Light Bulbs, Lighting, and Resistor Thread!

[gatrhumpy]

OK, so I bought a package of ten mini light bulbs, with a rating of 1.5 Volts per light bulb. I hooked one up to my regular power pack, and it blew, which was expected.

What resister should I use in line with each bulb to make sure they don't blow?

I know the power pack (I'm guessing) puts out 12 volts, so I would need a resistor to lower the voltage on the lightbulb to less than 1.5 volts, correct? What should be the value of the resistor? Would it help or would it make it worse if I hooked all the lights up together to the same terminal in the power pack (parallel connection)?

[rolleiman]

http://cs.trains.com/forums/1443077/ShowPost.aspx

Get out your calculator.. 

[loathar]

Not all power packs put out 12v. My MRC puts out 14v on the DC side and about 16v. on the AC side. I usually use 680ohm with 1.5v. bulb. It's a good starting point. If that's too bright, use a bigger one.

[gatrhumpy]

I have nine 1.5 volt bulbs that I want to hook up eventually to several buildings, but I don't know the amperage. I'm sure it's on the package. I can figure out the resistance from there. Thanks.

[jasperofzeal]

Most of the 1.5v bulbs I've used are either 15mA or 20mA, others may vary but I usually do the resistor math with these values.

I use this tool to help find the resistor values I need: http://ledcalc.com/.  This tool manily deals with calculations for LED's, but it works the same for bulbs.  There are four sections that do different things just look for the one you need: single LED, parallel, or series.

This other tool is useful if you have many a resistor to which you don't know the value: http://www.dannyg.com/examples/res2/resistor.htm

Hope you can find them useful.

[cacole]

As a good rule of thumb, start with a 1K Ohm resistor.  If the bulb is too dim, cut down the resistor value until you get close to what you want, but 1K is usually close enough and prolongs the bulb's life.

[betamax]

OK, so I bought a package of ten mini light bulbs, with a rating of 1.5 Volts per light bulb. I hooked one up to my regular power pack, and it blew, which was expected.

What resister should I use in line with each bulb to make sure they don't blow?

I know the power pack (I'm guessing) puts out 12 volts, so I would need a resistor to lower the voltage on the lightbulb to less than 1.5 volts, correct? What should be the value of the resistor? Would it help or would it make it worse if I hooked all the lights up together to the same terminal in the power pack (parallel connection)?

What is the current rating of the bulb?

The resistor must drop most of the voltage, so all that is left is the 1.5V needed by the bulb. If the bulbs are 50mA, then you need (for example) (12-1.5) divided by 0.05. Which results in 10.5/0.05, so the resistor needed is a minimum of 210 ohms. I think 220 is the closest match.

A bigger resistor will reduce the light output, but the lamp will last longer.

[Steve_F]

Here's an easy calculation site...

http://www.anderson-bolds.com/calculator.htm

I also use a resistance substitution wheel, starting at 1000 ohms and working my way down to the desired brightness, saved many a bulb and or LED.

[gatrhumpy]

The lightbulbs are 0.04 amps, 1.5 volts. I measured the voltage of the powerpack, and it was about 18.5 volts. So I would need (18.5-1.5) volts/0.04 ohms of resistance 425 ohms. I would need less resistance if I wired all the bulbs in parallel.

 Thanks for everyone's help!

[gatrhumpy]

OK, so I have a problem. I connected eight (8) 14V lightbulbs to a rocker switch, which is then connected to the accessories AC portion my Tech II power pack. Whenever the lightbulbs are off (hence the switch is set to OFF), the powerpack light for the OVERLOAD function lights up, and won't go away unless I turn the lights on. What the heck is going on?

I figured the OVERLOAD function would only light up if the lights were on.

[Rotorranch]

Sounds like the switch is either mis-wired, or is shorting internally.

Rotor

[rolleiman]

Yep. You have a short circuit somewhere..

Replace the Dimmer Pot with your rocker switch and it should work fine. 

[gatrhumpy]

Well, it shouldn't short out when connected to the accessories portion of the power pack right?

I connected all the lights in parallel. What I did was to connect  one wire of each of the lights together to one side of the rocker switch, and then connect the other side of each of the light wires to the other side of the rocker switch. I should not need a resistor because the lights are 14V and the package said they could be connected directly to a power pack.

What's going on?

[rolleiman]

So then what you have is This, Correct? (it's what you just described to us)

 

Try a single bulb and see what happens.

 

[gatrhumpy]

I'm an idiot.

The switch should be connected in series to ONE of the bungled leads, not both (parallel).

In other words, when I turn the switch off, it shorts, just like everyone said. I basically connects the circuit in a loop. It's the equivalent of putting a wire directly across both AC accessories terminals.

I need to unglue the switch from the fascia and unsolder the leads. Then I should connect the switch in series to one of the bungled wires leads, amd connect the other bungled wire lead directly to the other terminal of the accessories of the power pack.

Thanks for the help!

[topcopdoc]

Hey guys,

Thanks a lot for all the great info on lighting. I am just starting to install my lighting system, good timing.

Doc