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From: Lizard Blizzard
Subject: Re: V regulator input cap size?
Date: Tue, 22 Oct 2002 08:10:08 -0700
References: <0001HW.B9D991AD000C5917163F7590@news.covad.net> <firstname.lastname@example.org> <0001HW.B9D9CD54001A5D15163F7590@news.covad.net>
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> On Mon, 21 Oct 2002 12:42:42 -0700, John Fields wrote
> (in message <email@example.com>):
>>THis is what you have:
>> | | | 7812
>> | +-----------+---IN OUT--+--->>--+
>> | | | GND | |
>> +-[CR]-+-[CR]-+ [470µF] | [22µF] [RL]
>> | | | | |
>>Your 35VRMS is ~ 49.5V peak, and after two diode drops through the
>>bridge will wind up as about 48V across the input of the 7812.
>>Unless that KA7812 is rated for substantially more input voltage
>>than a standard 7812 (35V) you've been shot down before you've even
>>gotten off the ground.
> What to do? I'm using 35vac from an HVAC control-voltage transformer. The
> transformer is used for standard control circuits in the HVAC unit on the
> roof, so I can't change the transformer.
> Short of getting a 35v/20v transformer (or such unavailable combination), any
> suggestions how I can make this voltage useable for my 12vdc circuit?
> Two-diode rectification? Single diode?
The schematic at the top of page 12 of this URL
shows a current amplified zener on the input of the regulator to drop
the voltage. Use a power transistor on a heatsink and a 1W zener, say
12V for a V drop of about 12.6V. Better yet, use two 9V zeners for a
total V drop of about 19V. 100 ohms seems like a rasonable value for
the resistor. This should also cut down the power dissipation of the
You can also include a wirewound power resistor there, to drop some of
the voltage, but if the regulator will have very little current thru it,
then the power resistor will not drop much, so don't depend on it solely
to do the V drop job. You could instead put a handful of 1N4002
rectifier diodes in series with the input, at about .6 to .9V each;
looks like you'll need more than a dozen. Just find a place to put them
all and keep them cool because they will also dissipate a fair amount of
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