Conformity Magazine - July 2008 - (Page 53)

prevent this, and doing so will probably also be helpful to the load. Are you thinking about using output fuses? Don’t! Virtually all power supplies today have output current limiting circuits that will drop the output faster than the time required for a fuse to warm up and blow, so including a fuse won’t accomplish anything. And current limiting has the additional advantage over a fuse of automatically resetting after a surge, while a blown fuse is totally counterproductive to the goal of a redundant power system, which is to always have output present. Other Considerations As with any other power supply application, ventilation is important. Space the power supplies away from sources of heat. If convection air flow is restricted, use a fan. Overheating may cause output drift but, perhaps even more importantly in the case of a system intended to have long-term reliability, it dries out capacitors, probably the single greatest cause of power supply failure. And speaking of capacitor dryout, schedule testing of both supplies at least annually, to be certain that each is capable of functioning properly. If the capacitors are drying out (reducing the output current capability of the supplies) and the supplies are sharing the load, it’s possible that working together they can support the load. But, if one should fail, the other won’t be able to support it by itself. Or, if one supply is set slightly higher than the other, the first will provide all of the current because the isolation diode of the other won’t be forward biased. The voltmeter of the other may show that it’s maintaining its output voltage, but that doesn’t necessarily mean that it can support the entire load. Slightly increase the output voltage of the backup power supply occasionally to verify that it can support the load by itself. . Summary Configuring a redundant power system isn’t as simple as connecting two power supplies in parallel. It requires careful consideration not only of how the power supply outputs may be affected by the manner in which they are connected, but also of factors that may affect both short and long term performance, and a physical arrangement that permits safe and fast maintenance while the system remains on line. The time spent in properly configuring a redundant power system will be time well spent, greatly reducing the possibility of a failure in critical equipment at an inopportune time. Tom Skopal is the vice‑president of sales and marketing at Acopian, and can be reached at tom@acopian.com. FAST Link www.conformity.com/1970 http://www.atecorp.com http://www.atecorp.com http://www.conformity.com/1970

Table of Contents Feed for the Digital Edition of Conformity Magazine - July 2008

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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