If the load is of a very high current or voltage nature, such as motors, heaters etc, then the load current can be controlled via a suitable relay as shown. One difference to consider when using Darlington transistors over the conventional single bipolar types when using the transistor as a switch is that the Base-Emitter input voltage. docx), PDF File (.pdf), Text File (.txt) or read online for free. With inductive loads such as relays or solenoids a flywheel diode is placed across the load to dissipate the back EMF generated by the inducti ve load when the transistor switches “OFF” and so protect the transistor from damage. Transistor as a Switch - Free download as Word Doc (.doc /. An example of an NPN Transistor as a switch being used to operate a relay is given below.

When used in this way, the transistors open collector output can thus “sink” an externally supplied voltage to ground thereby controlling any connected load. There are two types of field-effect transistors, theJunction Field-Effect Transistor (JFET) and the Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET), or Insulated-Gate Field-Effect Transistor (IGFET). The simplest way to switch moderate to high amounts of power is to use the tra nsi sto r wi th an op en -co lle ct or ou tp ut an d the tra nsi sto rs Emi tt er ter mi nal connected directly to ground. With a positive signal applied to the Base of the transistor it turns “ON” acting like a closed switch and maximum circuit current flows through the device. With a zero signal applied to the Base of the transistor it turns “OFF” acting like an open switch and zero collector current flows.

Then the transistor operates as a “single-pole single-throw” (SPST) solid state switch. For a PNP transistor, the Emitter potential must be positive with respect to the Base.

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