Ohm's Law & Digital Multimeters Alex Mason If you understand Ohm's Law and know how to use a Digital MultiMeter (DMM), then your renewable energy system holds no mysteries. Through the application of Ohm's Law and accurate measurement with a DMM, we can find out just about everything we need or want to know about our system's operation and performance. Ohm's Law You loved it before, and we're serving up again by popular demand. All Nerds, Techies, and Electroweenies can skip this basic explanation of Ohm's Law and proceed directly to the DMM Stuff. Ohm's Law describes the relationship between three characteristics of electricity. The first characteristic is Voltage (abbreviated as "E" in the formulas). Voltage is a concept and measurement of electron pressure, similar to the idea of water pressure. The second electrical characteristic is Current, or the rate of flow of electrons. Current is measured in Amperes (abbreviated as "I" in the formulas). The third electrical characteristic is Resistance. Resistance is the property of all matter to resist the flow of electrons. Resistance is measured in Ohms (½) and abbreviated as "R" in the formulas. This then is our cast of characteristics: Voltage (E), Current (I), and Resistance (R). A particularly awake human named Georg Simon Ohm worked out the relationship between these characteristics in 1800s. His equation is: E=IR Which may not seem too earthshaking, but it allowed ancient electrotechies to use electricity as something more than a curiosity. This relationship between Voltage, Current and Resistance is sublime and unavoidable. This relationship is built into every device in our systems. All devices using electricity operate by the grace of Ohm's Law. Understand this relationship, learn to measure these characteristics with a DMM, and your system will talk to you. You will know all, and by knowing, you will be better able to use and maintain your system. Lets look at Ohm's Law more carefully. It is an algebraic equation. If we know two of the the three quantities, we can mathematically determine the third. Seem abstract? Well it isn't. Consider this example. We want to know how much current the inverter is consuming form our battery. Since direct measurement of the large currents (well over 500 Amperes) is difficult and expensive, we insert a small amount of resistance between one of the inverter cables and the battery. This small resistance is called a shunt and is precision made to have a very specific amount of resistance. In this case let's consider a shunt with a resistance of 0.001 Ohms. If we can accurately measure the Voltage drop across the shunt, and we know the Resistance, then we can calculate the amount of current (Amperes) flowing through the inverter. Very useful, this. And this is just one place that Ohm's Law and a DMM can provide accurate information about system performance. The three components of Ohm's Law can be used to calculate other electrical characteristics like Power. Electric power is measured in Watts and abbreviated as "P". Power is the product of Current and Voltage (P=IE). Consider the inverter shunt example. Since we've already determined the current flowing through the shunt, all we have to do is measure the battery's voltage, multiply the Current times the Voltage and presto we have the Power that the inverter is consuming in Watts. All this from two measurements, and the practical application of Ohm's Law. The graphic below show the mathematical relationship between Voltage, Current, Resistance, and Power. You can get the needed equation for any of the quantities shown in the center of the circle by looking to the outside rim. INSERTGRAPHIC This article is not intended to be a complete course on all the ramifications of Ohm's Law. Techies have written entire books of the subject. The serious renewable energy person will find that time spend learning and understanding Ohm's Law is time very well spent. See the following Home Power issues for more information: #1- pg.35, #3- pg.40, #4- pg.33, and #6- pg.35. Get a basic physics book and read up on electricity. My favorite is the one I used in high school, Modern Physics by Dull, Metcalfe, & Williams. It's simple, to the point, and understandable. Nerding Ohm's Law with a DMM Modern electronic instrumentation has reached the point were accuracy and utitlity are out of sight. For even a few bucks, the average user of electricity can buy a digital meter that will accurately measure all the three components of Ohm's Law. Twenty years ago, this equipment would have cost a fortune and filled a room. Now it is affordable and packs into a lunchbox. The best of the lot now are the Digital MultiMeters (DMMs). They all measure the following electric parameters-- Voltage, Curent, Resistance. Some models also measure Frequency, Capacitance, and Duty- Cycle. Accuracy varies from 3% to 0.1%, depending on the DMMs qualitity. They all display their information as numbers on an Liquid Crystal Display (LCD). The LCDs are rugged, and consume miniscule amounts of power, so the DMM is very portable and suited for backwoods use. What follows below are descriptions of three DMMs, of varying quality and cost. In the field of instrumentation, you get what you pay for. The Radio-Shack DMMs Radio Shack offers many DMMs in their catalog. These are entry level meters and we list them here because they are inexpensive and avaialble everywhere. The current model that works best and is most cost effective is their #22-185 for $59.95. It works and measures down into the milliVolt range, making it useful for shunts like our inverter expample. The qualitiy of the hardware used to make these Radio Shack meters is not very high. In our experience, two years of heavy use will wear out the rotary switches. We test their basic DC voltage accuracy at around 3%, which is not bad considering their price. One problem with the less expensive meters is that they will not be able to accurately measure either the ac Voltage or ac current produced by inverters. Accurate measurement of nonstandard ac waveforms (like inverter power) requires the use of a peak reading and true RMS reading meter. None of the Radio- Shack meters offer true ac Voltage measurement. The Fluke 77 This DMM is rugged and an entry level professional model. Basic DC voltage accuracy is 0.1% and that's very accurate. In fact, the Fluke DMMs mentioned here all have calibration tracable to the National Bureau of Standards (NBS). This means that your Fluke talked to a meter that talked to a meter that talked to THE METER. Cost is $150. This unit uses a microprocessor to run a self-check on all functions every time the 77 is turned on. The Fluke 77 is very well constructed of high quality components and will last many years of hard service. The unit comes in a rubber armored case to protect it from Life's little knocks. The Fluke 77 does not have true ac voltage measurement so it can't accurately measure inverter power output. That takes its big brother the "87". The Fluke 87 If this meter could cook I would marry it. The Fluke 87 offers all the standard DMM features of the Fluke 77 mentioned above, 0.1% DC accuracy, and it measures peak and true RMS voltages. The 87 can accurately measure all the electrical parameters of an inverter's output! And a host of other things, like: frequency down to 0.1Hz, duty-cycle, and capacitance. The Fluke 87 also records measurement data and can function as an Ampere-hour meter, among other things. The record feature works in most functions and detects maxima, minima and true average measurents. The meter will measure and record ac or DC data as fast as 1 millisecond. This makes it very useful for measuring surges and transients. It will record data for up to 35 hours. It is also capable of 4 1/2 digit operation, which gives the user even greater accuracy than either of the meters above. Cost isn't cheap, Å$280, but then this is a hell of a meter. Use it! It's not important which DMM you use, but if you are making your own electricity, you should and and know how to use A DMM. Understanding Ohm's Law, and armed with your DMM, you are ready for anything your system can throw at you. You can troubleshoot problems. And, most importantly, you can most effectively use your system. Access I will answer questions on instrumentation, write Alex Mason, C/O Homer Power, POB 130, Hornbrook, CA 96044. Radio Shack is everywhere and you don't need my help to find one. Call Fluke at 800-443-5852 for access data on your nearest Fluke dealer.