POWER SUPPLY

[Patrick Cobb]

Help every time I build a power supply the output voltage is 2X the input
and I don't want it that way. I connect a transformer to the mains line and
then put a bridge on the output and follow that with a 2200 microfarad cap
to soften the DC. The voltage will be twice what it is suppose to be e.g..
transformer is rated at 12V output I get around 20V why?

 

[Patrick Cobb]

IS THERE ANY WAY TO STOP THIS?
"Robert Morein" <nowhere@nospam.com> wrote in message
news:ZBKdnZh-qfbSUYCiXTWJjQ@comcast.com...

"Patrick Cobb" <patrick77598@hotmail.com> wrote in message
news:0OjTa.100121$xg5.93476@twister.austin.rr.com...
Help every time I build a power supply the output voltage is 2X the
input
and I don't want it that way. I connect a transformer to the mains line
and
then put a bridge on the output and follow that with a 2200 microfarad
cap
to soften the DC. The voltage will be twice what it is suppose to be
e.g..
transformer is rated at 12V output I get around 20V why?

Because the 12V is the "RMS", ie., "root-mean-square" value.
In the absence of a load, the capacitors charge up to the peak value.
This accounts for the discrepancy you see.

 

[wave_man]

Or you could build a simple switcher and not waste the heat. Check out
National ( www.national.com ) they make many, easy to implement, with great
apps notes.

JF
"Dan Barlow" <danbarlow@radix.net> wrote in message
news:3f1de280_1@vienna7.his.com...

Patrick Cobb wrote:
IS THERE ANY WAY TO STOP THIS?

Yes! You need a load resistor to absorb the extra peak voltage.
If you are going for 12V, use a replacement lamp from the auto
section of your local department store. You can get a tiny
dash light for a little current drain, or a headlight for around
6 Amps. It might be fun to get a few different small to medium
ones and chart how much voltage drop you get for a certain amount
of current through the bulb.

If you want a very close 12V you need a 7812 voltage regulator.
Radio Shack may(!) still carry them. They waste the extra volts
as heat so you need a metal tab on them so the air can carry the
heat away [called a "heat sink" also at R.S.].

Combining both can be very effective.
-Dan Barlow

 

[DarkMatter]

On Tue, 22 Jul 2003 23:30:04 GMT, "Patrick Cobb"
<patrick77598@hotmail.com> Gave us:

Help every time I build a power supply the output voltage is 2X the input
and I don't want it that way. I connect a transformer to the mains line and
then put a bridge on the output and follow that with a 2200 microfarad cap
to soften the DC. The voltage will be twice what it is suppose to be e.g..
transformer is rated at 12V output I get around 20V why?

Chief say you needum load regulated supply for this application.