Re: 19K6400

On Fri, 23 Jan 1998 11:55:02 -0800, Clay Cowgill <ClayC@diamondmm.com> wrote:

>> Another thing that would have to be added is an overvoltage clamp on
>> the
>> output of the regulator, since if the Vin to Vout voltage is greater
>> than
>> 40V, then the regulator will burn up. This will not be a problem with
>> installing it into a standard WG6100.
>>
>Been a while since I tried any LM317 tricks, but let's say we have
>40-0-40 input into a full-wave bridge rectifier. We'll get about +/-57V
>unfiltered DC out, yes? (.707*80) Hang a couple big 4700uF caps (ugh,
>I'd really say you wany 100V caps there) and figure our input to the
>voltage regulators is about 56V DC. We want +/-47V, so we need to burn
>off around 10V with a regulator setup. We want to use a standard LM317
>and LM337, so is the way to do that just to raise the "floor" the ADJ
>pin up to (let's say) 18V with a zener. So the input is 56V, the output
>is set for 46V, but the "adj" voltage sets virtual ground as +18 so the
>regulator "sees" Vin of 38V, output of 28V and it'll burn off the 10V
>delta. (Actually, you could probably play some input games and get an
>input voltage closer to the regulation voltage and save some heat...)
>Does that sound right? My power supply design skills haven't been
>exercised since high-school electronics class. ;-)

You don't even need the 18v zener. The LM317 is really nothing more than a 1.2v
regulator, it's the voltage divider on the input pin that allows the output to
regulate a higher voltage. By setting the ratio of the voltage divider
correctly you can regulate in voltage, just as long is the input is never 35
volts greater than the output. The REG pin will always be 1.2v lower than the
output voltage regardless of what that voltage is. The 7805 works well as a
variable regulator down to 5 volts instead of 1.2v, though the LM317 series has
been designed to limit the amount of current that flows from the REG pin which
allows for slightly better regulation.

All you need are the LM317, the two resistors that make up the voltage divider,
some bypass caps, and you can through in a protection diode or two. If you use
a variable resistor to set the regulation voltage you should add an extra
resistor between GND and the variable resistor to keep the output from ever
dropping 35v below the input. (It just limits the range of regulation.)

-Zonn

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Received on Fri Jan 23 13:11:28 1998