Hi, I was looking at the following circuit:
and I was wondering why the transistor is used in the first place. From what I can see, it is used to amplify the current in order to drive the motor.
Since the motor seems to require 470mA to run, why not simply use a current limiting resistor instead? With a 5V battery and a 10 ohm resistor, the current should be around 500mA which should be able to drive the motor, no?
What am I missing?
Thanks all, Martin
No. All the voltage ( and hence the power) would be across the resistor wouldn't it ?
The transistor isn't limiting the current. It's acting as a switch. It can be driven from logic level outputs.
10 ohms will draw 500ma from 5V, but what voltage does the motor need? Motors usually do better with a low-resistance constant-voltage source. The current will be high at start, then drop as the motor comes up to speed and develops back EMF. There are exceptions; some motors are designed to apply constant torque to a load whether it's turning or not, but those are usually AC induction motors.
Which is enough to fry most logic circuits. The transistor lets a small current (2.15 mA) control a larger current (470 mA) in cases where the control current is simply not powerful enough.
Most TTL/CMOS outputs are only rated for a few milliamps.
The "hfe" spec is the amplification factor - in this case, 300 means that a 470mA load needs (470/300 = ) at least 1.57 mA of input current. At 2.15 mA input, the load can draw up to 645 mA. Approximately. Good to leave some margin too.
The concept of a "relay" is well established in this field.
You are also ASSuMEing that *you* always get to decide on the nature of the originating source used to turn something on . We often have to tie into someone else's existing design.
wouldn't
Ok. As I understand it, the transistor provides a way to give more current without adding resistance in the circuit, therefore, keeping the potential 'intact'.
Is this correct?
Martin
wouldn't
No.
If whatever you have that turns on and off can only output 5mA, then it can never supply enough current for that motor that needs 10mA. A resistor will not do it, since a resistor can only limit current.
The transistor switches the current. It's an amplifier, taking the low current on/off signal from (presumably) the IC, and providing enough current to the motor.
Any limiting resistor in series with the motor at that point is to limit current to the motor. But that's because the transistor can pull a lot of current through the motor, unlike the output of the IC.
Michael
wouldn't
No.
The transistor when used as a switch here is either off ( no base current) or on ( base current applied ). It works like an on-off switch controlled by the input to the base resistor. So a logic 1 from typical digital lelectronics ( e.g. a microprocessor) will cause the motor to run and a logic 0 will turn it off.
Graham
Got it. Thanks all for your answers! Martin
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