resistors for 1.5V bulbs

[rws1225]

Since I'm spending more time at home these days I pulled out an unfinished loco project to work on.  The box of stuff I set aside for the loco includes a Digitrax DH-126D decoder and Circuitron 1.5 Volt miniature bulbs. According to the decoder instructions I need a 560 Ohm 1/4 Watt resistor and Circuitron has some available in either 12 volt or 16 volt.  However, there is no explanation of whats in the package as to ohms or watts. Anybody out there have any insight.  Thanks in advance.

[rrinker]

 Wattage of the light bulbs is what you need to make the calculation. Almost certainly 1/4 watt resistors are too small and will either get really hot or burn up.

 Instead of 12 or 16 volt light bulbs (which if in contact with the plastic of the loco, will end up melting it), switch to LEDs. Correct resistors are 1K, 1/8 watt is fine.

                                  --Randy

 

[RR_Mel]

I use a lot of Grain of Wheat bulbs. I have 1mm diameter 1½ volt @ 15ma and 3MM @ 100ma 1½ volt.  Assuming you have the high current 3mm bulbs this it what you would need.

 

 

Double Click on the picture to enlarge

 

Randy is correct about the heat, not only from the bulb but also the resistor.  1 Watt is quite a bit of heat in a compact area.  Also I would use a slightly larger value resistor (133Ω or 147Ω) to reduce the current, check the voltage across the bulb to make sure the bulb is a bit under 1½ volts they have a very short life running a max voltage.

 

As Randy suggested an LED with a 1000Ω resistor would be a much better choice.

 

 

 

Mel

 

 

 

My Model Railroad   

http://melvineperry.blogspot.com/

 

Bakersfield, California

 

I'm beginning to realize that aging is not for wimps.

 

[BigDaddy]

Mel why 11 volts?

[richhotrain]

At this stage of my modeling efforts, I would never put an incandescent bulb in a plastic housing, not even a 1.5 volt bulb. Resistored LEDs are really the only way to go.

Rich

[gregc]

i agree with Rich.    decoders are designed to work with LEDs which require 20 ma at most.    Not sure a decoder can supply sufficient current for an incandescnet bulb.  This 12V lamp requires 60 ma.  These 1.5V lamps require 0.3A.   These grain of wheat bulbs require 100ma.

 

 

one approach for determining a resistor value is to use a potentiometer and slowly reduce the resistance until there is 1.5V across the lamp.    With excess resistance, the lamp voltage will be less than 1.5V.   Only when the lamp current is at the level it was designed for will the voltage increase.

when you know the resistance, you can calculate the required wattage of the resistance from the voltage across the resistor (volt squared / R).    You can probably assume 1-2 Volts less than track voltage is output by the decoder.    For a track voltage of 15V, the wattage for a 560 Ohm resistor is 0.3W  ((15 -2) ^2 / 560), so a 1/2W 560 Ohm resistor (if 560 is the proper value).

[richhotrain]

Earlier in my modeling efforts, I discovered a burned out 1.5 volt incandescent bulb in a Proto Heritage Berkshire. At that time, I was clueless as to voltage, amps, etc. I unwittingly replaced the 1.5 volt burned out bulb with a 12 volt bulb. Once around the layout to test the new bulb. The photo shows the result.  

Rich

[gmpullman]

richhotrain

Once around the layout to test the new bulb. The photo shows the result.

Deja-vu, Rich!

This was the result of one of my early DCC installs and I followed the directions closely. No mention was made of the headlight bulb. I used a resistor but the heat still was too much for the plastic!

LL_Berk-melt by Edmund, on Flickr

Walthers had just taken over the Life-Like line and they graciously sent me the parts to repair the front end.

From that day forward = LED all the way!

Cheers, Ed

[RR_Mel]

BigDaddy

Mel why 11 volts?

 

Henry

 

11 volts + 1½ volt bulb = 12½ volts.  I operate all my incandescent bulbs at 70% voltage to greatly improve their life and I think it’s more realistic looking at reduced voltage so using the 147Ω resistor (74%) is what I would the best for me.

 

 

 

Mel

 

 

 

My Model Railroad   

http://melvineperry.blogspot.com/

 

Bakersfield, California

 

I'm beginning to realize that aging is not for wimps.

 

[richhotrain]

gmpullman

 

richhotrain

Once around the layout to test the new bulb. The photo shows the result. 

Deja-vu, Rich!

This was the result of one of my early DCC installs and I followed the directions closely. No mention was made of the headlight bulb. I used a resistor but the heat still was too much for the plastic!

LL_Berk-melt by Edmund, on Flickr

Walthers had just taken over the Life-Like line and they graciously sent me the parts to repair the front end.

From that day forward = LED all the way!

Cheers, Ed 

No way! What an unbelievable coincidence. I got no help from Walthers, but here is my fix with a generous donation of NKP numbers from Doc Wayne.

Rich

[gmpullman]

richhotrain

No way! What an unbelievable coincidence.

Those Life-Like Heritage locomotives are some excellent models!

My fiasco dates back to 2006 so the parts bin was probably not empty yet. I was lucky to get the parts from Walthers. I have not replaced the decals yet, my bad. Glad you got yours fixed up so nicely. Hat's off to the good Doctor, too.

NKP_765-pre_fame by Edmund, on Flickr

In 2006 there weren't many choices in decent LEDs, either. Kato was still using yellow/green colors in theirs. Some were only available in 5mm size which are gigantic by todays standards.

I actually have three other locos, all diesels, that suffered heat damage, too. I guess it took a while for me to learn

Regards, Ed

 

[richhotrain]

Yessir, I love the Proto 2000 Heritage Berkshires, NKP, PM and C&O in my case.

It only took me one meltdown to learn my lesson.

Rich

[richhotrain]

gmpullman

 

Those Life-Like Heritage locomotives are some excellent models!

My fiasco dates back to 2006 so the parts bin was probably not empty yet. I was lucky to get the parts from Walthers. I have not replaced the decals yet, my bad. Glad you got yours fixed up so nicely. Hat's off to the good Doctor, too.

Wayne would give you the shirt off his back. That said, no amount of begging, pleading, and/or bribing has been able to pry Bertrams off of the doctor's layout.

Rich

[santafe5000]

http://apnmrr.club/files/tech/LEDohmsCalc.htm

 

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