please check for me

[JC@]

I would like to use a 12 volt / 3watt bulb fed by a 43 volt ac power supply.
Is a 3k2 / .3watt the resistor I have to put in series?

Thanks a lot.

[Enzo]

I'd like to see how you got there.

I get 124 ohms dissipating 7.75 watts

Using 12v and 3w I get the following:

To make 3 watts, 12v across a bulb must be flowing at 0.25 amp. That implies a hot resistance of 48 ohms.

The supply is 43v, so we need to drop 31v across a series resistor. Current through the resistor wil be the same as through the bulb - 0.25 amp.

E=IxR, so 31v across R with 0.25A means R must be 31/0.25 = 124

Power? ExE/R = 31x31/124 = 7.75W So a 15 watt resistor would be about right.

ANyone spot a glaring error in my figures? My day is just starting.

Is your 43v supply up to providing 11 watts over and above whatever else it is doing?

Might a 31v zener work? Or a pair of them for AC.

What are you actually doing here?

[JC@]

I'd like to see how you got there.

Me too
I failed here: I=W/E, and for some reason I did 3/31=.0097 instead of 3/12=.25
I found the calculator just before I read your post, and of course you're right.
It is 124 ohms, 7.75 watt
Thank you.

What I try to use is a 12 volt/3W automotive bulb replacing the 48 volt/4W bulb in an ebs td600

[JC@]

I decided to order a replacement bulb. A lot easier, but I have to wait a few days.