Using a transistor as a switch

[Christo66]

I have a battery operated digital timer circuit board that will control a small motor, via a control board.
The timer runs on 3.6vdc (batteries). The timers Output Control Signal is 3.6vdc when on, and 0Vdc when off. The timer is replacing an old analogue timer.
The old timer had normally open switch contacts.
I think that I will need a transistor to act as a switch, can anyone recommend a transistor and resistor (if needed) that would be suitable for this scenario?

 

[uart]

What voltage and current levels do you need to switch Christo? You could probably use either a BJT or a low threshold MOSFET.

Low threshold mofets are designed to allow switching with gate voltages as low as 2.5 volts (some probably even lower). You may get a slightly better approximation to a short circuit with a mosfet compared with a BJT.

 

[Christo66]

Thanks for the reply uart

I will run some tests to check out the current and volts, but to give you an idea of a similar motor see below:

6 Vdc
70mA no load
640mA full load
700mA stall

Cheers
Chris

 

[uart]

Thanks for the reply uart

I will run some tests to check out the current and volts, but to give you an idea of a similar motor see below:

6 Vdc
70mA no load
640mA full load
700mA stall

Cheers
Chris

A small low threshold mosfet would be ideal. You can usually drive the gate directly directly from the 3.6 volt logic output.

 

[alphysicist]

Using a transistor as a switch is a common and effective method in electronic circuits. In this scenario, a transistor can be used to control the flow of current from the battery to the motor based on the output signal from the timer. The type of transistor and resistor needed will depend on the specific requirements of the circuit.

For this application, a small signal NPN transistor such as the 2N3904 or 2N2222 would be suitable. The base of the transistor can be connected to the output control signal from the timer, and the collector can be connected to the positive terminal of the battery. The emitter can then be connected to the motor, allowing current to flow through the motor when the transistor is turned on.

The value of the resistor needed will depend on the current requirements of the motor and the gain of the transistor. A general rule of thumb is to choose a resistor that will limit the base current to about 1/10th of the collector current. For example, if the motor requires 100mA of current, a resistor with a value of 1kΩ would be suitable.

It is important to note that the transistor should be chosen based on its maximum collector current and voltage ratings, which should be higher than the requirements of the motor. Additionally, proper heat sinking may be necessary to prevent the transistor from overheating.

In conclusion, using a transistor as a switch is a suitable solution for controlling the motor in this circuit. The specific transistor and resistor chosen will depend on the requirements of the circuit, and it is important to ensure that the components can handle the necessary current and voltage.