# NPN Transistor Help



## rchoi54 (Dec 5, 2008)

I'm currently trying to use my transistor as a switch for lights.

It was working fine the other day, and then I wired it a little different and the thing blew.
I got another of the same today, but I just can't get it to work at all.

Is there anything wrong with this??


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## magnethead (Mar 18, 2006)

what is "amp" Is it a switch or what?

Last I remember, the base works as a switch between collector and emitter. If base has power, C-E conducts, and vice versa. 

Remember though that a transistor is 2 diodes buted together..electrons only move certain ways


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## Done_Fishin (Oct 10, 2006)

how an NPN transistor works 

Positive voltage on Collector
Negative Voltage on Emitter

More commonly the LOAD is placed in the Collector to positive Voltage source path but it can be placed in the Emitter to Negative Voltage source path .. (IF YOU TAKE INTO ACCOUNT THE AFFECT OF BASE VOLTAGE SHIFT DUE TO EMITTER VOLTAGE SHIFT TOWARDS POSITIVE VOLTAGE RAIL WHEN CURRENT PASSES THROUGH LOAD)

as the voltage difference between Base and Emitter increases current will flow through your load .. whatever that might be (lamp , resistor , relay, coil, solenoid etc., etc)

when the voltage reaches approx 0.6 ~ 0.7 volts the transistor should be fully saturated which means that the voltage at the collector will be more or less at teh same voltage potential as the voltage at the emitter. If you have no load in either the emitter or collector circuit then the transistor fries and dies, so a load must always be placed somewhere that limits the current that may flow through the transistor to a value that is within the transistors power rating.

current will also flow through the base lead of the transistor, so you will also need to protect the base from being overdriven. this is done by placing a resistor in line from the source to the base pin. failure to do this may also result in a fried tranny or worse kill the signal source.

most transistors have gains and you need to have an approximate idea of the gain of your transistor. This will help you decide what base "current limiting" resistance is needed as well as configuring your overall circuit.

lte says that you are trying to activate a 100mA 12V relay using your NPN transistor 

1st the transistor must be able to cope with 12V off load ie no current flowing. 
2nd it must be able to cope with a 100mA current that will energise the relay.

lets say that the gain of the transistor (Hfe) is 100 
this means that for every 1mA of current that flows through the C-E circuit, 0.01mA is required through the base (AND note that the Emitter current = Ic + Ib)
when our transistor is energising the relay with 100mA the Emitter current will be passing 110mA.

in order for the transistor to be turned ON and energise the realy we require a voltage difference between the Base & the Emitter pins where the Base Voltage will be ~0.7V (silicon transistor) higher than the Emitter Voltage.

If the Voltage that is being sent to the Base pin starts at 0V and goes to 5V then a resistor must be placed in circuit that will limit our current to 10mA so that when the 5V level is reached only 0.7V is passed through to the transistor base pin.
the 4.3V that is in excess of requirements at 10mA will give us the resistance required to protect our transistor from frying.

you show a +/-3V supply for your circuit and the base being fed from AMP .. so I suspect that you are trying to use audio frequencies to make your lights glow.

in this case you are in great danger of blowing your output amplifier, since you MUST have isolated supplies to do this or ensure that the 0V of your +/-3V supply and the 0V of your final amp supply are at the same potential, normally Earth .. 

you will need to check what transistor you are using , find the specs, find the gain , check how much current your load is going to want etc, etc.

if you don't want to damage your Hi-Fi or amplifier I would suggest the use of opto - isolated couplings, this will separate the voltage sources of both sides and avoid conflicts that might destroy your property


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