# Transistor as a switch in active or saturation region?

I am working on a circuit in which a load (a DC motor at 6V and 1A max current) needs to switch on or off. The controlling voltage (voltage at base) will be 0 to 3.3V.

I am planning to use 2N4401 transistor. My question is I can use my transistor in active or saturation region when turned on. I can select the resistor values accordingly. However, I am not sure when one mode is preferred over other and why.

Does operating in active region will cause my transistor to overheat as Vce will be equal to excessive voltage left in the collector circuit. For example, if Ib = 1mA, Current Gain of transistor (Beta) = 100, Vcc = 10V, my load resistance = 10 ohms. Then, a current 0.1 A will flow in collector circuit and my load will have 1 V drop. Then, the excess voltage of about 9 V will be across Vce? Is this a problem for active region?. What if a connect a resistance of 50 ohm in series with my load in collector circuit? The resistance of 50 ohm will still keep the transistor in active region but it will reduce the Vce to 4V.

What if I have a load and I want to push only a specific current and design my circuit to operate in active region when turned on and Vce will be calculated to be low so that power dissipation will be quite less.

• If memory serves the 2N4401 is good up to 600mA, you might want to consider another device for 1A.
– user133493
Apr 25 '17 at 8:06
• Correct, this transistor is unsuitable for this. When a bipolar transistor is used as a switch, you must operate it in saturation region. When it is not in saturation, it will determine the current through the load. That is not what a switch does. A closed switch will have such a low resistance that the load determines the current, not the switch element. Apr 25 '17 at 8:22
• @replete: can you suggest a transistor at 1A? Apr 25 '17 at 8:29
• I would not use a transistor with a Ic_max of 1A to control a 1 A motor. I would use a 2 A transistor to have some margin. You can find a suitable transistor on the Farnell or Mouser website and filling in your criteria. And don't forget the flyback diode otherwise your transistor will be damaged. Apr 25 '17 at 8:58
• The flyback diode must be rated for the motor current and must have a sufficient recovery time for your switching rate, in both cases with decent margin. The first criterion rules out a junkbox 1N4148 in your case. The second criterion may rule out a junkbox 1N400x. Use any decent supplier's parametric search to find suitable devices.
– user133493
Apr 25 '17 at 9:43