WorldWide Drilling Resource

35 WorldWide Drilling Resource ® FEBRUARY 2015 Protect Your Investment by Britt Storkson Owner, P2FlowLLC Often, people have a lot of money invested in switches and relays. When switching inductive loads, like motors and solenoids, the switches must be able to take the “back EMF” (electro- motive force) generated when the switch opens and the current flow (amperage) is interrupted. When this happens, the energy stored in the coil is released as a high voltage which, over time, can damage switch contacts. If it’s dark, you can actually see the “spark” generated when the contacts open. That’s how mechan- ical “point type” ignition systems work. Amperage flowing through the ignition coil is released to the proper spark plug when the “breaker points” open. Usually, the way switch and relay manufacturers deal with this transient high-voltage condition is to make the switch contacts larger so the contacts can erode over time and the switch will still function. While this works, it still doesn’t solve the problem of high-voltage “spikes” coming through the wiring. The automotive ignition “breaker points” solved the contact erosion prob- lemby using a “condenser” or high-voltage capacitor which would “store” the high voltage generated by the points open- ing, and released it back into the circuit when the points closed. A better way to deal with this prob- lem with direct current (DC) circuits is to install a common rectifier diode in the “reverse biased” position. This is with the cathode of the diode connected to the positive part of the circuit and the anode of the diode connected to the negative, or ground part. This works best if it is mounted as close as possible to the coil or motor. When “back EMF” is generated by the coil or motor, the diode will block current flowing in one direction, but will “turn on” and conduct current flowing in the other direction. This keeps the voltage from rising to much more than about one volt above the system voltage. Use diode part number 1N4001 - 1N4007 or equivalent for this task. These diodes are cheap (about five cents) and very reliable. For years, General Motors mounted one of these diodes in the air conditional magnetic clutch mechanism electrical connector. It was a simple, cheap way to keep unwanted voltages out of the car electronics. For alternating current (AC) circuits, a voltage “snubber” needs to be used. Since AC current is flowing in both direc- tions, the current must be blocked in both directions. A voltage “snubber” is typically a resistor (usually 100 ohms) and a capacitor (usually 0.1 microfarad, 250 volts) connected in series. This circuit tends to “dampen” high voltages generated in the system, so they won’t cause a problem. This “snub- ber” network is cheap and reliable, and works with both AC and DC circuits. Since computer memory is voltage “stored” on silicon, any “stray” voltage may cause a problem if it alters the memory state of one or more memory “cells”. Manufacturers try to keep their circuits as stable as possible using a number of techniques, but it’s best to keep the “stray” voltages out of the cir- cuit instead of expecting the circuit to deal with them. Britt Britt Storkson may be contacted via e-mail at michele@ worldwidedrillingresource.com 3"- 0)'! +#0* /'! /,,). '+ ./,!( '4#. 6 /, 6 +(. ,1* + 13 -"#+ ))#3 5 2 - 0!&*$% !,* 5 - 0!&*$% $-,+/'#-+#/ !,* ,"#) '4#. /#+/ + " #+"'+%