Supplying 5v to A1 powers on arduino?

Projects General Guidance
1

Hey guys so I've been working on a project with a relay board and a pro mini and I'm having a small issue.

Some background....I have a 3, 12v-5v inverters in this setup. 1 powers the pro mini and the relay board. The other 2 step down a 12v signal to 5v.

When there is no power on the 12v-5v inverter powering the arduino and relay board and power is applied to either of the other inverters, the arduino powers up and runs it's program.

I do not want the other 2 inverters to power the arduino. I only want the inverter connected to VCC to power the arduino.

When power is applied to one of the step down inverters( using A1 and A2) , VCC on the pro mini is 3.xx volts.

Only the inverter powering the arduino and relay board is grounded to the pro mini. The other 2 inverters are not grounded on the 5v side; they are just supplying a signal to A1 and A2. (Grounding the step down inverters did not change anything)

All the invertors have the same ground.

The relay board is grounded to the arduino. The arduino is grounded to the 5v side of the inverter powering the pro mini and the relay board. They do not have a straight ground to the power supply. Should I run a straight ground to the power supply?

Thanks in advanced for any input!

2

I found that this issue is called back powering or parasitic powering.

I'm not sure what to do, to solve this issue though.

I read you can use a transistor but I fail to see why a transistor would work different than the 12v-5v inverters I am currently using

3

A1 and A2- analog inputs?

Simple - do not connect any external power source to an analog (or digital) input. Only connect inputs from devices that are themselves powered from the Vcc on the same Arduino.

4

To solve this issue, connect a resistor between the inverter and pin A1 instead of connecting them directly. Similarly for A2. Use at least 10K, preferably more e.g. 47K.

5

Thanks for the replies, guys.

I will try the resistor method and see if it works.

The pro mini and A2 get power at the same time however, there are some cases where A2 will have 5v and the pro mini will not.

But with the arduino only pulling about 10ma or so, I may just wire it directly to my battery and integrate the sleep command in my code to avoid this issue all together.

Right now, the arduino is being powered off a user-activated circuit in my car. After 15 minutes of no activity, the power source goes to sleep, with the vehicle unlocked. With the car locked, the supply sleeps within 5 minutes.

6

Paul__B:
A1 and A2- analog inputs?

Simple - do not connect any external power source to an analog (or digital) input. Only connect inputs from devices that are themselves powered from the Vcc on the same Arduino.

Yes, they are analog inputs.

Can you propose another method for me to trigger these circuits?

A2 is the interior lighting in my car.
A1 is ignition.

I am using these as triggers. So when they are present, the arduino will do xxxxx.

7

So you do not actually need an analog function; it just happens to be convenient to use those pins.

For a digital input, a 100k resistor would do just fine and feed negligible current in when the Arduino was powered off - quite safe.

You could however isolate the input with a simple NPN transistor, 47k to the base, a 10k pull-down on the base - which means there has to be at least 3½ volts to trip it, and use the internal pull-up of the Arduino (which is also present on "analog" inputs*) to bias the collector.

You could even use an opto-coupler.

(* - minor exception to this should not worry you)