# Newbie:Current and voltage draw through light bulb

Discussion in 'Electronic Basics' started by Michael, Jan 31, 2007.

1. ### MichaelGuest

I am fairly new to this and have several (simple) question:

I have a krypton flashlight bulb (markings on bulb say 4.8v .5A). The
original flashlight that the bulb was removed from used 4 AA batteries.

1. Wouldn't the 6v from the batteries be 'pushing' the bulb a bit and
reducing its life?

2. If I want to wall-wart power this bulb, what voltage wall wart would be
recommended?

3. Since most wall warts (unregulated) supply higher voltage than the
sticker says, what method
should I use to bring down the voltage to the recommended voltage
(regulator, resistor, etc)
(I remember once trying to do something similar to this project and using a
resistor. Even
though the resistor was a high wattage resistor it got REALLY hot)

4. I think this question probably will have the same answer as #3, but if I
want to dim the bulb what
method would be recommended?

Thanks for you assistance

2. ### John FieldsGuest

---
For alkalines, yes, but at 1/2 an amp the internal resistance of the
cells would drop the voltage somewhat, and as the batteries were
drained the voltage would also drop. For NiCd and NiMH
rechargeables I believe their "flat" voltage region is around
1.2V/cell, so that would work perfectly.
---
---
4.8V @ 0.5A, but since that's not a readily available value I'd get
one rated at 5V @ 0.5A and drop the voltage to the lamp with a
resistor:

Vin - Vlamp 5.0V - 3.8V
R = ------------- = ------------- = 2.4 ohms
Ilamp 0.5A

The resistor would need to dissipate:

P = IE = Ilamp * Vin - Vlamp = 0.5A * 1.2V = 0.6 watts

Which means you should use a resistor capable of dissipating at
least 1 watt; a higher wattage allowing the resistor to run cooler.
---
---
Most wall-warts put out their rated voltage when they're fully
requirement you'd want to pick a wall-wart with a slightly higher
voltage but which is rated for the same output current as your load,
then drop the extra voltage with a series resistor, as illustrated
above.

3. ### Homer J SimpsonGuest

When you draw .5 A from the batteries you wind up with 4.8 V (not strictly
true but it works).
4.5 V.
Use a multi voltage unit, run at 3 V.

4. ### jasenGuest

5V ones are fairly common, use one of them.
5V ones are usually regulated , or you could use a 4,5V unregulated one
that'd probably be close enough.
to drop 0.2V at 0.5A you need a ideally a 0.4 ohm resistor, 0.39 will
probably be will close enough, actually 5V would be close enough.

usually it's just easier to buy a nightlight,

Bye.
Jasen

Thanks

Thanks