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Low voltage shut off circuit question (mosfet + zener)

supak111

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Apr 29, 2012
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Hi guys I need to make a simple switch that turns OFF anytime the voltage is below 9v.

I was wondering if I took a mosfet to be the switch and put a 9v zener diode on the gate, would this circuit work well? In principle anytime the voltage is 9v zener brakes down and mosfet/switch would conduct. And if it goes below 9v the zener stops conducting and so does the mosfet right?

PS zener question, if a zener is rated at say 9v break down, will it conduct right at 9v and above? And when does it stop conducting 8.9v 8.8v? 8v?

My setup uses 10v and 8v, change between the two is fast. At 8v I need it to not conduct and at 10v circuit above needs to conduct. Would I then choose a 9v zener, right in between the 2 voltages?
 
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Harald Kapp

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  1. You'll have to take into account the threshold voltage of the MOSFET in addition to the zener voltage.
  2. A zener diode has a rather sharp knee from non-conducting to conducting, but not a clear on-off characteristic. The voltage across the zener diode will vary with current. The datasheet will tell you the detailed characteristic. It is up to the application (or you as the circuit designer) to define the point where current through the zender diode is small enough for the zener diode to be considered "off".
    For this reason you need to put a resistor from the aget tor the source of the MOSFET to turn it completely off by routing any residual current through the zener diode to the source (otherwise it will chage the gate and the MOSFET will not turn off completely).
  3. The diffference between your two voltages (8V and 10V) is probably not enough to reliably turn the MOSFET on and off under all conditions (tolerances, temperature etc.). You will fare much better by using a Schmitt-Trigger (Google) circuit to detect the change in voltage and create a clean on/off signal to the gate of the MOSFET.
 

supak111

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Apr 29, 2012
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Thank you, you've answered everything I was wondering about.
 
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