Hello people

I am a newbee.
All I just want to know how do I calculate what Resistor will I need.

Example: I got a 12v DC Adapter with a 3v Bulb. Now I want to put in a resistor to bring down the Voltage so the bulb can glo and not blow.
Thanks.

 

You use ohms law, V=I*R, you can rearrange it to R=V/I to find the resistance you need.

However, you need to know the current the bulb will take (I) to work it out, V is 12V-3V, giving 9V.

You will also need to work out the wattage of the resistor, this is done by W=V*I.

 

Hey nigel nice usage of watts law and ohm law but don't forget he can figure out power with Isquared*R!

 

Or Vsquared/R - but both are only a combination of the two formulas I gave - simple algebra.

 

but the Isquared*r is a little easier with the resister in the circuit because simple ohms law can be used to find your watts instead of having to use watts law. though I know both I personally prefer Isquared*R so we are both correct because both ways will work its just personal preference.

 

6 in one hand, 1/2 dozen in the other

 

Well I must admit, I've never seen it called 'watts law', but I suppose it's as good as anything else :lol:

But I'm totally baffled by your assertion that you can find power just from ohms law, particularly when you quote 'Isquared*r' which is a combination of ohms law and 'watt's law' exactly the same as Vsquared/r is :?

 

I want to point out that there is a problem in having too large a resistor in series with an incandescent lamp. The lamp resistance increases with temperature which increases the power dissapated which increases the resistance which --- you see where this is going?

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You would measure the current through the lamp at 3 Volt and when the lamp filament is hot. Then calculate the lamp resistance using this figure, this will be the "hot" resistance. Then calculate the series resistor using Ohm's law as Nigel said.

As for the resistor power calculation, it is easier, in this case, to use V * I since both are known. Why square current or voltage when you don't need to?

Len

 

Which was why I suggested it of course 8) but it's hardly a concern these days when a few button presses does it for you.

But I come from the days before calculators, and automatically tend to go for the easiest option :lol: