Reducing 25v to 12v DC

[stoj]

I am trying to reduce 25v DC to 12v DC form a AC/AC transformer and a bridge
rectifier but the current output of the transformer need to stay the same.
(the transformer properties are 18v @16.7Amps which is the only output).


Thanks in advanced

 

[cdb]

::I am trying to reduce 25v DC to 12v DC form a AC/AC transformer and a
bridge
::rectifier but the current output of the transformer need to stay the same.
:the transformer properties are 18v @16.7Amps which is the only output).

I take it that 25v, is PEAK? 18 *1.414 = 25.5v approx

You will lose approx 1.4v through the bridge, you will also get 0.62 * AC
current 10.35A . Or is that 16.7 amps per winding and you've got them in
parallel?

After this I'd suggest a switcher IC of some kind - have a look at the ST
series L296/4269 or similar (I think the 296 only does 4A, but they have
beefier ones).

www.us.st.com/stonline


colin



---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.544 / Virus Database: 338 - Release Date: 25.November.2003

 

[stoj]

That is correct the voltage is the peak voltage after the bridge is placed
in the circuit. The 16.7Amps is the output rating of the transformer at 18
volts (which has been stamped on the caseof the transformer)


"cdb" <barnDOTc@webcityuk.zznDOTcom> wrote in message
news:3fc5785f$0$20482$afc38c87@news.optusnet.com.au...

::I am trying to reduce 25v DC to 12v DC form a AC/AC transformer and a
bridge
::rectifier but the current output of the transformer need to stay the
same.
:the transformer properties are 18v @16.7Amps which is the only output).

I take it that 25v, is PEAK? 18 *1.414 = 25.5v approx

You will lose approx 1.4v through the bridge, you will also get 0.62 * AC
current 10.35A . Or is that 16.7 amps per winding and you've got them in
parallel?

After this I'd suggest a switcher IC of some kind - have a look at the ST
series L296/4269 or similar (I think the 296 only does 4A, but they have
beefier ones).

www.us.st.com/stonline


colin



---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.544 / Virus Database: 338 - Release Date: 25.November.2003

 

On Thu, 27 Nov 2003 03:46:57 GMT, "stoj" <stoj84@hotmail.com> wrote:

I am trying to reduce 25v DC to 12v DC form a AC/AC transformer and a bridge
rectifier

Can't be done this way. 25V AC to 12V DC would be ok.

You'll need some type of regulator circuit for this to work, cheapest
option would be to try somewhere like Repco and get a 24V -> 12V
regulator as used in trucks to power normal 12V items like radios,
phone charger etc.

 

[Martin, VK2UMJ]

<David Sauer> wrote in message
news:32bbsv8am1136msm1ji3u9b6og7c8tduon@4ax.com...

On Thu, 27 Nov 2003 03:46:57 GMT, "stoj" <stoj84@hotmail.com> wrote:


I am trying to reduce 25v DC to 12v DC form a AC/AC transformer and a
bridge
rectifier

Can't be done this way. 25V AC to 12V DC would be ok.

You'll need some type of regulator circuit for this to work, cheapest
option would be to try somewhere like Repco and get a 24V -> 12V
regulator as used in trucks to power normal 12V items like radios,
phone charger etc.

What's wrong with standard 7812 or LM723 regulator design with pass
transistors??? All standard stuff and shown in the back of DSE catalogues,
on the net, etc, etc, etc..... Not hard to find these designs - just try a
Google search.




---
Outgoing mail has been checked
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.543 / Virus Database: 337 - Release Date: 21/11/2003

 

[Leon.]

"Martin, VK2UMJ" <vk2umj@REMOVEyahoo.com.au> wrote in message
news:bq6qjg$v1$1@austar-news.austar.net.au...

David Sauer> wrote in message
news:32bbsv8am1136msm1ji3u9b6og7c8tduon@4ax.com...
On Thu, 27 Nov 2003 03:46:57 GMT, "stoj" <stoj84@hotmail.com> wrote:


I am trying to reduce 25v DC to 12v DC form a AC/AC transformer and a
bridge
rectifier

Can't be done this way. 25V AC to 12V DC would be ok.

You'll need some type of regulator circuit for this to work, cheapest
option would be to try somewhere like Repco and get a 24V -> 12V
regulator as used in trucks to power normal 12V items like radios,
phone charger etc.

What's wrong with standard 7812 or LM723 regulator design with pass
transistors??? All standard stuff and shown in the back of DSE
catalogues,
on the net, etc, etc, etc..... Not hard to find these designs - just try
a
Google search.

Well, the only problem is that for every watt your device uses, the
regulator consumes(turns to heat) a watt.

And that means the transistor regulator is limitted in power output by its
ability to avoid melt down.

And also by being half the power of the 25volt supply...

 

[Ken Taylor]

"Leon." <leon@noteon.net> wrote in message
news:3fca8abc$1@news.rivernet.com.au...

"Martin, VK2UMJ" <vk2umj@REMOVEyahoo.com.au> wrote in message
news:bq6qjg$v1$1@austar-news.austar.net.au...
David Sauer> wrote in message
news:32bbsv8am1136msm1ji3u9b6og7c8tduon@4ax.com...
On Thu, 27 Nov 2003 03:46:57 GMT, "stoj" <stoj84@hotmail.com> wrote:


I am trying to reduce 25v DC to 12v DC form a AC/AC transformer and a
bridge
rectifier

Can't be done this way. 25V AC to 12V DC would be ok.

You'll need some type of regulator circuit for this to work, cheapest
option would be to try somewhere like Repco and get a 24V -> 12V
regulator as used in trucks to power normal 12V items like radios,
phone charger etc.

What's wrong with standard 7812 or LM723 regulator design with pass
transistors??? All standard stuff and shown in the back of DSE
catalogues,
on the net, etc, etc, etc..... Not hard to find these designs - just
try
a
Google search.


Well, the only problem is that for every watt your device uses, the
regulator consumes(turns to heat) a watt.

And that means the transistor regulator is limitted in power output by its
ability to avoid melt down.

And also by being half the power of the 25volt supply...

Err, what do you mean by that last?

Ken