Hi, I am new to electronics and I am stuck on my circuit.
I have a 6 volt, 4 mA motor that I am running when a 3 volt timer comes on. I am using a TIP120 Mosfet to close the circuit when the timer comes on. I have another 6 volt, 4mA circuit that I want to open when the 3 volt timer comes on. I am trying to use a JFet to accomplish this. I don't know how to calculate what size JFet that I need. Thanks for any help you can give me.

 

Doors and windows open and close. Circuits turn on and turn off.

A TIP120 is not a Mosfet, instead it is a darlington transistor (look on its datasheet).

Why are you trying to use a Jfet to turn on the extremely low power motor?

 

I am wanting to turn off a 6 volt circuit while the 3 volt timer is on.

 

The junction gate electronic transistor is that the simplest variety of electronic transistor. It is used as associate electronically-controlled switch or as a voltage-controlled resistance. charge flows through a conductive channel between "source" and "drain" terminals. By applying a reverse bias voltage to a "gate" terminal, the channel is "pinched", in order that the electrical current is obstructed or changed fully.

 

Tina said "Junction gate electronic transistor" but in English it is a Junction Field Effect Transistor (Jfet).

 

So what size Jfet would I use to achieve a "pinch off" of 6 volts with -3volts to the gate?

 

Would you please post a schematic of your circuit? I think that will help us to help you.

 

You cannot buy Jfets with different "sizes".
You also cannot buy Jfets with a pinchoff voltage that you want.

Jfets have a wide range of pinchoff voltage for example the 2N5459 has a pinchoff voltage range from 2V to 8V. Some are 2V, some are 4V, some are 6V and some are 8V.

 

Where did you get a motor that only draws 4mA? And why do you want to use a JFET?

 

I am using a gear motor to turn a right angle drive that turns a shaft. When the timer comes on I want the shaft to turn to a certain point and then stop, and then when the timer comes on several hours later I want the shaft to turn to the next point and then stop. I am using a lever switch to turn off the circuit. But I need the shaft to be off of the lever switch to be ready for the next time the timer comes on. So my idea was that when the lever switch was activated it would switch to the other part of the circuit with the JFet. The voltage of the timer would block the 6 volts to the motor through the JFet. Once the timer ended (1 min.) The 6 volts would reach the motor, turning the shaft until it was off the lever switch. There may be other ways of doing this, but this is the only way that I could think of except of course using a stepper motor. The 4 mA is with no load. I do not know how much current is used with the load I am using. I apologize for my ignorance on electronics, but I am having fun learning.

 

Where did you get a motor that only draws 4mA?

I have an electric RC model airplane that flies for 10 minutes with a 3.7V 120mAh Li-Po battery cell. That is 20mA. The current is much less when the motor is not working hard.

The motor is 6mm in diameter and is 15mm long. It is a coreless brushed motor originally designed to open and close a CD tray. It costs $9.99.