Basic Electronics (Ohm's Law)

[gregw66]

If I have a 1.5v lamp and want to run 15ma through it according to ohm's law it will have 100 ohms of resistance.

If I put that in a 12v circuit I need a resister to drop the voltage and limit the current. Therefore, according to ohms law I need about a 700 ohm resistor.

This will run .1575 watts so a 1/4 watt resistor should do.

If my calculations of series circuits is correct, then I should brightly light a 1.5v 15ma light bulb with a 700 ohm resistor. However, my memory tells me that 700 is not a standard value so I would likely have to go to 750, thus not running the bulb at full voltage and current.

My point to all this is to check to see if anyone agrees with me and to confirm I haven't lost all my electronics knowledge. Reaching back 20+ years to recall this stuff.

[Mark R.]

Correct - Either a 680 or 750 ohm resistor will work just fine, but as you said, the 750 will put slightly less stress on the bulb.

You wattage rating should be 1/2 watt resistor though .... 1/4 watt is .125 which is less than the calculated .1575.


Mark.

[alco539]

Hi, Mark,

My Electrical theory is kind of "rusty", but in the my machine shop, .125 is 1/8 . A 1/4 is .250, so if he needs a value greater than .1575, Greg should be OK. Unless there is some kind of inverse law of resistance I've forgotten, I'm serious. Also, I'm not trying to put you down, like they do on some of the other forums, but that's my gentle correction, I hope. It wouldn't hurt to use 1/2 anyway. Charley.

[Mark R.]

Man, where did that come from ? I apparently looked at the "25", thinking it was one fourth of 100 and just stuck the common 1 in front !

They say the mind is the first thing to go !

You are correct Charley, as is the OP with all his calculations.


Mark.

[gregw66]

Thanks for your confirmations. Appreciated! ;D

[alco539]

I know the feeling Mark, and by the way, the mind isn't the first thing!
Regards Charley.

[nscaler711]

this helped me out too guys thanks!!
I'm 23 so I think it was the knees to go out first... sure doesn't help when your trying to pass your run on a APFT test lol.