Resistance is Futile, Unless Using Ohm's Law, Hak5 1719

• published: 24 Dec 2014
• views: 276

This week on Hak5, Shannon is calculating resistance with Ohm's Law, and explaining how resistors work in small electronic circuitry. Happy Holidays!! What is Ohms law? It comes from Georg Ohm, a scientist who defined resistance by calculating voltage and amps. V = I * R ---- V = voltage in volts, I = current in amps, and R = resistance in ohms. The voltage is the amount of electric current that's being held inide the circuit. Ohm is the resistance, the narrowness of a channel where the electricity flows through (like a bunch of traffic cones cutting off a lane - it creates some traffic). The flow (or speed, or current) of the electricity ends up being the amp. So if the resistance goes up in size (as in more lanes are cut off), then the amp speed goes down, because it becomes slower. Now, we can use this math to create future currents. If you have a 12v battery, and an LED light with 22mA current, then you'd do this: 12volts / 0.022A = ? ohms? = 545 ohms. So you'd want to use a resistor with 545ohms or higher of resistance. You can find HUGE packs of resistors on Amazon and tons of other places, and they usually have several different values in a pack. Common ones include: 0, 1.5, 4.7, 10, 22, 47, 100, 220, 330, 470, 680, 1k, 2.2k, 3.3k, 4.7k, 10k, 22k, 47k, 100k, 220k, 330k, 470k, and 1Mohms. If you need a specific resistance, you can either buy that one or if it's not available, you can stack them up together in a 'series' and the resistance value simply adds up. Resistance is measured in Ohms, which is a unit from the International System of Units. Ohms are usually spelled out in thousands- such as 1000ohms would be 1kilohm, or 3,000,000 ohms would be 3Megaohms. Resistors have a never-changing electrical resistance. The resistor's resistance limits the flow of electrons through a circuit. They only consume power, they can't create it, so they are called Passive components. Resistors are usually added to circuits where they complement active components like op-amps, microcontrollers, and other integrated circuits. Commonly resistors are used to limit current, divide voltages, and pull-up I/O lines. Resistors are shown as squigglies or a rectangle with two straight lines on both ends whenever on a schematic. They'll usually be called R# for the name, and under it you'll see the value in ohms, like 1kohm. Choosing a resistor is important: there are several different types - vary in value, size, and look. Through-hole resistors are like mine. They have long leads that you can cut shorter as needed. You can solder the leads too. The "axial package" is the plastic part. The bigger ones are 1/2W, smaller ones are 1/4W. This is called the "power rating". A power rating tells you tells you what rate the electric current is turned into something else. If you have a 75W light bulb, it's turning electricity into light at 75 watts. With resistors, the electricity is turned into heat. If you have too much heat going to a component, you get fire. Calculate the power rating, if it's not obvious from the size or if you can't find it listed, by taking P (power) = I (amps or current) * R (resistance). Surface-mount resistors are the other kind. These are tiny black rectangles that have silver solder points on their edges. These are more common in mass production, where a robot does all the work. Resistors are made up of carbon, metal, or metal-oxide film. If we stripped off the outside part of a resistor, you'd see the carbon-film wrapped around an insulator. The more wraps you get, the more resistance it has. Color coding matches the resistance and tolerance of the resistor. A good resistor pack will come with a color code graph for the bands. Mine has up to 5 bands, with the first three being the first 3 values, the 4th being the multiplier, and lastly being the tolerance (+ or - a percentage, because no resistor if going to be made absolutely perfectly, they may be off by a bit of an ohm). So match the first three bands... then multiply (reminder: 10 to the power of 2 means you add two 0's. 10 to the power of 7 means you add seven 0's to the end). Surface mount resistors use a different type of code, so refer to online guides for those. We'll discuss parallel resistors and resistor networks later on as we build complicated robots. These are the Sunfounder kits I've been using: here and here.