S3 Design basic circuits using discrete components

Posted: October 10, 2010 in Systems and Control

Initial knowledge – 0

Current working knowledge – 2

Again designing circuits with the use of discrete components is not something I have a great deal of knowledge in.  The ED217 project has certainly been a bit of an eye opener to me over a vast range of new information.  I have found it interesting though and certain things have decided to remove themselves from the dark depths of my memory bank.  Circuit design is one of these things and after making the PCB and designing my circuit on circuit wizard I had to make the circuit itself.

A discrete component is basically a singular component such as a resistor, a capacitor, diode  etc these would be classed as passive or a transistor or vacuum tube known as active.  It only has one circuit element unlike an integrated circuit like a 555 timer which is made of lots of discrete components combined onto a small chip.  The 2 can be combined and in ED216 I used discrete components with the PIC micro controller.

I was using 3 leds for my ambiance light so each of these would need a resistor placed before it in the circuit, likewise with the switches, I needed a resistor before the switch and then one before the switch is grounded back at the PCB

I was using a 9v battery to test the circuit,  and so there were 9 volts of potential energy,  The current then flows through the circuit from + ve to – ve and is measured in amps.    All materials have a resistance (R) and this is measured in ohms.

As I only used the standard PIC AXE 08 chip for my light the circuitry is relatively simple and there was no need for an additional biscuit board.

The only thing I needed to consider was the value of resistor I would need prior to going through the positive leg of the led.  The value of the resistor is worked out using ohms law.

To make a current flow through a resistance there must be a voltage across that resistance. Ohm’s Law shows the relationship between the voltage (V), current (I) and resistance (R). It can be written in three ways:

V = I × R or
I = V
R = V

For most electronic circuits the amp is too large and the ohm is too small, so you often measure current in milliamps (mA) and resistance in kilohms (kohm). 1 mA = 0.001 A and 1 kohm = 1000 ohm.

where: V = voltage in volts (V)
I  = current in amps (A)
R = resistance in ohms (ohm)
or: V = voltage in volts (V)
I  = current in milliamps (mA)
R = resistance in kilohms (kohm)

To use the ohms law triangle above you can follow these equations:

  • To calculate voltage, V: put your finger over V,
    this leaves you with I R, so the equation is V = I × R
  • To calculate current, I: put your finger over I,
    this leaves you with V over R, so the equation is I = V/R
  • To calculate resistance, R: put your finger over R,
    this leaves you with V over I, so the equation is R = V/I

So in the case of my ambiance light
V= 3.0-3.2v
I= 20-30mA or 0.02-0.03mA
(4.5-3.1) divided by 0.02 = 56R, as I am just using the pic axe 08 board I have decided to use 100 ohm resistors.

Also because I used switches I needed to include two resistors to the value of 1 k and a 10k to go to ground. This is included because if the input is not tied to V via the 10k pull down resistor it may float and cause the input to trigger randomly.

When doing my competency module I again have been using discrete components and using them in conjunction with integrated circuits.  I have chosen to look into the discrete components featured within digital electronics in a more detailed way as I have found that I have learnt and gained a better understanding of electronics by stripping it back to basics and looking at why and how things work to gain a better understanding.  By using certain discrete components such as capacitors it is important to understand that if I change the resistor before the capacitor it will have an effect on the time and therefore in the components section of   http://aidanross2.wordpress.com/category/components/  it includes not only details and functions of the main discrete components that are used but also equations needed for working out resistance and capacitance amongst other things.  By researching the components at a basic level I now have a better knowledge of why they fit into circuits and where they will fit into circuits and what functions they will carry out so i will use aidanross2.wordpress as a useful revision guide in the future.


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