When designing transistor circuits, the manufacturer's data sheet is an invaluable resource. The data sheet for the 2N303055 transistor indicates that this is an NPN transistor that can handle up to 15 amps of current and 60 volts. As such, it is useful for automotive switching applications. Unlike most of the accessories in your car, which are permanently connected to ground and switched by the positive power supply wire, an NPN transistor will work the opposite way. That is, the device to be controlled by the transistor is connected permanently to the +12 V supply and the ground side is switched. If this is acceptable for your application, then this transistor may be used.
- Skill level:
Things you need
- Insulated wire
- Soldering iron
- Flux core solder
Use the manufacture's data sheet to identify the base, emitter and collector connections on the transistor. If you are planning on switching a heavy load (five amps of higher), mount the transistor in an appropriate heat-sinking mount.
Use a suitable length of wire to solder the emitter of the transistor to an appropriate ground connection of the automobile.
Connect the base of the transistor to the control source signal responsible for operating the device. If you are using a transistor switch, it is because you are trying to control some load from a source that is incapable of driving it directly, such as a micro-controller project or other light electronic source. Note that since this is an NPN transistor, the source will have to provide a positive voltage to the base to switch the transistor on.
Connect the power side of the device to be controlled by the transistor directly to an appropriate supply voltage, and connect the ground side of the device to the collector of the transistor. Since the collector connection is direct to the case of the 2N3055, you will have to use a nut and bolt through the transistor's mounting hole and slip the wire under the nut before tightening it. The case of the transistor is not designed to be soldered, leaving this as the only proper option for connecting to it.
Test the circuit by having your controlling source provide power to the base of the transistor. The device being controlled by the transistor should now become active. When the control voltage is removed, the device should turn back off.
Tips and warnings
- According to the data sheet, the 2N3055 requires 1.5V on the base to turn it on and the current that will flow in the collector will be on the order of 100 times the base current. Therefore your controller will have to output at least 1.5V and must be able to supply at least 1 per cent of whatever amount of current you are attempting to switch with the transistor.
- For safety, always remember that the power source should be disconnected while performing any electrical connections to a vehicle.
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