Re: +/- 18VDC PS basic questions

On 4 Nov 2005 10:34:31 -0800, jpopelish@xxxxxxxx wrote:

>
>John Popelish wrote:
>> Dave wrote:
>(snip)
>> > 120VAC --> transformer --> 12VAC --> rectifier --> 18VDC --> 2 sets of
>> > caps --> two 15-18VDC power supplies, one pos, one neg which use the CT lead
>> > for ground (common).
>> >
>> > Do I need to start with a 12.6-0-12.6 transformer? This puts 25VAC into my
>> > rectifier, does it not? That's >35VAC out!
>
>That's ~35 VDC out, from the most positive point ot the most negative
>point.
>
>> > It will cook any 30V regulator
>> > UNLESS by using the center tap for gorund you are cutting the voltage in
>> > half.
>>
>> This is what you need, and each regulator will see half of the 25 *
>> 1.414 = 35 or about 17 or 18 volts between one end of that 35 volts
>> and the middle voltage of the center tap.
>
>I should add, that each of the two storage capacitors connects between
>one output from the bridge rectifier and the center tap, to make two,
>stacked DC supplies.


---
Like this, in Courier:


12.6VRMS
/
+--[CR1>]-----+---+--------> +17VDC
| | |
+--+--[<CR2]--+--|---|-----+--> -17VDC
||(So | | |+ |
120AC>---+ ||(E | | [C1] [C1]
P)||(C | | | |+
R)|| +-CT----------|--|---+-----+--> GND/0V
I)||(So | |
120AC>---+ ||(E | |
||(C | |
+--+--[<CR3]--+ |
| |
+--[CR4>]-----+
\
12.6VRMS


--
John Fields
Professional Circuit Designer
.

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