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]

Despite that the current is low you could have fried your device testing like that.

[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

[JustMichael]

RayB,  when you have a break in the circuit and then stick the meter in the break to meaure the voltage you will get almost the whole supply voltage.  This is because the several meg-ohm resistor inside the meter completes the circuit and is also where most of the voltage drop occurs.

To measure the voltage drop of a resistor in a circuit, first the circuit must be operating.  Second set the meter to measure dc voltage (in this case), also set the proper range too.  Touch the probes to opposite sides of the resistor while it is working.  This will give you the voltage dropped by this resistor.

Here is how to calculate the resistance needed for a led:
 (Supply voltage - voltage required by led [if a range of values is listed, use the smallest value of the range]) / (max current led is supposed to receive) = least amount of resistance needed

Example:

Supply voltage is 5V
Led's voltage is .7V
Led max current is 20mA

 (5 - .7 ) / (.020 [.020 is the same as 20mA])
(4.3) / (.02)
and it equals 215.  So in this case we would need at least a 215 ohms resistor.  A common value resistor near this is the 220ohm resistor which will work just fine.

I actually have my atari volcano buttons thooked up to the Ipac using one 220 ohm resistor on each led.

[RayB]

Very educational. Thanks all of you.

[mahuti]

I guess I should really learn how to use my multimeter. I still can't figure out how to use it on my pinball machine coils to see how jacked up they are.

One of these days I guess I should get online and look at a tutorial on electrical junk.