But WHY do I have to ?!

[hydrated-narwhal]

Yo fam, I have a high current circuit that uses a Arduino to control a transistor. Why do i have to plug the power supply of the high current circuit to the GND pin of the Arduino to make it work?

My thinking is, Since the high current LED circuit has a transistor on it, it cant have current flowing through the Arduino because of the transistor. So why do I need to connect the arduino to the negative side of the high current circuit even though the arduino is already being powered up by a USB?

Sorry guys, Im a newbie here. I started learning about this stuff 2 weeks ago...

Thanks!

[sterretje]

The signal that the Arduino sends to the transistor needs a return path to the Arduino.

There are a few things wrong with your diagram

1. No resistor on the base of the transistor
2. All LEDs in parallel is not advisable.
3. No current limiting resistors for the LEDs
4. Probably the wrong type of transistor (NPN vs PNP).

[Grumpy_Mike]

@hydrated-narwhal

Why do i have to plug the power supply of the high current circuit to the GND pin of the Arduino to make it work?

To provide a common reference point to allow the current a path to flow through..
See:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

[DrDiettrich]

@hydrated-narwhal

why do I need to connect the arduino to the negative side of the high current circuit

Simply understand "electronic circuit". If control current shall flow through the BE diode of the transistor then there must exist a way from Arduino (pin) to base and back from emitter to the Arduino (GND).

[Geek_Emeritus]

the key word is circuit. a circuit is a loop. the electrons going out have to cause an equal number of electrons to return to complete the circuit.

things which are part of a circuit have one common connection point that point is ground.

[JohnRob]

To answer your specific question:

For a transistor to turn "ON" you must pass a current from the base to the emitter. If you only connect the base then the current cannot go out the emitter as it would have no where to go.

Since the UNO is supplying that current, the emitter must connect to the UNO ground to "accept" the current out of the emitter.

You could think of it as a light bulb, if you only connect one wire to a light bulb it will not light. Also the same with your LEDs, needs two wires.

[PerryBebbington]

Common ground and why you need one

A common mistake for people new to electronics is to have 2 circuits fed from different power supplies with signals passing between the circuits, but with the grounds not connected. This leads to questions asking why the project doesn't work. You don't have to spend long reading the questions on this forum to see this is asked often. Here I attempt to explain the problem and why it matters to connect the grounds. Note that in all of the schematics that follow I have used a battery as a power sup…

[hydrated-narwhal]

Wont the current from the high current circuit reach the arduino through the ground pin.?

[hydrated-narwhal]

Lets say a power supply on the has 8amp. Will the 8 amps reach arduino through the ground pin.?

Thank you all for the responses

[b707]

@hydrated-narwhal

Lets say a power supply on the has 8amp. Will the 8 amps reach arduino through the ground pin.?

You connected you phone charger (0.5A max) to the wall socket with rated current 10-16A. Why didn't you afraid that the 16A current reach and burn your phone?

I would recommend you to read something about electricity. The forum is not the place where you need to study the basics of the school physics.

[Grumpy_Mike]

@hydrated-narwhal

Will the 8 amps reach arduino through the ground pin.?

No.
Only the current needed to power your load will flow.

But you still need to correct those other problems like having a current limiting resistor on each individual LED and a resistor in the base of your transistor.