Hello all,
I have an electronic ignition system for an R/C gas engine and want to monitor the voltage on the battery that powers the sparks, so that I can know when to stop flying and change the batteries. Obviously, I cannot connect an Arduino on the same power supply as the ignition system (the sparks are ~20,000 volts, while the input into the ignition system is 4.8-8.4 V).
So, I want to input the voltage from the power supply into an analog input pin on the Arduino (I will use a simple voltage divider system to read voltages over 5V), and power the Arduino on a different source. But in order to get a meaningful read they need to have the same ground. Can I have them connected to the same ground without transferring the noise? This must be a very easy question.
Also, I would need to filter the voltage level input into the Arduino (or does Arduino have built in protection?). Would a simple low pass filter do?
Use a common ground and run the signal + ground on a twisted pair or shielded
cable from ignition battery to the Arduino/voltage divider (well the divider can be at
either end).
A 100nF capacitor between the analog pin and ground at the Arduino end will
help reject RFI from the spark - keeping the Arduino away from the ignition and
preferably shielded will help.
There are three ways that you can get coupling between the ignition circuit and the monitoring circuit.
One is resistive, where the current in the ignition circuit produces a voltage drop between two points which become part of the monitoring circuit. Essentially, you want the monitoring leads to be directly across the battery terminals - or at least at the battery connector for a model.
One is inductive, where the current flows in a loop and another loop exists in the monitoring circuit and the two loops couple like a transformer. All wires in each circuit should run parallel and close (but separate for the two circuits), at least as "ribbon" or "figure 8" wire, possibly twisted as MarkT points out. Preferably both the (low voltage) wires in the ignition circuit and the monitoring circuit.
The third is capacitive, where the electrostatic field is coupled particularly from the high voltage circuit to the monitoring circuit (note that both the monitoring wires and the power wires to the monitoring system are concerned in all three scenarios ). This is dealt with by keeping high voltage wiring as far away from the monitoring circuit as possible, and interposing a shield (connected to the ground or negative wire of the supply) between the high voltage circuit and the monitoring circuit. One approach of this is indeed, to use shielded wire from the monitoring point to inside the case shielding of the monitoring unit.