hi,

i have a 12v 3W lamp on hand. The 12V is refer to the RMS voltage or the Vpeak? if i have a Vrms = 15Vrms output which i measure using a DMM, will it blow the 12V 3W lamp i bought?

thank you.

 

It almost certainly refers to 12 Vrms. You might be able to run the light at 15 Vrms, but you'll reduce the life of the filament, perhaps drastically.

If you have to use the 15 Vrms, consider finding a 3 Vrms lamp and put it in series. Then the 12 Vrms lamp should work fine.

Now 12 V at 3 W is 1/4 A, so you could also get away with using a resistor. The needed resistance is 3/(1/4) = 12 ohms. The power is (1/16)12 = 3/4 W, so a 1 or 2 W resistor would work fine. You could put four 48 ohm 1/4 W resistors in parallel.

 

If you have to use the 15 Vrms, consider finding a 3 Vrms lamp and put it in series. Then the 12 Vrms lamp should work fine.

You need to make sure that the 3v lamp has the right wattage rating to maintain that 3v drop. Higher wattage means lower resistance which puts more than 12v on your lamp. Too low of a wattage means higher resistance which will put less than 12v across your lamp. I think a 3/4 watt, 3v lamp is what you want if you want to go this way.