Conformity Magazine - July 2008 - (Page 52)

But doing so still doesn’t eliminate the need to allow for diode drop when selecting a supply. Keep in mind that the supply must be capable of providing a voltage equal to the sum of the voltage required across the load, the diode drop and the drops (round trip!) in the wiring. So don’t determine the wire gauge to use based only upon safety considerations. Particularly at low voltages, the lower drop of a larger gauge wire can make the difference between a supply working well, just marginally, or not at all. A typical power supply can compensate up to a volt or so of drops in the wiring, but may not be capable of compensating for both the wiring and the diode drops of a redundant system. And if you’re using remote sensing to regulate the voltage across the load, you might not be able to solve this problem by simply stepping up to a supply with a higher nominal output voltage (for example, going from a 5 volt supply to a 6 volt supply), because then the sense lines of that supply would try to maintain 6 volts across the load rather than 5. Therefore, be sure to use a supply that is capable of putting out a voltage high enough to compensate for both the diode and wiring drops under worst-case conditions (usually, at low line voltage and with maximum rated load current being drawn), and which also has the desired load voltage within its adjustment range. A supply’s maximum output voltage is usually considered to be the high end of its adjustment range. For example, a supply with an output specified as 24V +/-1V could be relied upon to provide a maximum of 25 volts. So, if the load requires 24 volts and if the combined drops will be no more than 1 volt, you’re in good shape. But if the sum of the diode drop and wiring drop exceeds what the high end of the voltage adjustment range would guarantee, it’s a good idea to contact the manufacturer of the supply and ask if you can count on any more voltage being available to compensate for the drops. If not, a ‘special’ may be required. Sometimes the solution is to use a wide range power supply; in the above example, a 0-30 volt supply adjusted to 24 volts would be capable of compensating ‘round trip’ drops of up to 6 volts. Perfectly functioning power supplies are of no value when they’re not receiving input power. However, if two sources of AC power are available, providing separate AC wiring for each power supply permits using a different source of input power for each supply, resulting in the additional advantage of input power redundancy. Even using two different branches of the same building power source will offer improved input redundancy. A battery-backup UPS may also be used in series with one of the inputs, further improving overall reliability by permitting continued normal operation of the load, even if both of the AC sources should fail simultaneously. Monitoring Output Constantly monitoring each power supply’s output is very important. Otherwise, you might not even know that one power supply is out until the other goes, and then it’s too late. Voltmeters and/or indicator lights are helpful, of course, but they don’t command attention and may not be checked regularly. However, an audible alarm can’t be easily ignored. Include an undervoltage alarm circuit on the output of each supply to detect if its output is lower than normal (or a relay can be used if you simply wish to know if an output is there or not), and use it to control an audible alarm, either built into the assembly containing the power supplies or remotely located where it will be heard. The contact wiring for two or more relays can be cascaded so that only one audible alarm is required. Checking the meters or indicator lights will then disclose which of the power supplies is low. Also be aware that power supply outputs don’t always go low when they fail. With linearly regulated supplies, the series pass transistors can short and the voltage can instead go high (by 50% or more in some cases) and quickly fry the load. Therefore, it’s vitally important that power supplies used in redundant applications always be equipped with fastacting overvoltage protection to assure that the output voltage can’t go much higher than the nominal under any circumstances. But that may in turn introduce a problem if the electronic environment harbors transients (particularly from electromechanical devices such as motors, solenoids or high current contactors); transients which are being radiated or conducted into the system wiring may cause frequent nuisance tripping of the overvoltage protectors. So it may be necessary to include line filters, bypass capacitors or other transient suppression to Figure 2: Simplified diagram for redundant power packages 52 Conformity JULy 2008

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Conformity Magazine - July 2008 Contents Editor's Note FCC Adopts 6 Million Dollars in DTV Enforcement Orders Commission Enforcement Bureau Adopts V-Chip Consent Decree FDA Seeks 2 Million Dollars from Medical Device Manufacturer FDA Answers Questions About CFLs and UV Radiation EPA Issues Energy Star Specs for TV, Video Boxes Canada Unveils Sweeping Product Safety Legislation How the Evolution of CISPR Standards Continues to Shape the Requirements for Anechoic Chambers ESD Open Forum Reach Compliance for Non-EU Article Manufacturers The iNARTE Informer Preamplifier Considerations for Emission Measurement Systems Effective Shielding in Medical Electronics Applications Product News Focus On...Power Components Buyer's Guide Updated Standards List for the EU's ATEX Directive EOS/ESD Symposium Set for September CPSC Actions in the News Researchers Work to Develop Non-Flammable Batteries Safety Link Launches Redesigned, Enhanced Web Site IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - July 2008

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