resistor

[plen]

what is the best resistor to use for a pilot light in a clone shin ei univibe. i think the bulb is 6.2v.

thanks.

[Boner]

V+ ----- *resistor* ------*bulb* ------- 0V
current through resistor = current through bulb, lets say its 10mA (current required to turn on bulb) and your high rail is 18.


(V+ - 6.2)/r = 0.010   ---> (18-6.2)/x=0.010, then solve for x

I think.

[antonis]

Excuse me but why you need a resistor..??
As far as I know, pilot bulb on Univibe is directly fed from a separate secondary winding of 6.3V..

If you refer on +9V supply effect, consider remaining voltage drop (9 - 6.2 = 2.8V) divided by current drawn by bulb..
(roughly estimated by dividing its power rating with voltage rating..)

[plen]

i am using an 18v wall power supply.

[antonis]

In such a case, there are about 12V (11.8 to be precise) to be dropped across the resistor..
(as Boner edited..)

So, check your bulb power rating and divide it with voltage rating to estimate current..
e.g. for 25W, current is 4A.. for 15W, current is 2.4A and so on..

Resistor value is set by Ohm's law, V = I X R, where V = voltage drop (12V in present case) and I = Current through resistor (the one calculated above from bulb's Wattage & Voltage..)

Beware of resistor power rating which MUST be higher enough than  V X I or I² X R or V²/R..
For a closed enclosure I'd suggest about twice the calculated power rating..

P.S.
I'd suggest the use of 12V bulb for resistor of lower wattage - you may end up with 24V bulb directly connected to 18V with enough light..
Alternatively, use a 10mA LED with 1k5 resistor of 1/4W and earn peace of mind..