Author Topic: Zener clamping at microamp supply levels  (Read 177 times)

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Offline jmoschetti45

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Zener clamping at microamp supply levels
« on: September 19, 2019, 06:44:29 pm »
Long story short, I have an input of 4-125VAC I need to measure with a comparator. Preliminary testing shows I'll need my threshold to be around 12V. The comparator portion is easy.

My problem is the variable voltage, or I'd just use a cap dropper to drive the opto.

My thoughts are a simple full bridge with a series resistor with a zener. To keep the dissipation down, I'd shoot for something like 220k or 330k. I don't remember the op amp's input current requirements off the top of my head (LM358 ....have a ton laying around).

Usually, I'd just use a zener to limit the voltage, but will the 1mA or less cause issues with the zener?
 

Offline blueskull

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Re: Zener clamping at microamp supply levels
« Reply #1 on: September 19, 2019, 06:51:33 pm »
Use a bandgap. TLV431 (the V version) goes down to 100uA.
 

Offline jmoschetti45

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Re: Zener clamping at microamp supply levels
« Reply #2 on: September 19, 2019, 07:05:55 pm »
Use a bandgap. TLV431 (the V version) goes down to 100uA.

I'm seeing a max of 7V in the datasheet?
 

Offline blueskull

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Re: Zener clamping at microamp supply levels
« Reply #3 on: September 19, 2019, 07:09:10 pm »
Use a bandgap. TLV431 (the V version) goes down to 100uA.

I'm seeing a max of 7V in the datasheet?

A resistor between 12v and cathode drops a few volts.
 

Offline magic

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Re: Zener clamping at microamp supply levels
« Reply #4 on: September 19, 2019, 07:26:12 pm »
There is a million xx431x parts differing in reference voltage (1.24/2.495/2.5), operating current range and abs maximum voltage.

Quick parametric search brings: SC431, LMV431
 


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