arduino - How can a higher power circuit share the ground with a lower powered circuit?

Just when you think you understand electricity, it throws another curve ball at you.

The following circuit confuses me. The Arduino has it's own power circuit supplied by either batteries or a transformer at +3V. The light bulb is powered somewhere between 0 and 60V by a separate circuit. Its ground, however, is connected to the Arduino's ground pin. To further confuse me, the gate is being driven by the Arduino, but its pulled down resistor is also tied to that ground.

I was under the impression that high powered circuits should be electrically isolated from the lower powered digital circuits. How can this work without destroying the Arduino? Why would you want to do this and where does the current sink to? The Arduino circuit or the external circuit?

Answer

Because of a basic rule of electricity, current only flows around circuit loops.

When you have two circuits, with no other connection between them, you can tie them together at ANY point to form a common reference voltage as shown below. No current will actually pass thought that connection without a second connection to complete a loop.

^{simulate this circuit – Schematic created using CircuitLab}

Once you have created that common reference it is then possible to pass signal voltages between each side as shown here.

^{simulate this circuit}

If you notice, the current driving the lamp only returns on the 60V side, while the base current returns on the Arduino side.

This is quite safe and normal.

The issue with mixing grounds occcurs when you pass the returning high current through the control sides low current circuits as shown below.

That creates noise and reference level changes on the control side which can cause unpredictable behaviour in the low voltage electronics.

The circuit in your image falls into the correct wiring category.

ADDITION FOR COMPLETENESS.

Note I said... "At any point". The following circuit is ALSO acceptable.

^{simulate this circuit}