Chloroplast type 1 SE

Posted on Feb 6, 2014

The MC34164 VS used in the Chloroplast SE is different in one respect from the 1381 VS (used in the VTSE and 1381 SEs) in that the MC34164 has an `open drain` output which requires a pull up resistor to the positive supply for the output to go positive. The CSE uses a resistor in series with the 34164 chip input and takes advantage of the fact tha

Chloroplast type 1 SE
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t the quiescent supply current for the 34164 is higher when the output is low and decreases to almost zero when the output is high. That means the voltage drop across that series input resistor, generated by the quiescent 34164 current, must be added to the rated "turn on" voltage which raises the voltage on the supply cap required to turn on the 34164 (ie to 7V). After the 34164 turns on, the quiscent 34164 current drops to zero and the voltage drop across the series resistor also drops to zero, so that the voltage at the 34164 input pin suddenly jumps up to the supply cap voltage. The voltage at the "open" 34164 output pin is now pulled up to the supply through the output resistor, but the output pin voltage only rises to the forward voltage of the darlington transistor base emitter junction or about 1. 2V. The darlington turns on and the collector supplies current to the rest of the circuit. The supply cap voltage starts to drop as the motor load etc depletes the cap charge until the supply voltage reaches the 34164 rated turn off voltage (ie 5V) and the process repeats In one of my earlier MC34164 hysteresis SE designs (hySE), the 34164 upper threshold was raised by inserting a resistor in the ground lead and using the voltage drop generated from quiesent current and sinking the current from the output pull up resistor. Craig Maynard greatly improved on that design using the same principle but simplifying the circuit in...

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