I will be programming an ATtiny85v, and I want to measure the current so that I can visibly "see" the power savings when I implement various sleeping functions and power saving tricks. So I am first trying to make a simple measurement of a simple circuit to prove that all is working as expected. But it is not.
I have a breadboard with an LED and a 2.2K (2200Ohm) resistor. I have the positive, long wire of the LED going to the 5V on the Arduino for a power supply. I then have the negative (flat side) of the LED connected to the resistor, and then the back side of that connected to the GRND on the Arduino. I have successfully measured the voltage coming from the Arduino 5V as 4.8V or so. I have measured the resistance of the resistor to 2,150Ohms. I am therefore expecting 2.23mA, but when I measure the current inline with the circuit between the 5V of the Arduino and the positive pin of the LED, I get 1.35mA. When I try using the 3.3V (3.28V measured) instead, I get 0.569mA instead of the expected 1.52mA. So my measurements are almost exactly 1mA lower than expected. Any ideas WHY? I am not confident in making accurate readings later if I cannot see this work as expected. Very strange. I may next need to try it out with a battery supply instead. Could my multimeter be faulty? As previously stated, I am getting correct results when measuring resistors and voltage. I am moving the red wire over to the "mA" port on the multimeter.
jrburke99,
You are forgetting the voltage drop of the led in your calculations.
A link to the LED datasheet or it's specs would help people help you.
TomJ
Does your multimeter have a range that lets you beep for continuity ?
The pictogram of that range most of the time also shows a diode (take a look at the attachment which shows what it looks like on my Amprobe multimeter).
If so, you can find the forward voltage of your LED by testing it with this range.
Now take the power supply voltage, and subtract the LED's forward voltage from that.
Use the result for your calculation.
If the current is 1mA less than you would get by connecting just the resistor across the power supply, then the voltage drop of your LED is about 1mA * 2.2K = 2.2V.
I don't know what DMM you are using but don't forget to factor in burden voltage into your equation if you want your results to be accurate. Current measurement on a device works by running your voltage across a resistor and measuring the voltage drop. At these low of currents, if you were using the 200mA range then likely you are fine as the voltage drop is on the order of a few mV. If you were using a 20mV range or a uV range then the voltage drop could be significant (up to a few hundred mV)
