Circuit Desing question

[nirVaan]

in designing circuits do you ever come across situations where you must have a certain voltage across as well as a certain current through a circuit component at the same time.if so how would you achieve this?

by using resisters you can one control either the required voltage or the required current at one given time right?

 

[DigiTan]

All the time. Say you have a 12V battery and a 3V lightbulb. When the light's instructions ask for a certain number of volts and amps, you've gotta use resistors or anything else you can throw at the problem to get things balanced right. Ohm's Law, Kirchoff's Law, Series and Parallel...they're all just tools for getting the right voltage and right current to the right location.

Hands down, my best recommendation is allaboutcircuits.com. I don't know what your experience level is, but if you can get through Ohm's Law to Series/Parallel circuits...and practice it enough for it be second nature...you'll have no problem with DC systems.

 

[ExtravDreams]

Well, a lightbulb has a specific impedance, which when a voltage is applied across automatically dictates the current.

The only example I can think of is a diode/transistor, where you need a minimum voltage to activate the device, yet need to pay attention to the maximum current rating.

 

[DigiTan]

Of course. Whether it's something linear like a bulb, or something with a 'success window' like a diode, you've got to work with what the circuit demands.

The other thing I forgot to mention is the Shaum's Outline textbook series. It's basically the "Cliff Notes" of electrical theory and also works out problems step-by-step.

 

[nirVaan]

well im just a beginner. thankz for the web site link.
if i take what you have said

you have a 12V battery and a 3V lightbulb and say for example it needs 2 A to fully light it self then i should have a resister that has a 9 V drop across it in series with the bulb that would provide 2A current

so according to V=IR

9=2*R

then R should be 4.5 ohms is this correct
if so how do i account for the internal resistance of the battery

 

[sniper007]

if so how do i account for the internal resistance of the battery

Why exactly you need resistance of the battery?
However one simply method is the DC load test which applies a discharge current to the battery while measuring the voltage drop. Voltage over current provides the internal resistance.

**broken link removed**

 

[audioguru]

If the internal resistance of a battery affects the voltage to your circuit then the battery is way too small or it is dead.

 

[DigiTan]

then R should be 4.5 ohms is this correct
if so how do i account for the internal resistance of the battery

That's correct. Generally, battery internal resistance is assumed to be negligible. But if it's a need-to-know matter: you can either (1) locate the battery's datasheet, or (2) discharge the battery in the lab to plot the voltage decay over time.

 

[nirVaan]

thankz guyz