I have been playing with an ohms calculator as I am currently working on a project requiring external power. I have a 9 volt dc plug that I want to use. I only need 7 volts. When plugging in numbers on the ohms calc, this is what I get:
20ma from arduino
So I assume that if I set this up on a breadboard, I will have a 7v power source.
I don't understand this. You have a 9vDC power plug, you want 7v. I'm not sure what you were calculating or what a 20mA Arduino has to do with it. ??
Like others have said, if you're thinking of "shedding two volts by using a resistor", it doesn't work that way. Well... according to Ohm's Law it does, but the load (your servo in this case) would have to present a 100% consistent current draw to the power supply, which rarely ever is the case. Thus, the voltage will not be 7v except when the current demand is exactly where you calculated it. Less current, and the voltage will be much higher. More current, and it will be much lower. When the servo is not "on", its current draw will be almost nil and the input voltage will be nearly 9v.
Here is the riddle: I set up the arduino just to light up an led with the 5 volt pin and ground.
Again, I'm not following. You put an LED between the 5v power supply and ground? BTW, that's a good way to turn your LED into a broken LED. Don't do that without a current-limiting resistor between the LED and either (doesn't matter which) power rail.
I do a read with the MM and it reads 4.98, great. So I set up a 330 resistor, but the voltage is the same. I set up 2 10ohm resistors with it for 350 and same voltage. So what the heck is the calculator telling me? Is the voltage less even though the MM says it is still 4.98 volts?
You need to be more specific with the phrasing. You "set up a resistor" how/where? In series, between the power and the Arduino? Adding resistance from the power supply is almost never a good thing. Usually, it just prevents the load device from working properly, and possibly heats up and ruins the resistor (depending on how much voltage it drops as heat). Hardly ever useful though.
P.s. Is it possible that even though the MM reads the same volts, the actual voltage is different via by way of reducing amps?
No. If I'm following your experiment correctly, and you have 5vDC to a resistor, and the resistor to your Arduino, and the Arduino to ground.... what happens is you have two resistors (one a constant value physical resistor, and the Arduino, whose resistance will vary based on its load) forming a voltage divider. The voltage at the point where they meet will vary as the current demands placed on the Arduino vary. With the Arduino doing nothing (not driving any LEDs or anything), its current draw will be very minimal, but would likely still cause a current-limited power supply voltage to drop a little. The voltage at the power supply terminal (north of your resistor) should stay constant though.