Re: Newbie:Current and voltage draw through light bulb

On Wed, 31 Jan 2007 09:59:23 -0500, "Michael" <N@xxxxxxxxxx> wrote:

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?

---
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.
---

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

---
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.
---

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)

---
Most wall-warts put out their rated voltage when they're fully
loaded, so in your case, since your load has an odd voltage
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.
---

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?

---
If I wanted to dim the bulb I'd start with a higher supply voltage
and use an LM317 and a pot to do the dimming.
---

Thanks for you assistance

---
:-)


--
JF
.

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