## Posts Tagged ‘alternating current’

### Gear Reduction Worked Backwards

Sunday, March 9th, 2014
 Last time we saw how a gear reduction does as its name implies, reduces the speed of the driven gear with respect to the driving gear within a gear train.   Today we’ll see how to work the problem in reverse, so to speak, by determining how many teeth a driven gear must have within a given gear train to operate at a particular desired revolutions per minute (RPM).       For our example we’ll use a gear train whose driving gear has 18 teeth.  It’s mounted on an alternating current (AC) motor turning at 3600 (RPM).   The equipment it’s attached to requires a speed of 1800 RPM to operate correctly.   What number of teeth must the driven gear have in order to pull this off?   If you’ve identified this to be a word problem, you’re correct.       Let’s first review the gear ratio formulas introduced in my previous two articles: R = nDriving ÷ nDriven             (1) R = NDriven ÷ NDriving             (2)       Our word problem provides us with enough information so that we’re able to use Formula (1) to calculate the gear ratio required: R = nDriving ÷ nDriven = 3600 RPM ÷ 1800 RPM = 2       This equation tells us that to reduce the speed of the 3600 RPM motor to the required 1800 RPM, we need a gear train with a gear ratio of 2:1.   Stated another way, for every two revolutions of the driving gear, we must have one revolution of the driven gear.       Now that we know the required gear ratio, R, we can use Formula (2) to determine how many teeth the driven gear must have to turn at the required 1800 RPM: R = 2 = NDriven ÷ NDriving 2 = NDriven ÷ 18 Teeth NDriven = 2 × 18 Teeth = 36 Teeth       The driven gear requires 36 teeth to allow the gear train to operate equipment properly, that is to say, enable the gear train it’s attached to provide a speed reduction of 1800 RPM, down from the 3600 RPM that is being put out from the driving gear.       But gear ratio isn’t just about changing speeds of the driven gear relative to the driving gear.   Next time we’ll see how it works together with the concept of torque, thus enabling small motors to do big jobs. _______________________________________

### 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. ____________________________________________

### 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. ____________________________________________

### Ever Had a Wall Wart?

Sunday, August 28th, 2011

### Electrocution by Microwave Oven

Sunday, August 21st, 2011