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LowPower

Transistors

Transistors can be used to amplify current (so it is always partially on), or they can be used as a switch (either fully on with

the maximum current or fully off with no current).

BJT (Bipolar Junction Transistors)

Theory: There are two types of BJT transistors;however, most BJT transistors are NPN because they are the easiest to make from silicon. A transistor has three leads: base(B) , collector (C), and emitter (E). In a transistor, the base current (IB) only flows when the voltage across the base emitter junction (VBE) is 0.7V or more. This very small base current(IB) can control a very large collector current(IC). The amount of current that will flow through the collector is determined by the gain, (hFE) which you find specified on a data sheet, but can vary greatly even in transistors that are the same type. You can test the gain of a transistor using your multimeter.

Devices:

• 2N2222A NPN General Purpose Amplifier (1A Max collector current)
• 2N3904 NPN General Purpose Amplifier (200mA Max collector current)

Implementation: Once you select an appropriate transistor, your main concern to make this circuit function correctly is to select the appropriate values for RL and RB. The current flowing through the load resistor is controlled by the source voltage (VS) and the load resistor (RL)
IC = VS / RL
The maximum value of the base resistor can be found by considering the gain of the amplifier(hFE) and the desired collector current (IC). RB = (Voltage from the chip - 0.7) / (IB)
Where IB=IC/hFE

JFET

Theory:

This type of transistor is controlled by an external voltage rather than an external current. This type of transistors also has three leads, but they are labeled gate(G), source (S), and drain (D). The location of these legs is shown on the circuit diagram.

Devices:

• IRF520 N-Channel Power MOSFET (9.2A)
• IRF610 N-Channel Power MOSFET (3.3A)
  

Implementation:

To use a 555 timer to drive a MOSFET, you can connect the gate to the pin it will be controlled by, and then connect the circuit you are driving to the remainder of the circuit, similar to a BJT.

Relays

Theory: A relay consists of a coil and at least one other set of contacts. Often a relays connections will be obvious. The coil can be connected either direction. This type of connection allows one part of the circuit to have no electrical connection to the other part. When the coil has current flowing through it, it will attract an arm in the relay. This will make a connection between the other terminals of the relay. Note: Often times, it will still be necessary to use a transistor to drive a relay.

Devices: We have several relays in stock. This relay is convienent because it has both normally open and normally closed contacts, and requires relatively low voltage.

• KS2E-M-3DC Data Sheet
  

Implementation:

The diagram shows how you can drive a relay using a transistor. Relay coils produce voltage spikes, so you use should use a diode across the coil of a relay to help protect transistors and ICs in the circuit. A signal diode (such as the 1N4148) should be connected “backwards” so it is not normally conducting. In the diagram this is the "Protection diode".

When selecting a relay, you should consider:

• The size of the relay and its pin arrangement
• The coil voltage
• The coil resistance

The circuit needs to be able to supply enough current to the coil. You can use Ohm’s law to find this current.

Relay coil current = VS / RC where VS is the supply voltage and RC¬ is the resistance of the coil.

For example: A 12V supply relay with a 400 Ohm; coil resistance will requre a coil current of 30mA. This will be fine with a 555 timer, but is too much current for most ICs. They would require a transistor to amplify the current.

Disadvantages of relays: Relays are often large, and they cannot switch rapidly. They use more power than a transistor and need more current than some chips can provide. (Luckily the 555 has enough output current (200mA) to be able to be directly connected to a relay)