## Posts Tagged ‘electronics’

### Transistors – Voltage Regulation Part V

Sunday, August 19th, 2012
I’m sure you’ve seen the television commercials warning about harmful interactions between prescription medications.  By the same token electronic circuitry can also be adversely affected by certain combinations of electrical components, as we’ll discuss in today’s blog.

Last time we looked at a circuit schematic containing an unregulated power supply.  This power supply was connected to an external supply circuit containing a number of components such as electric relays, buzzers, and lights.  Each of these components has a resistance factor, and combined they have a total resistance of RTotal.  We saw that when RTotal increases, the electrical current, I, decreases, and when RTotal decreases, I increases.

In contrast to this increasing/decreasing activity of the total resistance RTotal,  the fixed internal resistance of the unregulated power supply, RInternal, doesn’t fluctuate.  Let’s explore Ohm’s Law further to see how the static effect of RInternal  combines with the changing resistance present in RTotal to adversely affect the unregulated power supply output voltage, VOutput, causing it to fluctuate.

## Figure 1

In Figure 1 RTotal and RInternal are operating in series, meaning they are connected together like sausage links.  In this configuration their two resistances add together as if they were one larger resistor.

Generally speaking, Ohm’s Law sets out that the current, I, flowing through a resistor in an electrical circuit equals the voltage, V, applied to the resistor divided by the resistance R, or:

I = V ÷ R

In the case of the circuit represented in Figure 1, the resistors RInternal and RTotal are connected in series within the circuit, so their resistances must be added together to arrive at a total power demand.  Voltage is applied to these two resistors by the same voltage source, VDC.  So, for the circuit as a whole Ohm’s Law would be written as:

I = VDC ÷ (RInternal + RTotal)

But, Ohm’s Law can also be applied to individual parts within the circuit, just as it can be applied to a single kitchen appliance being operated on a circuit shared with other appliances.  Let’s see how this applies to our example circuit’s RTotal next week.

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### Transistors – Voltage Regulation Part II

Sunday, July 29th, 2012
I joined the Boy Scouts of America as a high schooler, mainly so I could participate in their Explorer Scout program and learn about electronics.  I will forever be grateful to the Western Electric engineers who volunteered their personal time to stay after work and help me and my fellow Scouts build electronic projects.  The neatest part of the whole experience was when I built my first regulated power supply with their assistance inside their lab.  But in order to appreciate the beauty of a regulated power supply we must first understand the shortcomings of an unregulated one, which we’ll begin to do here.

Last time we began to discuss how the output voltage of an unregulated power supply can vary in response to power demand, just as when sprinklers don’t have sufficient water flow to cover a section of lawn.  Let’s explore this concept further.

## Figure 1

Figure 1 shows a very basic representation of a microprocessor control system that operates three components, an electric relay (shown in the blue box), buzzer, and light.  These three components have a certain degree of internal electrical resistance, annotated as RR, RB, and RL respectively.  This is because they are made of materials with inherent imperfections which tend to resist the flow of electric current.  Imperfections such as these are unavoidable in any electronic device made by humans, due to impurities within metals and irregularities in molecular structure.  When the three components are activated by the microprocessor chip via field effect transistors, denoted as FET 1, 2 and 3 in the diagram, their resistances are connected to the supply circuit.

In other words, RR, RB, and RL create a combined level of resistance in the supply circuit by their connectivity to it.  If a single component were to be removed from the circuit, its internal resistance would also be removed, resulting in a commensurate decrease in total resistance.  The greater the total resistance, the more restriction there is to current flow, denoted as I.  The greater the resistance, the more I is caused to decrease.  In contrast, if there is less total resistance, I increases.

The result of changing current flow resistance is that it causes the unregulated power supply output voltage to change.  This is all due to an interesting phenomenon known as Ohm’s Law, represented as this within engineering circles:

V = I × R

where, V is the voltage supplied to a circuit, I is the electrical current flowing through the circuit, and R is the total electrical resistance of the circuit.  So, according to Ohm’s Law, when I and R change, then V changes.

Next time we’ll apply Ohm’s Law to a simplified unregulated power supply circuit schematic.  In so doing we’ll discover the mathematical explanation to the change in current flow and accompanying change in power supply output voltage we’ve been discussing.

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### Transistors – Voltage Regulation

Sunday, July 22nd, 2012
 Electrical voltage flow and water flow have a lot in common.  They’re both affected by fluctuations in supply, fluctuations which can adversely impact both performance and equipment integrity.  Take for example a sprinkler that fails to cover a designated section of lawn due to heavy neighborhood demand.  Everybody wants to water on the weekend when it’s been 90 degrees all week, and low water pressure is the result.  There are times when it’s hard to get a glass of water.  By contrast in the winter months, when water demands tend to be lower, water supplies are plentiful.  This scenario of varying water pressure is analogous to what sometimes occurs within electric circuits.      In my previous blog article on wall warts, I described the operation of a simple power supply consisting of a transformer, diode bridge, and capacitor.  Together, these components converted 120 volts alternating current (VAC) to 12 volts direct current (VDC).  The wall wart power supply is fine for many applications, however it is unregulated, meaning if there are any sudden surges in power, such as spikes or dips caused by lightning strikes or other disturbances on the electric utility system, there could be problems.      Take for example a power supply that is used in conjunction with sensitive digital logic chips, like the one used in my x-ray film processor design shown in my last blog article.  These chips are designed to run optimally on a constant voltage, like 5 VDC, and when that doesn’t happen input signals can fail to register with the computer program and cause a variety of problems, such as output signals turning on and off at will.  In the film processor the drive motor may start at the wrong time or get stuck in an on modality.  If power surges are high enough, microprocessor chips can get damaged, compromising the entire working unit.      The output voltage of an unregulated power supply can also vary in response to power demand, just as when sprinklers don’t have sufficient water flow to cover a section of lawn.  Demand for power can change within a circuit when electrical components like relays, lights, and buzzers are turned on and off by digital logic chips.      Next time we’ll take a look at a basic concept of electrical engineering known as “Ohm’s Law” and how it governs the variable output voltage response of unregulated power supplies. ____________________________________________

### Transistors

Sunday, June 10th, 2012

Back in the 60s my dad spent about \$25 to buy a small transistor radio.  That was a lot of money in those days, but well worth it.  The new transistor technology allowed for a much less cumbersome radio to be produced.  No more lugging around big radios armed with heavy vacuum tubes.  In the years that followed the word transistor became a household word.  They were employed in a variety of ways within televisions and other electronic devices, increasing both their reliability and functionality.

So what is a transistor and what does it do?  It’s an electronic component, developed in the late 1940s.  The first transistor was about as big as a softball and crudely made.  As such, it was too impractical for commercial use.  Then in the l950s technological advancements made commercial production of smaller, high-quality transistors possible.  Transistors enjoyed widespread introduction to the consuming mainstream in the l960s, and since then they’ve been made in many different types, shapes, and sizes.  Some are shown in Figure 1 below.

## Figure 1

A commonly used type of transistor is called a field effect transistor, or FET, one of which is shown in Figure 2.  The FET has three metal leads which allow it to be connected into electrical circuits.  These leads are referred to as the drain (D), the source (S), and the gate (G).

## Figure 2

FET’s control the flow of current within an electronic circuit.  A good way to understand what they do is to consider the analogy of water flowing through a faucet.

## Figure 3

Figure 3 shows a faucet, complete with valve and handle.  With the valve closed the flow of water is completely shut off.  If the valve is opened partway by rotation of the handle, a trickle of water emerges.  The more the handle is turned and valve is opened, the greater the flow of water.

The FET shown in Figure 4 operates a lot like a faucet, but with regard to electrical current.

## Figure 4

The FET controls the flow of current flowing through its D and S leads, but it does not employ a valve or handle to do it.  Rather, flow rate is controlled by application of a small amount of voltage to the G lead.  The voltage’s influence on the G lead influences the FET to permit current to flow in through the D lead, then out through the S lead.  The amount of voltage applied to the G lead is directly related to how much current will be allowed to flow.

In this example the D lead on the FET is connected to a 10 volt direct current (VDC) power supply.  The S lead is connected to a flashlight bulb which is connected to electrical ground.  If you will remember from previous blogs, electric current naturally wants to flow from the supply source to ground, much like water wants to naturally flow downhill.

If the bulb was connected directly to the 10 VDC power supply, current would flow through unimpeded and the bulb would light.  However, in Figure 4 the FET acts as a regulating device.  It’s connected between the 10 VDC power supply and the bulb.  When no voltage is applied to the G lead the FET acts like a closed valve and current is unable to flow.  Without current we, of course, have no light.

When a low amount of voltage, say one volt, is applied to the G lead, the FET acts like a partially opened valve.  It permits a trickle of current to flow from the 10 VDC supply to the bulb, and the bulb glows dimly.  As voltage to G increases the FET valve opens further, permitting more current to flow.  The bulb glows with increasing brightness.

When the voltage applied to G increases to the point the FET valve is opened fully, in our example that is 2 volts, full current is allowed to flow from the 10 VDC supply to the bulb.  The bulb glows brightly.  Generally speaking, the voltage required to be applied to G for control of current flow through an FET depends on overall design and the particular application within an electrical circuit.

FETs are often used within electronic devices to turn things on and off, with no other function in between.  Next time we’ll look at some example circuits to see how it’s done.

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### Inside The Wall Wart

Monday, September 5th, 2011
 What would a cop show be without a crime scene, or better yet the obligatory dissection at the morgue?  Forensic doctors performing autopsies have become commonplace, the clues they provide indispensable.  Forensic engineers such as myself do much of the same thing, working our way backwards through time by dissecting industrial equipment and consumer products left in the wake of fires, injuries, and deaths.       Let’s do some forensic dissecting now to see what’s in a wall wart and how it works.  The inside of a basic wall wart is shown in Figure 1. Figure 1 – Inside The Wall Wart       You’ll note that a wall wart has four main components:  a transformer, diode bridge, capacitor, and a printed circuit board (PCB).  The PCB is constructed of plastic resin upon which is mounted copper strips.  This makes a rigid platform base upon which electronic components are attached, namely the transformer, diode bridge, and capacitor.  These components are soldered to the PCB, tying them together both mechanically and electrically.  Now let’s see how the components of the wall wart work together to change the 120 volts coming from your standard wall outlet into the 12 volts needed to power a typical electronic device.   We’ll use an instrument known as an oscilloscope to help us visualize what’s going on.   See Figure 2. Figure 2 – The Workings of the Wall Wart Transformer      What is depicted in the graph above is the oscilloscope’s ability to receive an electronic signal, measure it, graph it, and then display it on a screen.  This enables us to see how the signal changes over time.  At Point A, which represents the wall wart plugged into a wall outlet, the voltage alternates between positive 120 volts and negative 120 volts upon entering the wall wart, which will now act as a transformer.      The wall wart transformer then does as its name suggests, it transforms the 120 volts coming from the outlet into the 12 volts shown at Point B.  You will note that this lower voltage also alternates between positive and negative values, just as the original 120 volts emanating from the wall outlet did.  In one of my earlier blogs I explained that transformers only work when the electricity passing through them alternates over time.  (Click here for a refresher: Transformers )   High voltage alternating electricity in one transformer coil creates magnetic fields that induce alternating electricity at a different voltage in a second transformer coil.  So when you put alternating voltage into the transformer, you get alternating voltage out.  But that’s not the end of the story.  Many electronic devices operate on voltage that doesn’t alternate.  What then?  Will our handy wall wart still be able to bridge the electrical gap to fill our needs?      Next time we’ll see how the diode bridge and capacitor come into play to deal with the alternating voltage from the transformer in a manner eerily similar to a microwave oven’s high voltage circuit. ____________________________________________