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Resistor woes
RayB:
OK so today I decided I would test out LED lighting off of my JPAC. I have Atari lit buttons, and I also bought a 20-pack of resistors from Radio Shack. I had already determined that I need 330 ohm resistors.
The 20-pack comes with 4 different resistors. 100, 220, 330 and 470 ohm. So I picked out 4 resistors that where different (they label these things with colored bands on the resistor). Not being online at the time, I figured I could just test each one with a volt meter.
So first I plugged the little harness that came with my JPAC. There are two wires on it. I figured out which was ground and which was voltage. I measured 4.78vdc.
So next step, I attached a resistor to that powered wire, and read the voltage coming out. Still 4.77. I thought maybe resistors are one-way? So I flipped it aroundand measured voltage again. Still 4.77. I triedall 4 varieties of resistor I have and all 4 resulted in readings of 4.77vdc.
So of course I never got around to attaching/testing one of the lit Atari buttons. I didn't want to risk blowing the bulb.
So what's going on here?
SirPeale:
First, you should have printed out a color chart before you started a stunt like that. Despite that the current is low you could have fried your device testing like that. Five minutes online to print it out would save you a lot of heartache.
Since you didn't blow it, however, the proper way to test a resistor is to set your multimeter to its Ohms setting. Grasp the probe in one hand and touch it to an end. Touch *only* the other probe to the other end (don't contact it with your skin, you'll throw the resistance off) and you'll get your Ohms reading.
JustMichael:
Also do not have the resistor connected to anything when checking its resistance.
RayB:
--- Quote from: Peale on October 02, 2005, 07:41:00 am ---Despite that the current is low you could have fried your device testing like that.
--- End quote ---
BobA:
Since you did not close the loop with the LED there was almost no current flow thru the resister. Only the current needed to take the voltage reading was there so as a result you were measuring the terminal volatage of your 5V supply thru the IPAC. Any resistor would give you the same voltage reading.
Read the resistance not the voltage. Voltage reading will only be valid with the proper current thru the LED and resistor in series. In which case the resistor must be properly sized or excessive current could pop you LED or even damage your IPAC.
BobA