Charles Bannister wrote:
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Some more information about how a transistor could wipe out a lot of boards might be interesting to the rest of the forum community.


In a circuit where I drive a lot of LEDs in a 5 x 7 matrix (105 LEDS, 2.4v forward volts, one pixel = three LEDs), I buffered the output of a MAX 7219 with a failry sophisticated current mirror circuit. The circuit used a small transistor pair as a mirror, and an op-amp to form the gain part of the system. The output of the opamp had to drive a bipolar transistor which was to deliver 100mA (peak) into the LEDs. To get 100mA through the BJT needs 100mA/ hFE mA from the base drive. My problem was that we ran out of production stock and I needed to order more BJTs in an awful hurry. In my rush, I misread the spec as I said. The result was that to get the deamnded current, the op-amp would fight very hard to achieve it, beyond its output current rating. The op-amp overheated in the rated 60 Centigrade ambient. POP !

Steve