ohm's law

[induction1]

i want to limit the current for a dc circuit, but my book is confusing concerning ohm's law. it leads me to two different equations. it's a 12 volt, 4 amp device. anyway, it's volts/ohms = current right? 12 v/4 ohms = 3 amps ??????? please help clear this up, thanks.

 

[Roff]

True, but it may not be that simple. If the device you are trying to limit the current to has resistance Rload, and you add Rlimit in series with it, the current I will be

I=Vsupply/(Rload+Rlimit)

If you know 3 of the 4 variables, you can solve for the 4th. For example, if you know you have Rload=3 ohms, a 12v supply, and you want to limit the current to 3 amps, then Rlimit=1 ohm.

 

[maria]

From ur question, i can only find this out that may be the device u are dealing with to limit the current have 4 ohms of resistance, thats why in ur book 12 v/4 ohms = 3 amps is given.so far as the question is related that y its using 3 amps and not 4 amps, so it may be due to safety factor.
according to a rule for safety, one shouldnt use the extreme value for any thing but should go for a little smaller value. here ur extereme value is 4 amps and the moderate good value is 3 amps.
maria

 

[Klaus]

Read your own question again, carefully, and you might spot your own error.
12V, 4 AMP device

Resistance= 12V/4A = 3 OHM

Current = 12V/ 3 Ohm= 4Amp

Clear as mud??? :?