When power supplies don't meet specs
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 6/1/2008
Randy Jones, a senior telecom specialist with the Washington State Department of Natural Resources, found that power supplies don't always meet their output specs. After some investigation, Jones uncovered the problem and a solution.
The department uses 13.8-VDC-powered VHF radio transceivers to assist firefighters with communications when they are fighting wildfires on the 12.7 million acres the department protects. Jones discovered that the radios, when transmitting, would force some of the power supplies to shut down from current overload.
 Transceivers draw 12 A for 90 ms before dropping to 6 A while transmitting. When receiving, the transceiver draws 1 A. |
“When a radio was keyed to transmit, its current consumption would jump from 1 A to 12 A,” Jones said. “Although 12 A was below the maximum current rating of the power supply, the supply would shut down.” The oscilloscope plot (figure) shows the power from the supply. When a user keyed a transmitter, the current draw jumped to 12 A before settling at 6 A.
A call to his power-supply supplier revealed that the manufacturer had published incorrect specifications. Jones purchased higher-rated power supplies, and the failures ceased. The manufacturer refunded the department for the cost of the original supplies.
Today, Jones doesn't buy power supplies based on manufacturer's specifications alone. He tests all new supplies before deploying them. Using an electronic load, Jones tests power supplies by setting the load to constant current. For example, one supply is tested 50% at 25 A and 50% at 35 A to ensure it operates at its rated specifications. Jones uses a logging digital multimeter to monitor the output voltage during a test.
