Transistors, Diodes, etc.

Transistors

Transistors are three termainal devices where one terminal controls the power through the other two.

Transistors Bipolar

Bipolar transistors are basically current controlled devices. There are two basic types of Bipolar transistors: NPN and PNP.

In bipolars, the collector current (Ic) is controlled by the base current (Ib). The collector current equals the base current times a variable called "Hfe" (often listed in transistor datasheets). In other words, Ic = Ib * Hfe.

NPN: To turn it on and get Ic flowing, a small base current (often microamps) must be squirted INTO the base. If this current is too small (or zero), the collector current shuts off. When you squirt enough current into the base, the base-emitter acts like a diode. The way you know you are squirting enough current into the base is by checking the voltage between base and emitter. When turned on full, the base will be several hundred millivolts above the emitter voltage (< 500 mV if barely on, usually 600-900 mV when full on).

Normal NPN operation for hobbiests: Collector voltage >= base voltage. Current squirting into the base. Base is 600-900 mV above the emitter. When collector drops below base voltage (saturation), the base will start sucking in more current and the transistor will no longer do its job.

PNP: To turn it on and get Ic flowing, a small base current (often microamps) must be pulled OUT OF the base. If this current is too small (or zero), the collector current shuts off. When you pull enough current out of the base, the emitter-base acts like a diode. The way you know you are pulling enough current out of the base is by checking the voltage between emitter and base. When turned on full, the emitter will be several hundred millivolts above the base voltage (< 500 mV if barely on, usually 600-900 mV when full on).

Bipolar Smoke Warning: You must limit the curent in the device. Two ways to do that. (1) Use a resistor feeding into the base to limit Ib. Since Ib controls Ic, that will limit Ic. (2) Use a resistor in series with the collector to limit Ic. If you don't limit one or the other, you will get smoke.

BIPOLAR SWITCHES vs AMPLIFIER: NPNs and PNPs can be used as switches, buffers or amplifiers. When used as a switch (the easiest), you're cranking the device hard-on or hard-off, nothing in between. Using a transistor as an amplifier requires significantly more design finesse to get linear gain and proper behavior.

NPN SWITCH DESCRIPTION: Emitter to ground. Resistor (e.g. 10k) between collector and Vcc. Resistor (e.g. 3k) between input voltage and base. Output = collector voltage. When input goes high (5V, 12V, whatever), transistor turns on, pulls current through collector resistor, and output goes low. When input goes to zero, no current can go into the NPN base. Transistor shuts off, collector voltage pulled to Vcc.

PNP SWITCH DESCRIPTION: Emitter to Vcc. Resistor (e.g. 10k) between collector and Ground. Resistor (e.g. 3k) between input voltage and base. Output = collector voltage. When input goes to ground, transistor turns on, pushes current through collector resistor, and output goes high. When input goes to Vcc, no current can flow out of the PNP base. Transistor shuts off, collector voltage pulled to Ground.

. Ignoring the current gain a transistor acts like two diodes connected to a common terminal. This is useful in sorting out transistor leads with an ohmmeter. Two general classes of bipolar transistors are NPN and PNP.

Links:

Transistor Components

Transistors FET MOSFET

Metal-Oxide Semi transistors are basically voltage controlled devices. The current is controlled by the voltage at the gate.

Transistors Transdiode

This is a connection of a transistor to use it as a diode.

Diodes

Diodes are two terminal devices that conduct very differently in one direction verses another. The basic use takes advantage of this property, but the are other characetistics that are also useful and special diodes that take advantage of these properties.

General

A diode lets current through in one direction but not another. It acts somewhat an infinite resistance in one direction, and a 0 resistance in the other direction. A more accurate description ( but not complet ) says that in the low resistance direction there must be a .6 v drop before much current flows.

Other Reading

• Diode From Wikipedia, the free encyclopedia

Use: Rectifier

Changing alternating current to direct current. A standard in almost all plug in power supplies and most electronic circuits run on DC.

Use: Detector

As in an amplitude modulated radio ( AM ) where the radio signal is changed to an audio signal.

Use: Snubber

Some circuits, typically those with inductance like inductors, motors, relays and solenoids, generate a large back or reverse voltage when they turn off. Often a diode will be inserted to "short circuit" this voltage/current. This can prevent dammage to other circuit components.

Use: Steering

Sending a voltage/current in a particular direction in a circuit

Use: Expotential/Logeritmitc Converter

The current in the foward direction is an exponential function of the voltage. Together with an op amp this can be used to convert voltages to via an expoential or logeritmitc function. These in turn can be used for multiplication, division, powers and roots. See OpAmp Links

Use: Clipping, Bias Voltage ( Temperature Sensing )

In the forward direction a junction has about .6 volts when conducting ( as does the base emitter junction of a bipolar transistor ). This voltage is useful as a small well defined voltage for bias in a transistor circuit. Often 2 or more will be used in series for a higher voltage. If the input is a varing voltage the output is equal to the input and then begins to clip ( stop rising ) at about .6 volts. Feeding in a triangel wave at the right amplitude you get out a triangle wave with the points rounded off, an approximate sine wave. The bias voltage is somewhat temperature sensitive, you can used this in an electronic therometer circuit.

Use: Over Voltage Protection

Many circuits cannot tollerate voltage over a certain limit ( often the power supply voltage ). Connecting a diode from the circuit input to the power supply can "short out" the over voltage. Make sure the diode is connected in the proper direction.

Use: Bridge

This is an arrangement that is used for full wave rectification and some other circuits that are a bit tricky. Not explained here and now but google will help you out

Use: Reverse Protection

Many circuits destroy themselves when connected backwards to a voltage source. Connecting a diode in series with the circuit blocks the reverse voltage.

Special Types

LED, Shockley Junction, Tunnell, variable capacitance.....

Photo

see Sensors photo

Reverse Protection Diodes - PTH and SMD diodes to protect against reverse polarization.