External supply

Hello fellows,

I have just created a DC-motor circuit with an external 8.4 volts battery supply to run it. Via puls-with-modulation over (pin 9, which applies at maximum 5 volts) the transistor (http://www.mouser.at/ProductDetail/ON-Semiconductor/P2N2222AG/?qs=ZXBb0xZ9WeAcMztsfBZpJQ%3D%3D) is controlled to adjust the level of current, which flows through the motor. The problem is that it doesn´t work and I can´t figure it out since I´m an electronics-starter. What basically happens is that when I use the Arduino 5 volts circuit the motor works perfectly but if I connect the external power-supply instead it does nothing, althought the data-sheet of the transistor allows the voltage. It seems like the transistor blocks for some reason.

Thanks for your help!

Where is your circuit and program?

Unless we know how it is connected, we are only guessing.

Have you joined the ground connection from the external supply to the ground of the Arduino?

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The program works for sure, because it works when I use the Arduino power supply. No I haven´t but why do I need it. I ground it with the battery package (sorry for maybe sounding stupid : )) Basically the only connection to the Arduino is via Pin 9, which applies a voltage to the base of the transistor if necessary. Why do I have to ground the battery on the Arduino and how should I do this?

p.s. sorry for the shitty sketch

Circuit.png

Electric current can only flow if there is a circuit. The Arduino cannot cause any current to flow in or out of the transistor when there is only one wire.

Ground on the Arduino must be connected to the ground (battery negative) of this output circuit.

It is a shitty sketch but it’s perfectly adequate to get the point across.

I don´t understand. The transistor base just applies a voltage in order to let the electrons cross the p-n junction. why do I need a ground for the voltage?

Because you can't have a voltage at one point. Voltage has to be measured with reference to a second point, usually called 'ground.' A voltage meter must have two probes - it doesn't measure anything when one probe is attached to your circuit and the other one is in mid-air.

that´s logical, by the way I tried it and it works! : ) thanks so much man!

everything works perfectly but another question poped up. Bascially the transistor I have (http://www.mouser.at/ProductDetail/ON-Semiconductor/P2N2222AG/?qs=ZXBb0xZ9WeAcMztsfBZpJQ%3D%3D) shows a VEBO-value of 6 volts. However, I now use a battery pack (8.4 volts) to run the Arduino Uno and when I apply the voltage (Pin 3) to the transistor-base it doesn´t get destroyed, althought the voltage would be 8.4 volts. Does the Arduino transform the voltage always to 5 volts for operation and hence also applies at maxium five volts at the pins?

I presume you have applied the 8.4v via the power socket. This regulates the voltage fed to the arduino to 5v. The output pin then applies 5v to the transistor.

And a subtle correction to an earlier post, a transistor is a current driven device not voltage driven. It is current flow through base and emitter junction that causes current to flow through the collector and emitter.

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JohnLincoln:
Vebo is the maximum voltage that can be applied to the base - emitter in the reverse direction. Exceeding this voltage will permanently damage the transistor.

As you are applying voltage/current in the forwards direction, you don’t need to be concerned with this parameter.

Of course! Was too late in evening to think!

Weedpharma

weedpharma:
And a subtle correction to an earlier post, a transistor is a current driven device not voltage driven. It is current flow through base and emitter junction that causes current to flow through the collector and emitter.

Is it not that the BJT transistor is current driven, but a MOSFET is a voltage driven transistor? I realize that the transistor that the OP is using is a BJT transistor, so in this case the correction was warranted, but didn’t want to leave the impression that ALL transistors were current driven.

Theoretically, yes the MOSFET is purely a voltage-sensitive device. But all practical devices have some capacitance so it does require a small amount of current flowing 'into' the MOSFET to turn it on. That means your formulas for calculating which MOSFET to use can often ignore the current flow but you still need to have a ground connection to make a circuit, which was the original problem this thread was created to solve.