I am currently building a circuit consisting of:
- I2C Extender with P82B715
- Shift register SN74HC595
- ADC MCP3208
- NTC 10k Thermistor
- A few LED with 220Ohm resistors
- 5V Power relay
The arduino NANO is powered and read from using a serial USB connection, where peripheriel parts are powered using an external power module. The datasheet says it can produce up to 700mA and its connected via a 220V AC-> 12V DC 1A power plug. (see schematic). In this test setup I use a breadboard and ordinary jumper cables (-> noisy but in tolerance). I also used the same COM (GND) for the Arduino and the circuit.
Now to my problem. When I only measure the temperature (everything else is idle) I get correct values for it. After switching on the LED I notice a voltage drop from the +5V-GND supply on my breadboard of 1mV (not too bad maybe noise). But when I switch the coil of the power relay the voltage drop is 240mV (drawing 57mA).
Since the drop is across the board the thermistor also notices it and reduces its voltage, therefore I get a different reading at the same temperature (this is how I noticed the drop in the first place).
The idle circuit draws 132mA, with the relay on 183mA, and with the LEDs 195mA.
Why is there even a voltage drop? Did I miss something in the wiring?
Any help is appreciated.
Testcurcuit.pdf (493 KB)