Transistor Design

Transistor Design ... Simple Approach


The following is a rundown of how to design simple NPN transistor circuits.
Basically, a transistor sets the Collector current (Ic) to be equal to the Base current (Ib) multiplied by the transistor's Gain "Hfe" (typically around 100). [IC = Ib*Hfe]

This lets you turn a large device (such as a motor or a Laser) from a small signal (such as from a microcontroller or a sensor). However, the transistor also requires a voltage Vbe between its Base and Emitter (typically around 0.7V). Here is an example circuit:



1st step is to determine how much current & power your load will take. If the load is a Laser or a Relay, then it should only be about 100mA at 5 or 12V, which is upto 1.2Watts (Power = Volts * Current).

2nd step is to determine the maximum Base current (Ib) you will have. If the Base resistor (Rb) is connected to a microcontroller, then this is generally a maximum of 20mA (enough to power a simple LED but not much more).

From these 2 steps you should find a transistor that has Max Collector Current (Ic) and Max Power Dissipation (Pd) ratings that are atleast twice your requirements (for safety), and has a DC Gain (Hfe) large enough to convert your Ib into your Ic. To amplify small signals or to turn on a small Relay / Motor / Laser under 100mA using a microcontroller, use a small transistor (eg: BC548) in the TO-92 package, but for any larger currents or power you will need a larger transistor (eg: TO-220 package), possibly with a metal heatsink.

3rd step is to determine the Base resistor value (Rb) to give the correct voltage Vbe (typically around 0.7V) for the transistor, based on what you are connecting it to. Use Ohm's Law (V=I*R) to work out the resistor that would give you the desired current (Ib) between the resistor's voltages (Vin and Vbe). Ideally, Ic = Hfe * Ib = Hfe * (Vin - Vbe) / Rb. However, transistors aren't perfectly linear amplifiers, so you wont get quite this much current output.

When you are trying to either switch something to be completely ON or completely OFF (which is a good idea for Relays, Lasers and Motor PWM), then you want to use the transistor at its "Saturation" limits, which means making Ib about twice as much as you originally calculated, so that it is definitely ON or OFF. In these cases, you can think of the transistor as a switch, that is open (not letting any current through, therefore Vce = Vcc) when Ib = 0, and closed (letting all the current through, therefore Vce = 0 or 'short circuit') when Ib = max. Just be aware that even when you have saturated the transistor on (Ib is max), Vce wont actually reach 0volts, but will usually be between 0.2 - 0.7V depending on the transistor.

Transistor Leads:

source:http://www.itee.uq.edu.au/~emami/metr2800/

Why Pullup Resistors ?

The Pull-up Resistor

This time in Very Basic Circuits, I would like to talk about pull-up resistors. The basic function of a pull-up resistor is to insure that given no other input, a circuit assumes a default value. Actually, there are two flavors of this circuit. A pull-up and a pull-down. Their function is the same, to create a default value for a circuit, but one pulls the line high, the other pulls it low.

A floating input gate. Not Good!

Consider this schematic. The gate U1A has an input (pin 1) and an output (pin 2). The input state of most logic gates is called a high impedance. This means it provides no real power of its own. Therefore, if nothing is connected to pin 1, the value of the input is considered to be floating. Most gates will float towards a high state. This is a very weak condition, and any electrical noise could cause the input to go low. When switch S1 is closed (on), the input state at pin1 goes low. Since there is a definite connection to an electrical potential (in this case ground), the state of the pin is stable. When switch S1 is open (off), then input pin 1 is susceptible to a wide array of electrical problems. The traces or wires connected to pin 1 may very well allow enough electrical noise in (by acting as little antennas) to cause pin 1 to incorrectly switch states. What is needed here is a way to connect pin 1 to an electrical potential that can be removed when the switch is closed. This electrical potential will allow the pin to keep a steady state.

One thought is to tie the pin to Vcc (+5 volts) to insure that pin 1 doesn't float. The circuit to the right certainly does that. With pin 1 tied directly to Vcc, the line does not float, and has an ON state. The problem with this circuit is what happens when switch S1 is closed. This creates a direct electrical connection between Vcc and GND. In other words, it will short out the circuit. If you are lucky, it will just stop your entire system from working. If you are unlucky, it will burn up the wires! The problem with short circuits is they allow too much current to flow from Vcc to GND. This causes heat to be generated, which can sometimes burn parts, wires, or even start fires. In addition, most circuits fail to function correctly because the voltage at the power supply drops to zero. In general, this is a bad situation!

A very bad idea!

Pull-up resistor limits the current

Now consider the next schematic, which is similar to the first but has added a pull-up resistor. This resistors function is to limit the amount of current that can flow through the circuit. When switch S1 is open (off), pin 1 is tied to Vcc through the resistor. Since pin1 is a high impedance input, a voltage meter or logic probe placed on pin 1 will show Vcc (+5v) if connected to pin 1. When switch S1 is closed (on), pin 1 has a direct connection to GND, which takes it to the low state. The pin1 side of R1 also has a direct connection to ground. Current will flow from Vcc, through R1, and to ground. It isn't considered a short, however, because R1 will limit the amount of current that can flow to a very small amount. In fact, you can compute this using Ohms law. I = V / R I = 5v / 10,000ohms I = .0005A (.5mA)

A variation on this them is a pull-down resistor. Just like the pull-up resistor, it is used to limit the current that can flow between Vcc and ground. Though less often used, it is still a valid thing to do. Most digital circuits use a 10k or a 47k resistor for pullups. The exact value doesn't actually matter, as long as it is high enough to prevent too much current from flowing. 10k seems to be the most common, but if you are hoping to save as much power as possible, the a 47k resistor may be right for your application. In some cases, you can go higher, but then you are depending on characteristics of the pins on the chip. In Summary You will find that pull-up resistors are extremely common is most digital circuits. The key function for a pull-up is to prevent input lines from floating. The key function for the resistor itself is to prevent too much current from flowing through the pull-up circuit. Source:  http://www.seattlerobotics.org/encoder/mar97/basics.html

Electronics Helpful Websites

Welcome to the Electronics Club

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Understanding Electronics Components
link : http://www.mikroe.com/en/books/keu/00.ht

Description :-
teaches the basic components and how the are used in circuits starting with resistors till transistors and ICs and other components like speakers and microphones .It also include some practical circuits using these components

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added: 20-8-2008

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