Author Topic: Zener based pre-regulator  (Read 1044 times)

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Offline infoTopic starter

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Zener based pre-regulator
« on: February 12, 2019, 02:41:16 pm »
Hello there, I was looking around for a simplistic pre-regulator for a project I'm working on and stumbled upon Ian Johnston's power supply project which has (had) a pre-regulator built using a zener.

I am pretty much new at simulating in LTspice so I may have done this wrong but here goes (attached image).

Is this a viable option for a linear regulator like the LM317 or LT3080? The voltage on the adjust pin is simulating a pot wiper.

Regards
 

Offline schmitt trigger

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Re: Zener based pre-regulator
« Reply #1 on: February 12, 2019, 03:21:26 pm »
Discrete pre-regulators have traditionally been used in places where the supply voltage may exceed the main regulator's maximum voltage rating.

For instance, in large trucks with a nominal 24 volt bus, the load dump transients may exceed 90 volts (if I remember correctly), so it was customary to use a pre-regulator for the purpose.

Another reason was to lower the total power dissipation in the main regulator.

 

Offline infoTopic starter

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Re: Zener based pre-regulator
« Reply #2 on: February 12, 2019, 03:36:16 pm »
Yes, that is why I was looking into a simple but working pre-regulator so as to reduce the dissipation in the main regulator.
 

Online David Hess

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Re: Zener based pre-regulator
« Reply #3 on: February 13, 2019, 07:33:18 pm »
It will work just fine and I have used the same circuit with the LM317 to protect against input voltage spikes.  Pick the right depletion mode power MOSFET and no other parts are required.

A more comprehensive circuit uses the input voltage to implement fold-back current limiting so that the regulator shuts down if the input voltage remains high for too long protecting the input transistor also.
 
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