Trying (and failing) to test a circuit

MillenniumFalsehood · Post by **MillenniumFalsehood** » Fri Mar 30, 2012 5:02 pm

I am embarrassed to say that I just had electronics in class a couple weeks ago, but this stuff is not my forte so once again I need help (I'm mechanically inclined, not electrically inclined; electronics will probably always be a mystery to me).

What I want to do is determine what size resistor to put on this circuit. I've got a couple strings of LEDs wired in parallel to a pair of nodes where I can hook them up to my power source. Each circuit is for two different types of LEDs, red and white, so they need different resistors because their power draws are different. The power source will be either 6v or 12v, and I need to put a big resistor on it to take the power down for each circuit of LEDs.

Thing is, I'm trying to get my little Craftsman circuit tester to tell me how much resistance the circuits have in order that I can plug the numbers into Ohm's Law (V=IR) and find out how big a resistor I need to put on the LEDs.

rayra · Post by **rayra** » Fri Mar 30, 2012 7:50 pm

I just put an extensive post on just that subject in one of the stuck topics, one week ago. Ohm's Law is the equation you need. My post is near the bottomg, on the 23rd
http://www.starshipmodeler.net/talk/vie ... hp?t=38441

eta - you already have values for V and I, you're solving for R. You're working it backwards. Just use basic algebra to plug in the already known values. I spell it out in that post.

vixr · Post by **vixr** » Fri Mar 30, 2012 10:37 pm

Sparky · Post by **Sparky** » Thu Apr 05, 2012 3:28 am

you will not be able to measure resistance of the LEDs to tell you how much resistance you need to consume the over age power.

The info you need still involves Ohm's Law:
First for parallel circuits know that current is cumulative.
So if you have say 4 white LEDs in parallel each drawing 0.02 Amps (20 mA) then the main feed line will be passing 0.08 Amps.

Next you need to know that the voltage drop across the parallel feed is not cumulative, it will be constant, so if the LEDs want 3.3 volts each the main feed better have 3.3 volts at the node.

So now you can plug in your numbers into Ohm's law to find out how much voltage needs to be dropped by the resistor:
(Power supply (6 or 12 volts) -3.3 volts for the node)/ total current (0.08) = Resistor value in ohm's

In this configuration it is better to have lower voltage power supply with more current (bigger battery).

For the other parallel circuit you can apply the same technique. However the resistor values you end up with should not be placed in parallel with each other, treat the resistor to parallel bundle as one feed going directly to the power supply.

You will not be able to have separate voltage levels at the node, if the whites want 3.3 and reds want something else they will each need their own resistor so that the voltage delivered to the LED bundle node is correct for each LED type.

Sparky · Post by **Sparky** » Thu Apr 05, 2012 4:15 am

If that's not clear, I can draw something up tomorrow at work.

qaz111 · Post by **qaz111** » Mon Apr 16, 2012 12:20 am

Just ignore: SPAM-bot