24VAC to 12VDC Power Supply

[Dean]

Hi,
I am trying to build a small power supply to take 24 volts AC down to
12 volts DC. The load is about 200mA at 12 volts. I have tried a
traditional bridge rectifier --> capacitor --> linear regulator. This
works but generates alot of heat as the input to the regulator is
about 33 volts.

Is there a more efficient way of doing this? Ideally, I would like to
fit this in a very small ( lipstick sized?) package and for under $10.
I have seen complete units that do this for sale on the web but would
like to learn how they work so I can build my own to suit the load.

Thanks for any help or suggestions.

Dean

 

[John Popelish]

Dean wrote:

Hi,
I am trying to build a small power supply to take 24 volts AC down to
12 volts DC. The load is about 200mA at 12 volts. I have tried a
traditional bridge rectifier --> capacitor --> linear regulator. This
works but generates alot of heat as the input to the regulator is
about 33 volts.

Is there a more efficient way of doing this? Ideally, I would like to
fit this in a very small ( lipstick sized?) package and for under $10.
I have seen complete units that do this for sale on the web but would
like to learn how they work so I can build my own to suit the load.

Thanks for any help or suggestions.

Dean

When size and heat matter, you go to a switching regulator. These
chop the input voltage (turn it off and on very quickly) and average
the pulses with an inductive input filter to produce the output
voltage. See:
An introduction to power supplies:
http://www.national.com/an/AN/AN-556.pdf
An example of a buck regulator control chip and how to design a
regulator around it:
http://www.national.com/an/AN/AN-711.pdf
--
John Popelish

 

[Dan Dunphy]

If your transformer has a center tap, ground it, and use 2 diodes, or
half the bridge rectifier.
Tie the cathodes together and the anodes to the transformer, for
+16.5V, or reverse the diodes for -16.5. Then your linear regulator
won;t dissipate near as much heat.
Dan

On Thu, 13 Nov 2003 16:39:47 GMT, Dean <dean.m@rogers.com> wrote:

Hi,
I am trying to build a small power supply to take 24 volts AC down to
12 volts DC. The load is about 200mA at 12 volts. I have tried a
traditional bridge rectifier --> capacitor --> linear regulator. This
works but generates alot of heat as the input to the regulator is
about 33 volts.

Is there a more efficient way of doing this? Ideally, I would like to
fit this in a very small ( lipstick sized?) package and for under $10.
I have seen complete units that do this for sale on the web but would
like to learn how they work so I can build my own to suit the load.

Thanks for any help or suggestions.

Dean

Colorado Springs, CO
My advice may be worth what you paid for it.

 

[Watson A.Name - "Watt Sun]

In article <e9c7rv0o9pkqg8j9nlhvs73174mhjkdcir@4ax.com>,
dean.m@rogers.com mentioned...

Hi,
I am trying to build a small power supply to take 24 volts AC down to
12 volts DC. The load is about 200mA at 12 volts. I have tried a
traditional bridge rectifier --> capacitor --> linear regulator. This
works but generates alot of heat as the input to the regulator is
about 33 volts.

Is there a more efficient way of doing this? Ideally, I would like to
fit this in a very small ( lipstick sized?) package and for under $10.
I have seen complete units that do this for sale on the web but would
like to learn how they work so I can build my own to suit the load.

The two transistor Black Regulator at www.romanblack.com may be able
to handle that voltage and current with the right transistors.
Bcause of the higher voltages, you would probably have to use a fast
recovery rectifier in place of the Schottky diode.

Thanks for any help or suggestions.

Dean

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[Brian]

Dean wrote:

Hi,
I am trying to build a small power supply to take 24 volts AC down to
12 volts DC. The load is about 200mA at 12 volts. I have tried a
traditional bridge rectifier --> capacitor --> linear regulator. This
works but generates alot of heat as the input to the regulator is
about 33 volts.

Is there a more efficient way of doing this? Ideally, I would like to
fit this in a very small ( lipstick sized?) package and for under $10.
I have seen complete units that do this for sale on the web but would
like to learn how they work so I can build my own to suit the load.

Thanks for any help or suggestions.

Dean

Hi Dean,

I drew out a switching power supply for you (24V. to 12V.). You can see it
at http://www.fncwired.com/12VoltExample/
Brian


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