Hey guys,
Sorry if this is a stupid question, I am absolutely new to electronics.
I am missing roughly 20 mA in my circuit. There should be 160 mA going in and out of my board considering that I connected 16 LEDs each drawing 10 mA(+-0,2mA). But I measure only 140 mA.
Is this just an inaccuracy of my multimeter? Its tolerance in the range of 160mA says 2%+30 which I interpret as 33 mA. This seems like way too much though? Or are these kind of inaccuracies in that range normal for multimeters?
Another anomaly is that when I disconnect the power from the 5V of the arduino to the VCC of the shift registers the LEDs are still on, albeit much dimmer. I then measure ca. 30 mA in the cable going from a digital pin to the ST_CP pins. The LEDs are getting 3-4 mA which again doesn't add up. I don't understand why or how or where from exactly the LEDs are still getting their power.
I'd appreciate any help, thank you.
Hi,
Measure the voltage across the series resistors to the LEDs and calculate the actual current.
Your DMM will add some resistance to the circuit when you break the circuit and insert it.
Another anomaly is that when I disconnect the power from the 5V of the arduino to the VCC of the shift registers the LEDs are still on, albeit much dimmer. I then measure ca. 30 mA in the cable going from a digital pin to the ST_CP pins. The LEDs are getting 3-4 mA which again doesn't add up. I don't understand why or how or where from exactly the LEDs are still getting their power.
This because the Arduino is still powered up and current is being drawn through the control wires, as you have measured.
The LEDs can still produce light output at low currents.
It sounds like your LEDs are of the high efficiency type.
This is normal and why most circuits have an ON/OFF switch that disconnects at the power source rather than at each individual major component.
If you want to check your LEDs, then power one up with series resistors higher than what you are currently using.
See how small amount of current is needed to give you a reasonable light output.
Use OHMs law to calculate current through the LED.
Tom.. 
shockfield:
Sorry if this is a stupid question, I am absolutely new to electronics.
I am missing roughly 20 mA in my circuit. There should be 160 mA going in and out of my board considering that I connected 16 LEDs each drawing 10 mA(+-0,2mA). But I measure only 140 mA.
That's right. Going into the board will be the amount of current that goes through the +ve supply wire. And the total current coming out from the board will be the amount of current that goes through the wire that's directly connected to the -ve terminal of your power supply. Whatever amount of current comes out of your power supply (through one terminal) will return to the power supply (through the other terminal). The currents through those particular LEDs contribute to the returning current.....but they're not the only returning currents, since the circuit boards connected to the power supply has circuit branches (with currents) that also contribute to the total return current.
