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Tuesday, 17 June 2014

What is an Open Collector or Open Drain in a Transistor ?

by realfinetime  |  in transistor at  11:51

An open collector or open drain output is an output from a transistor with no connection on collector as shown in the following image. 

This circuit will work. only if an external pull up resistor is added. One end of the resistor is connected to the collector and the other end is connected to the positive voltage.
When the transistor is on( Control Signal is HIGH ), it puts ground on the output (Vout = 0 ). When the transistor is off ( Control Signal is LOW ), since one end of pull up resistor is connected to the positive voltage, output will be 1 ( Vout = 1 ). Main advantage of the open collector circuit is, by varying the value of resistance, we can change the output current. Also it is compatible with I2C communication protocol. Usually the pull up resistance will be 10K, if it is used for General Purpose Input/Output (GPIO) and the pull up resistance will be 2K, if it is used for Inter-Integrated Circuit (I2C).

3 comments:

  1. i had been scratching my head over it for two days and i did not complete my assignment on it. your post helped me in an hour and i am glad it did

    ReplyDelete

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