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mrquincle/dimming.md

Last active July 2, 2019 15:48
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Crownstone dimming
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dimming.md

The Crownstone dimming uses a trick to make sure the IGBTs are not turned on partly when the voltage at the input of the gate driver is not yet up to around 15V.

How this is done is to place a 8V2 Zener back-to-back with the 15V Zener to be able to generate a negative 8.2V (give or take a diode offset of 0.7V). Subsequently, there is a voltage divider R2, R4 which divides this by two. The voltage of around -4V is fed into a depletion mode n-mosfet. The latter will be conducting till its negative threshold has been reached (-2.8V typical). If this has been reached it will be off. At the moments before that it will regularly conduct and short the output of the IGBT driver.

Circuit

The problem is that we - supposedly - don't see the voltage going down to -4V, but to only -2V. What's going on?

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