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You know what, Kyle? I've learned something today. - Adventures in Engineering — LiveJournal
The wanderings of a modern ronin.

Ben Cantrick
  Date: 2006-01-15 23:47
  Subject:   You know what, Kyle? I've learned something today.
Public
  Mood:stupid
Today I learned that op-amps (expect special, expensive ones) won't assert their full supply voltage(s) to their output. Moreover, the difference between the best possible output and the supply voltage is usually greater than 0.7 volts. In practical terms, this means that an op-amp output can be pushing itself as far down as it goes, and the voltage it's asserting still may not be low enough to turn off your transistor that has a 0.7V base-to-emitter threshold.

I've also learned that electronic circuits in general work a whole lot better if you actually put the zener diode in place with the correct lead pointing towards ground!
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Alex Belits: iskra
  User: abelits
  Date: 2006-01-16 01:48 (UTC)
  Subject:   (no subject)
Keyword:iskra
1. Base to emitter, not base to collector.

2. If you can't create an offset by other means, just divide the output voltage with resistors.
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Ben Cantrick
  User: mackys
  Date: 2006-01-16 07:55 (UTC)
  Subject:   (no subject)
1. Base to emitter, not base to collector.

Right you are. Thanks for the correction.

2. If you can't create an offset by other means, just divide the output voltage with resistors.

Thought about that, but decided that I would rather make the upper resistor in the pair a zener diode, and the lower one a 1Meg resistance. You can see how the circuit works now, I bet...
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