Load Control with Transistor

Load Control with Transistor

Introduction:- To control any load on/off we can use Transistor as a control element. As per rating of transistor we can directly control dc load. for higher load we can use suitable relay and Contactor.

Relay:- A relay is electro mechanical device. It consist of a electromagnet, moving armature with moving contacts and fixed contacts. When supply is given to coil electromagnet attract the armature and  moving contact of armature touches to fixed contact on relay body. This make no contact changes to closed contact. Simply saying a relay has a coil and set of change over contacts. Coil is rated for some voltage like 12 v dc 24 v dc and coil resistance like 50 ohm 100 ohms etc. For example a coil is 24 volt 100 ohms resistance. Again the contact are also rated for current and voltage for example 250 v dc 10 amp. It means contacts are suitable for 250 v dc voltage up to 10 amp load.
now if we want to control a load of 230 volt 20 amp we need to select relay whose contact rating is more than 250 v 20 amp. For safety reason we should choose slightly higher rating. Now comes to coil voltage we can make circuit of our choice either 12 volt, 24 volt or 5 volt. For  small current we can get relay with 5 v dc coil voltage or 12 v dc coil voltage. But for larger load we may get coil voltage 48 v or 230 ac/dc.in such case we use an intermediate relay to control main / Contactor.

Now we are starting to calculation.

Suppose we have a relay with coil voltage 12 vdc and coil resistance 120 ohms. This is the starting point of calculation and circuit design.
coil current is equal to coil voltage/coil resistznce
coil current = 12/120 = 0.1 amp or 0.1×1000 =100 ma.
here coil volage is 12 volt coil current is 100 ma. so coil power = coil current ×coil voltage = 12×0.1 =1.2 watts.
Now see for the transistor selection:- As coil current is 100 ma there fore transistor collector current must be more than 100 ma and collector voltage must be more than 20 volt. By seeing transistor data we should select a transistor with collector current 150 ma and collector voltage 30 volt. higher value may serve. With transistor selection we will get the transistor current gain alfa.
With that we can calculate base current. base current is equal to collector current/current gain. here we assume current gain is 80 then base current = collector current/current gain= 100/80 =1.25 ma. Now if we want to control this transistor with 5 volt output of a micro controller then we have to give a resistance from micro controller to base of transistor.  The value of this resistance is equal to voltage 5 volt divide by base current 1.25 ma. (5/1.25)×1000 = 4 k ohms. In this way we can set the components. Main thing is to find out collector current, current gain of transistor, base current and base biasing resistance. While selecting transistor we should see for collector current collector voltage and current gain to optimize the design. these  calculation are based on practical experience basis. 

Circuit diagram

Small loads can be put in collector circuit directly.

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also visit links given below

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