Resistor value for 5v bulb?

[richg1998]

With a bunch of clip leads, you might have to wrap a little electrical tape around the exposed metal. The test setup can get messy at times.

If working with DCC and decoders, the decoders are sensitive to even an intermittent short.

Maybe you will not mess with DCC but some reading this post will be aware of the issue. I read every so often of someone in a hurry not insulating exposed wire and smoking a decoder.

Rich

[rrinker]

 One cna never have too many of those cheap Harbor Freight meters. I have them all over the place so I never have to go search, and they are plenty adequate and accurate for model railroad use. I have a super fancy benchtop meter that I wouldn't have if I hadn't gotten it for free, but it's totally beyond what is needed for anything MRR related.

 And you cna never have too many of those alligator clip leads, either. Just don't allow them to become semi-permanent parts of your layout wiring, as I've seen happen. When you only have 2 hands and need to hold 5 wires to various test leads - the alligator clips are worth their weight in gold. Pick up two packs when you get them,

          --Randy

 

[Southwest Chief]

Thanks for the info on meters Rich.

Alligator clips seem like a must have for easy use.

[richg1998]

I have used the below meters and clip leads for some years. The meters do not measure AC current but I have never found that to be an issues in model railroading.

Some rivet counters will insist on a $200 meter but have never seen the need for an expensive one though I do have an expensive one.

If you are going to work at what I call the component level, you need a meter and clip leads.

http://www.harborfreight.com/7-function-digital-multimeter-90899.html

http://www.harborfreight.com/18-inch-low-voltage-multi-colored-test-leads-66717.html

I use two. That way I can read voltage on one and current on the other.

I actually have three of these meters.

Get a box of fuses at Radio Shack in case you burn out a fuse. There is a protective fuse inside.

Rich

 

[Seamonster]

Try contacting Accucraft and see if they know what current that bulb draws.

 

[Southwest Chief]

I appreciate the information.  Thanks

Unfortunately I don't have a meter to check on the draw.  I should really get one.

The bulb is actually a 6 volt bulb made by Accucraft for G scale trains.  But they don't mention any milliamps in their documentation (which I have) or online. 

Accucraft 6v bulb

[CSX Robert]

Southwest Chief

...

Unfortunately I do not know the milliamp rating for this bulb.

...

You need that information to calculate the correct resistor value.  Doy you have a 5 volt supply and a meter that you can test it with to determine it's current draw?

Southwest Chief

...

Resistor calculators are so easy to find for LEDs, but I always have difficulty finding one for bulbs.

...

You use the same formula for bulbs and LED's.  With LED's, you are given a voltage drop and a maximum current, and you are calculating the resistor that will limit the current to below the maximum.  With bulbs, you are given a current and maximum voltage, and you are calculating the resistor that will limit the voltage to below the maximum.  In both situations, however, you use the same formula: V=IR (voltage = current x resistor), or, solving for R, R=V/I.

 

For example: 5 volt, 40 ma bulb, 14.8 volt supply.  V = 14.8 - 5 = 9.8(because the resistor has to drop the supply voltage - bulb voltage). R = 9.8 / .04 =  245 ohms.

[Southwest Chief]

Looking for the correct resistor value for a 5 volt bulb with a supply voltage of 14.8 volts?

Unfortunately I do not know the milliamp rating for this bulb.

Resistor calculators are so easy to find for LEDs, but I always have difficulty finding one for bulbs.

Note I also have to use resistors, no room for a voltage regulator.