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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.

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    \$\begingroup\$ If memory serves the 2N4401 is good up to 600mA, you might want to consider another device for 1A. \$\endgroup\$ – replete Apr 25 '17 at 8:06
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    \$\begingroup\$ 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. \$\endgroup\$ – Bimpelrekkie Apr 25 '17 at 8:22
  • \$\begingroup\$ @replete: can you suggest a transistor at 1A? \$\endgroup\$ – abhiarora Apr 25 '17 at 8:29
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    \$\begingroup\$ 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. \$\endgroup\$ – Bimpelrekkie Apr 25 '17 at 8:58
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    \$\begingroup\$ 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. \$\endgroup\$ – replete Apr 25 '17 at 9:43
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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.

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  • \$\begingroup\$ Can you explain your last point with a shunting transistor example? \$\endgroup\$ – abhiarora Apr 29 '17 at 22:01
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    \$\begingroup\$ A reverse diode only clamps back EMF above the supply, but when the supply drive is removed, it coasts, thus, a PWM controlled brake switch to ground controls the brake force. \$\endgroup\$ – Sunnyskyguy EE75 Apr 30 '17 at 6:43
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When looking at current loads, as safe rule is to use the use not more than the rms value (0,707) of the supply available to the load as your guide. Motors are electromagnetic devices like transformers. Reactance during on and off state to the coils of the motor, places "funny" variances on the suppy line being switched and behave like flyback transformers. Therefore snubber cct are required. Easier to switch with relay. Snub cct I easiest. A bit noisy though. Transistors switching motors is common in small toy or devices. Remember if no brake is required, dc motors tends to generate small currents while turned off and slowing down. So far the 1N4148 has been doing IT for me. Let's assume the 1 amp motor is used because of the torque required, look at stepper motors. Experiment safely.

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