WorldWide Drilling Resource

Don’t Put it in Reverse by Britt Storkson Owner, P2FlowLLC There are a lot of things that work fine going one direction, but not so great going in reverse. Your car, for example, goes pretty fast going forward, but not so fast going in reverse. Then again, we don’t want to go too fast in reverse because we cannot easily see where we are going, and it’s just safer that way. Electric current is the same way. Most electronic components will not tolerate the reverse flow of cur- rent. If current flows through an electronic part the “wrong way”, you probably won’t have the part anymore. So it is very important to keep the current flow in one direction. Fortunately, there are numerous electronic components capable of doing this. The most common type of current flow reg- ulator is a rectifier diode which acts like an electrical “check valve”. A diode allows current flow in one direction, but blocks current flow in the other direction. However, there is a penalty - a “trade off” so to speak - as each diode requires about 0.7 volts to “turn on”. This means no (or very little) current will flow if the voltage is less than 0.7 volts. When the voltage does rise above 0.7 volts, current will flow, but you will have about a 0.7 volt loss. For example if you feed 12 volts into a diode you will have 12 - 0.7 volts or 11.3 volts com- ing out of the diode. Two diodes connected in series doubles the voltage loss, so instead of a 0.7 volt loss, you have a 1.4 volt loss. Three diodes connected in series triples the voltage loss and so on. The voltage does not just disappear. It’s dissipated as heat proportional to the current passing though it. So if you have a 0.7 voltage loss at 1 amp, you will have 0.7 x 1 = 0.7 watts of heat dissipated by the device. So there’s an energy penalty for the ability to block the flow of current. So we “trade” some energy for the ability to control the current direction. When diodes are arranged in a “bridge rectifier” configuration (see illustration) you can place the current source on either of the “~” terminals and the diodes will make sure the current flows correctly to the + and - terminals. This is very useful for installations where you tolerate a reverse current condition. They also work for battery charging where you want to put current into a battery for charging, but don’t want it coming out the same direction. I’ve found some electronic equipment makers are not very careful about pro- tecting from reverse voltage. All it takes is a split-second to do permanent damage. I bought a trailer brake control which had the power wires color-coded in a way most people would consider “backwards”. With a direct current (DC) system, the red is typically the “power” or “hot” wire, and the black is the ground wire. But this brake system wasn’t color-coded the usual way, making it very easy to hook up wrong, resulting in a dam- aged-beyond-repair brake control unit. One wonders if it wasn’t deliberately made like this just to sell more of the product. Reverse current circuit protection in the form of diodes or other components can easily and inexpensively be built into the product, but often isn’t. In any case, it’s easy to protect against this condition by installing a bridge rectifier. They are inexpensive and readily available. One should always buy a part is rated for about five times the actual amperage it will be expected to handle. For example, if you need to handle 2 amps, then buy a 10-amp bridge rectifier. There’s a reason we do this, which is beyond the scope of this article, but if the actual current flow is about 20% of what the bridge rectifier is rated for, you will be fine. Bridge rectifiers also need to be rated for reverse blocking voltage. With 12-volt automotive electrical systems, a 50-volt blocking voltage is usually more than ade- quate. There isn’t much of a price difference between a 50-volt and a 1000-volt blocking voltage, so I usually buy the 1000-volt part. Remember, with a bridge rectifier the current flows through two diodes in series so your voltage loss will be about 1.4 volts. One must also be aware of the heat rejection as a bridge rectifier will dissipate about 1.4 watts per amp. Volts x Amps = Watts. Higher current bridge rectifiers come in a metal case or have a metal surface which can be mounted to another metal surface or heat sink for cooling. Bridge rectifiers also help isolate the part from the rest of the circuit, and high- blocking voltage rectifiers tend to protect the part from voltage “spikes” which can come from a variety of sources. So they are cheap insurance in this sense. Properly designed bridge rectifier circuits are rugged and reliable, and will probably outlast the equipment they’re connected to. Britt Britt Storkson may be contacted via e-mail to michele@worldwidedrillingresource.com 302-684-3197 FAX: 302-384-0643 142 Broadkill Rd. • Milton, DE 19968 www.atlantic-screen.com email: atlantic@ce.net Manufacturers of Slotted & Perforated Pipe ranging from ½” to 24” diameter Atlantic Screen & Mfg., Inc. • Well Rehab. Products • Manholes • Bentonite • Filter Sock • Inline Chemical Mixers • Sampling Bailers • Clear PVC Pipe • Locking Caps 19 WorldWide Drilling Resource ® OCTOBER 2017