3v Grain of Wheat Bulbs on 18v AC: What size resistor is needed?

[M. Mitchell Marmel]

The bulb in question checks out at about 2.5 ohms at 3 volts, so any idea what size resistor will work at regular operating voltages (up to 18v)? 

Tried asking this question on the OGRR forum, and got fifteen miles of bad road instead. 

Help?

Mitch

(NB:  The answer isn't diodes and LEDs, thanks; I want to do this old school.) 

[lionelsoni]

12.5 ohms.  But are you sure about the lamp's resistance?  That's a 3.6-watt lamp if you measured the resistance correctly.  You'll need an 18-watt resistor, and you'll have to supply 1.2 amperes at 18 volts.  How do you know that the lamp resistance is 2.5 ohms?

[M. Mitchell Marmel]

Well, that's what my multimeter says, although I'm willing to be convinced otherwise.  'Course, that is with the lamp cold, so I daresay the resistance will drop considerably once it fires up. 

So, off to Radio Shack for some 15 ohm resistors!  Thanks! 

Mitch

[lionelsoni]

Stop.  Your resistance measurement is wrong.  I was afraid that you had used an ohmmeter like that, which is why I inquired.  The resistance will increase as it warms up, not drop.  Your lamp will burn out at 18 volts if you use that series resistance.  What you need to do is to measure the current through the lamp when the voltage across it is 3 volts.  Then the resistance (in ohms) is 3 divided by whatever current (in amperes) that you measure.  I'm sure it will be substantially more than 2.5 ohms.

But you can calculate the series resistance you need directly from the current measurement.  Just divide 15 by the measured current (in amperes) to get the needed resistance in ohms.  The power rating needed is 15 multiplied by the measured current (in amperes).  It's a good idea to use a resistor with a higher power rating than you need, even as much as twice the expected power.

When you measure the lamp current at 3 volts, you will need to have your ammeter in series with the lamp.  There's a good chance that there will be a significant voltage drop across the ammeter.  So you really need a separate voltmeter so that you can adjust the lamp voltage (as opposed to the supply voltage) to 3 volts, with the ammeter in place.

You should be warned that, although it will be fully bright at 18 volts, a lamp connected this way will dim even more severely than a higher-voltage lamp would when the voltage is reduced below 18.

[M. Mitchell Marmel]

(does measurements per instructions)  OK, I get 130mA at 3 volts.  So, that means 115 ohms resistance...

I should THINK this will do the trick? 

http://www.radioshack.com/product/index.jsp?productId=2062316

Mitch

[lionelsoni]

The series resistance is okay; but you need a much higher power rating.  If you had gone with 115 ohms, you would have needed a power rating of .13 * 15 = 1.95 ~= 2 watts minimum.  By going up to 150 ohms, your power rating drops to V * V / R = 15 * 15 / 150 = 1.5 watts.  But the resistor you selected is rated at only 1/2 watt.

Since you get 5 in a package, you could use a series-parallel connection of 4 of them to increase the total rating to 2 watts, or just look for a 2 watt (or better, 5 watt) resistor.

(The series-parallel connection just means, put two resistors in parallel, which drops their combined resistance by half, then put those two together in series with another pair also in parallel, which doubles the combined resistance again.  You get back to the original resistance, but you can handle 4 times the power.)

How about 2 of these in parallel for 150 ohms, 4 watts:  http://www.radioshack.com/product/index.jsp?productId=50559836&filterName=Type&filterValue=2-watt+resistors

You may not want to increase the resistance from 115 to 150 ohms however.  That decreases the light put out fairly drastically, by a factor of about 5.  You can get close to full brightness at 18 volts with 2 of these in parallel for 120 ohms, 4 watts:  http://www.radioshack.com/product/index.jsp?productId=50559816&filterName=Type&filterValue=2-watt+resistors

[M. Mitchell Marmel]

Having noodled about briefly with one of the 1/4 watt resistors, I can see a) what you mean about quick voltage drop and b) how those resistors get real hot real fast. 

Think I'll just go shopping for 18v grain of wheat bulbs.    

Mitch

[lionelsoni]

Good move.

[dbaker48]

M. Mitchell Marmel

Think I'll just go shopping for 18v grain of wheat bulbs.    

Mitch

Check out hobby shops or train meets where G-Scale is sold.  I typically find them at 50% off, just bought some this week-end (3 for $2) red, green, yellow, clear .