Subject: From: Date:

Re: [ECS] Telephone Deadman switch Steven Wegman Thu, 27 Aug 1998 20:05:58 -0400

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At 07:38 AM 8/27/98 -0700, you wrote:
>I have looked in my big book of electronic parts and it seems that
>voltage regulators allow input voltages up to about 35 volts.  If the
>phone system runs at 48 vdc and rings at 90 vac, any ideas as to how to
>limit the voltage that ends up at the regulator so as to not blow the
>thing?

	You "float" the ground of the IC regulator with a zener diode.  Based on
the zener selected, you can use an adjustible voltage regulator to regulate
a few hundred volts (I did this with a vacuum tube preamp I built).  Just
remember to make such that the difference between the input and output does
not exceed the design limit of the regulator (usually 40 volts).  Details
on this technique may be found in a National Semiconductor voltage
regulator spec book.

steve

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