I am trying to drive some small water pumps with a 74HC595, I added transistors to the output of the 595 in a common emmiter configuration, I used BD135 transistors and 700 ohm resistors for the base of the transistors.
The way I did the Resistor calculations is as follows, the expected output voltage from the 595 outputs are 5V, and the datasheet tells that a good current per pin output is 6mA, the transistor VBE on is 1V. Hence:
R = (5-1)/0.006 = 666 ohms -> 680 ohms resistor used.
BD135 has a minimum beta(HFE) of 25 , so should I expect to be able to handle at least 150 mA per pin output?
I have done a board with this schematic and it results on 595 heating until it is dead.
Can someone point me where do this went terribly wrong?