# Electrical – Transistor as a switch in active or saturation region

circuit analysisswitchestransistors

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.

#### Best Answer

DC motors have a DC resistance about 10% of rated V/I which means start surge can be up to 10x rated current.

Saturated mode at rated Vce(sat) is always rated for hFE=10, which has is due loss in hFE when Vce drops below 2V towards Vce(sat).

You must comprehend the Pd implications of both in your load factors thus your Rth-ja heat sink requirements and Ic/Ib ratios when Vce<2V,<1V and Vce(sat)vs Ic and Ic/Ib ratios at all times.

Back EMF overvoltage may be clamped with the same current rated diode and a additional shunting transistor will act as a dynamic brake, so long as both transistors are off during crossover from ON to OFF to prevent shoot-thru.