Current Limiter - Support Material

  
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This circuit places a maximum limit on the current available from the output terminals. It does this by limiting the maximum current that can flow through Q1. Because the output terminals are in series with Q1 - limiting the Q1 current also limits the output current. = The negative output terminal is connected to the collector of Q1. So any curren
Current Limiter - Support Material - schematic

t drawn from the positive terminal - flows into the collector of Q1. = Thus - the Q1 collector current is supplied by the output terminals. The Q1 base current is supplied by R1. And the Q1 emitter current - flowing to ground through R2 - is the sum of the collector current and the base current. Because the base current is relatively small - for present purposes - it can be ignored. And you can think of the emitter current as being equal to the collector current. = Or - to put it the other way round - the collector current is equal to the emitter current. So - if you set a limit on the size of the emitter current - you also set a limit on the size of the collector current. That is - you set a limit on the amount of current available from the output terminals. = The key to limiting the Q1 emitter current - is to set a maximum limit on the transistor`s base voltage. That`s the purpose of D1 & D2 Each diode has a forward voltage drop of about 0v7. The two together prevent the base voltage of Q1 from rising above about 1v4. It may be lower than 1v4 - but it cannot go higher than 1v4. In effect - D1 & D2 are acting like a 1v4 zener diode. = The voltage on the base of a silicon NPN transistor is always about 0v7 higher than the voltage on its emitter. So - when D1 & D2 prevent the voltage on the base of Q1 from rising above 1v4 - they also prevent the voltage across R2 from rising above about 0v7. It may be lower than 0v7 - but...



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