WorldWide Drilling Resource

19 WorldWide Drilling Resource ® JULY 2015 The Resistance Movement by Britt Storkson Owner, P2FlowLLC Those who have worked with AC (alternating current) power transformers know if you place an ohmmeter across both the transformer primary and second- ary - when the power is off, of course - you will measure a very low resistance. This is because the wiring looks like a “short circuit” to an ohmmeter. In fact, the transformer primary is indeed a short circuit for a fraction of a second when first turned on. The AC voltage oscilla- tions create the impedance needed to limit the current flow into the transformer. So how does one know the differ- ence between the normal transformer wiring and a real short circuit? Both look the same when tested with an ohmme- ter. One way to do this is to use a resistor in series and measure the difference in voltage across the resistor when the power is turned on. First, disconnect the secondary loads on the transformer and determine the no-load current on the pri- mary connection. The transformer manu- facturer should have the information. Let’s say the no-load transformer primary current is 1 amp. Since Voltage = Current x Resistance (Ohm’s law), if we have 1 amp of current flowing and use a 5-ohm resistor in series with the trans- former primary, we will measure 5 volts difference from one end of the resistor referenced to ground. 1 amp x 5 ohms = 5 volts. This would verify the transformer primary circuit is working properly and there are no wires “shorted” to ground. For safety, make sure the resistor is rated for the voltage you wish to meas- ure and all connections are properly in- sulated. One must also size the resistor for wattage output. Since Volts x Amps = Watts (Ohm’s law again), 1 amp with a 5-volt “loss” through the resistor would be 5 watts. So the resistor wattage rat- ing for this application must be 5 watts or higher to safely dissipate the heat which will be generated. Now if there is a short circuit, the voltage difference in this example will be greater than 5 volts and the resistor can get very hot, very fast. However, the current will be limited and the resistor will protect the rest of the wiring. Most electronic equipment can withstand an overload for a short period of time with- out damage. With no resistor in series, the only indicator you will have something is wrong is a blown fuse or circuit breaker. One can get away with almost any- thing if you use a resistor in series to protect your circuit. I can run 240 volts AC into my microprocessor if I use a high value (>1 million ohms) resistor in series to protect it. A resistor limits the voltage which also limits the current into the cir- cuit. Voltage + Current Flow = Energy, which is what damages things. Voltage is what gives you the static electricity “shock” when you shuffle your feet across the carpet during the winter when there is low humidity. Electrons are stored on the surface of an insulator since they cannot easily pass through the material. They are quickly released when you touch something conductive, which provides the electrons with a path to ground. The reason you are not elec- trocuted by this is because while there is very high voltage present there is very little current, and therefore very little ener- gy to cause damage. A resistor limits voltage which lim- its current. So next time you need to limit the current to a circuit, connect a resis- tor in series. The voltage will be limited to the equipment and it may not work, but you will get an idea of whether it is “pulling” the correct amount of current. Resistors are cheap, readily available, and even come in fusing types so they will interrupt current just like a fuse. A little research “up-front” can often save a lot of headaches down the road. Britt Britt Storkson may be contacted via e-mail at michele@ worldwidedrillingresource.com Do you have a special person, product, or project you would like to see featured in WWDR ? We want to hear about them! September is Spotlight Month... Act now and don’t miss out! E-mail bonnie@ worldwidedrillingresource.com or give us a call. (850) 547-0102 DEADLINE Is JULY 15 th .