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