark R. said:


> Now, the big question .... if for some reason this dummy engine ever gets set on live tracks with DCC, will it damage it ? - or will it work just the same ?


Should work fine as DCC uses AC on the rails.


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## Mark R. (Jan 26, 2013)

John, you say a 42 ohm value for R1. I used a 1000 ohm value for R1 and netted the correct voltage output I was after. 

Is this a case of it working because I didn't know it shouldn't ? 

Mark.


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## trains galore (Jul 22, 2013)

Well if it works then don't worry as long as you get 1.5v it's fine. The value of the resistor might also depend on how many lights you want (the current draw will be different depending on how may lights so the final voltage would be affected)
If you have lots of lights you need only a smaller resistor as the voltage will drop as you add more lights in parallel. But if you only have a small current draw, then you need a bigger resistor or else the voltage will go over


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## gunrunnerjohn (Nov 10, 2010)

I don't know how that's working with a 1k resistor there, but I actually said 47 ohms, not 42 ohms. A resistor of 1K for R1 with the 240 for R2 should give you around 6.5 volts out.


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## Mark R. (Jan 26, 2013)

I don't know how / why it works either John. I can follow diagrams, but I don't necessarily understand them. Maybe to be more specific, I was playing with an LM317LZ regulator .... would that have something to do with it ? Does the 80ma load between the two bulbs also affect the calculations ?

According to this calculator ....

http://www.electronics-lab.com/articles/LM317/

A typical value of 240 ohms for R1 calculates out a value of 47 ohms for a net output of 1.49 volts. Any combination I make always seems to have the higher value on R1. (?)

What kind of current are these good for ? I have to build another one for a powered engine that will need to run four 30ma bulbs. 

Mark.


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## gunrunnerjohn (Nov 10, 2010)

You're at the limits with the LM317LZ, as it's rated at 100ma max, and that's with some additional heatsinking. Try the LM317T for 80ma with no heatsink, it'll work a lot better.


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