mosfet bias question

I plan to use a mosfet in a dawn to dusk switching circuit with a 12V regulated power supply. In the application notes they give the gate source absolute maximum voltage as +/- 10V Gate to Source. (page 1)

On page 6 of the same application note they show a "Gate charge test circuit." with a 12 volt battery between the gate and source with a 50 Kohm variable resistor for the current source circuit.

Aren't they exceeding their own absolute maximum G-SV when the wiper is turned to the +12 side of the battery?

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My Circuit is simple with just a photo transistor gate to source and a

4-10 megohm resistor to the gate and +12V biasing it on when the photo transistor is dark. The load is a 12V ten watt LED spotlight.

I've incorporated several of these and never had a failure and they've been working outdoors for >5 years and I just now noticed that I'm doing something illegal according to the app note.

As I understand mosfets a G-S breakdown, even through a megohm value resistor, would kill it forever. Is this wrong?

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Only of they turn the pot all the way up, which they wouldn't. That's just a test circuit.

Most mosfets die at around 70 volts g-s, except that the protected-gate ones zener in the low 40's. 12 volts won't hurt your fet.

But you might fry it from power dissipation. Check your worst-case dissipation and heat sinking.

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

Worst case is when it is in the linear region. Heating is insignificant. While daylight is fading or the sun is rising, full on to full off and visa versa takes about 15-20 seconds. The difference in light is imperceptible, it surprised me. I figured it would dim gradually but it is a lot faster than I was expecting.

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Years ago, QST had a Class E amplifier (Author Sokal) I think they used a +/- 18Vpp signal to turn the FETs on and off, even though they were rated at 10V.

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