Hi!

[Nils Magnus Englund]

I have a 10W 12V light bulb which I want to install a "fader" on - so when
the switch is turned on, the bulb uses around 1-2 seconds to fade up, and
then fades down again in the same amount of time when you turn the switch
off.

How could I do this? Shouldn't I be able to use a capacitor? (How much
capacitance would it need?) Any other things I could do?


Regards,
Nils Magnus Englund

 

[Graham Knott]

Jamie wrote:

use a common NPN tranny found at radio shaft that can handle
at least 2 amps or more.
most likely found in a T-220 style case.
use a heatShink on it.
you will need a resistor and CAP on the Base .
12 Volts into the resistor.
12 Volts to one side of the Bulb.
the other side of the resistor is connected to the
Base of the tranny.
the Cap (+) side is also connected to the base of the
tranny..
the emitter to ground, the collector to the other side of the
bulb.

first the size of the resistor..
since its obvious the bulb is going to draw aprox.
.8 amps and the average Hfe on a Npn transistor is around
45..70.
so i am will assume 50.
after doing some cals i can see that using a common
560 Ohm resistor should do it around a 1/4 watt or maybe
a 1/2 watt to be safe.
and using a 1000 Uf should be enough to give you aprox
2..3 second rate.

P.s.
you may want to consider running the Transistor in/near saturation
level if you plan on keeping the bulb on.
this will help reduce heating in the transistor.
using a 470 ohm resistor may be better.
as it is basically near that point already.
you should get 0.6 volts less than your source at the bulb if all
goes well.

this is off the top of my head but should do you.

Wont you have to discharge the capacitor thro a resistor at switch off?

Nils Magnus Englund wrote:

I have a 10W 12V light bulb which I want to install a "fader" on -
so when the switch is turned on, the bulb uses around 1-2 seconds to
fade up, and then fades down again in the same amount of time when
you turn the switch off.

How could I do this? Shouldn't I be able to use a capacitor? (How
much capacitance would it need?) Any other things I could do?


Regards,
Nils Magnus Englund

--
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Electronics for Beginners & Intermediate Electronics
http://homepage.ntlworld.com/g.knott/index.htm