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October 2017

Temperature Rise Of A Heatsink In An Amateur Radio Transceiver

I recently did a “tear-down” of a Baofeng BL-8 battery eliminator (for use with a UV82 dual-band tranceiver; see www.hamguyparts.com, articles). The tear-down revealed that the eight volt battery eliminator actually used 2X L7808CV TO-220 voltage regulators connected pin for pin in parallel, which is somewhat unconventional.

Both regularors were mounted to a common small (20 x 34 x 1.8 mm) aluminum heatsink and all enclosed within the plastic housing which was sized to resemble the battery it was replacing.

In the ideal case, each regulator would carry 1/2 the total maximum current taken by the transceiver, which is approx. 0.775 amps. Each regulator produces a nominal 8.0 volt DC output and has to dissipate approx 4.6 watts of heat. The thermal resistance of the TO220 package is 5°C per watt, junction to case (or mounting base).

They are mounted dry, which adds another 1°C per watt for mounting the base to the heatsink.
  Q1: What is the temperature rise in the aluminum plate?
  Q2: Since there is no direct path to air, what further temperature rise may take place inside the plastic housing?

#10173
Don Dorward
Pickering, ONT Canada

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