# Voltage dropping resistor for bulb & finding parts

Thread Starter

#### Chris Wright

Joined Jul 26, 2006
62
I have an old micro-fiche reader that uses a low voltage halogen bulb that is marked 50 watts and 13.8 volts. They are getting harder to find and when I do find them online they want $25; but I can go down anywhere and buy the same physical size bulb rated 50 watts and 12 volts for$6. I have two questions:

1) Is the 1.8 volt difference worth worrying about? i.e. I calculate the 12v bulb will theoretically be putting out 66 watts instead of the 50 watts it is rated at; will it burn out in short order or just reduce it's life some ? (I can buy 4 of the cheap bulbs and still break even). I also know that the resistance can increase with an increase in temperature, which a bulb filament certainly does. So is a simple ohms law calculation even valid for a light bulb? I guess I need to think about the increased heat in the machine also. Which brings me to my next question.

2) If I use a simple voltage dropping resistor to drop the 1.8 volts is the following simple ohms law calculation correct? Or is there another formula that takes into account the heated filament in the bulb?

P=IE or P/E=I .... 50w / 12v = 4.1666 amps
E=IR or E/I=R .... 1.8v / 4.1666a = .432 ohm resistor

3) Let's make that three questions. Where do hobbyist find parts? RadioShack is a joke, and the online website's have good prices until you factor in the $7/8 shipping with a$30 minimum order for $2 or$3 worth of parts.
As well as the .43 ohm resistor for the bulb, I have an old Simpson Meter and need a 1% tolerance 11.5 ohm resistor to repair it. I've called around and all of the repair shops in the area have closed up; seems no body fixes anything any more!

Thanks in advance
Chris

#### Gadget

Joined Jan 10, 2006
614
Another option may be to to remove 3 or 4 turns from the secondary of the mains transformer, or even replace the transformer. Running a high wattage 12 v bulb at 13.8v will definately shorten its life, even if you do get a little more brightness.

As for you high tolerance resistor, do what I do..(my version of laser trimming)... Get a 10 ohm resistor and hook up to an accurate meter. With a sharp edged file, cut a small nick across the body until the resistance is where you want it. NB, doing this will reduce it's wattage rating a bit.
Otherwise you could get a few cheap 20% tolerance 12 ohm resistors, measure em, and choose the one that is closest to 11.5 ohms.

#### mrmeval

Joined Jun 30, 2006
833
Digikey is good but they charge an extra 5 for orders under 25.00. I've ordered from them with no problem
http://www.digikey.com/
This one looks good
http://www.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=TWW5JR43-ND

Jameco is supposed to be good but I've not ordered from them
http://www.jameco.com/
As good
http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=660033

Mix and match series/parallel resistors to get 11.5. If it's an extreme precision one you'll want metal oxide film and 1 percent resistors. The filing method is acceptable but seal the resistor with clear nail polish or they suck up moisture.

#### pebe

Joined Oct 11, 2004
626
A 10ohm and a 1.5ohm resistor in series.

#### thingmaker3

Joined May 16, 2005
5,084
3) Let's make that three questions. Where do hobbyist find parts? [/quote]

Surplus electronics places can be found in most medium or larger cities. One can also hook up with radio clubs, robot clubs, and the like to find out when they hold their swap-meets. Ebay works for some items.

#### Syed

Joined Dec 5, 2005
6
3 to 4 diodes(connected in series) can drop the required voltage. For AC one can use 3 for one half cycle and 3 for the other.
Happy micro-fiching,
Syed

#### Gadget

Joined Jan 10, 2006
614
Thats over 4 amps thru them diodes. They should be rated to suit, and they may get a little hot. Not the method I would have used.

#### mrmeval

Joined Jun 30, 2006
833
Can you open that up and take a look inside. I suspect there is a 120v to 13.8v transformer in there.

12V at 6.6 amps
Digikey part# 237-1276-ND \$26.84 Triad part# VPS24-3300
I think that is with the output wired in parallel but I'm not sure how that is supposed to be done.
Ah here's how it's done
http://rocky.digikey.com/WebLib/Triad Magnetics/Web Data/VPS24-3300.pdf

For the money it should do the job and easily handle the current.

Thread Starter

#### Chris Wright

Joined Jul 26, 2006
62
Thanks Guys for all of the great responses!

If I were using the fiche reader daily, the transformer would be the way to go. But I got to thinking, I only use it for an hour or so once every couple of weeks. So I fell back on a tried and true method: I plugged in the bulb and smoke tested it. I'll see how long it lasts and then decide.

For the 1% 11.5 ohm resistor I've been sorting through a pile of resistors looking for a 10% 11 or 12 ohm out of tolerance or a combination of values to make 11.5. No luck yet and I haven't tried nicking a 10 ohm to make an 11.5 yet, that's next. Why does nicking the surface cause the resistance to increase anyway?

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

It only works for carbon resistors. The cylinder of carbon composition inside the phenolic cover is a bulk resistance, so the cross-sectional area bears on total resistance.

There is a joke that says that you can make a 1 meg resistor starting with a 1 ohm 2 watt carbon resistor and a triangular file. Just keep filing the notch until the meter reads 1,000,000 ohms.

#### rukrazy?

Joined Mar 5, 2005
21
Why not replace the lamp assembly with one from auto parts store. They have small halogen bulbs that must be cheaper.

Thread Starter

#### Chris Wright

Joined Jul 26, 2006
62
Why not replace the lamp assembly with one from auto parts store. They have small halogen bulbs that must be cheaper.
There is no lamp assembly or socket per se, The bulb is an MR 16 form factor, with a small low voltage bulb permanently mounted in a glass reflector whose flanges slide into a couple of wire bails that hold it in place.

The auto bulb would still only be rated at 12 volts. This is one of the reasons I just put the bulb in and tried it. Autos run between 13 to 14 volts to recharge the battery, so unless they are under rating their bulbs, they last at this voltage.

#### Joe S.

Joined Aug 28, 2006
10
Go to an auto parts store, cars are only 12 volt in theroy, when running they will go 13.8 up to 14.2 volts, and I doubt it would make all that large of a difference if you used a 12 volt lamp. Hella makes all sorts of lamps and they are marketed for use in fog lamps, driving lights, police car light bars, marine applications, aircraft lights. Federal sign uses Halogen and they market through most police equipment dealers (check Pursuit Specialties), there are others.
these lamps are available in all sorts of bases, and sizes, and wattages, and are also available as spotlights and other type of sealed beam lamps.Aircraft Spruce may sell them check their website. http://www.aircraftspruce.com/

Happy Hunting

#### Joe S.

Joined Aug 28, 2006
10
a final thought,
Home Depot and probably everyone else sells light fixtures which use very similar lamps, buy one - gut it for the transformer, lamp and socket and then use cheap lamps forever.

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