Battery Charger for power supply.

J

Jack P. Flash

Guest
Hi,
I am attempting to build a power supply that will supply 6 to 14v DC at
around 20amps. I'm going to parallel some LM338's for the regulator side
and now am looking for a transformer.
Is there any reason I couldn't use a transformer out of a battery
charger? I've got some large caps, 22,000uf 50v electorlytics and a
large bridge rectifier, 40amps at 50v I believe.

Any help?

Thanks

James
 
On Fri, 04 Feb 2005 13:01:52 -0600, "Jack P. Flash" <jack@flash.net>
wrote:

Hi,
I am attempting to build a power supply that will supply 6 to 14v DC at
around 20amps. I'm going to parallel some LM338's for the regulator side
and now am looking for a transformer.
Is there any reason I couldn't use a transformer out of a battery
charger? I've got some large caps, 22,000uf 50v electorlytics and a
large bridge rectifier, 40amps at 50v I believe.

Any help?
Click to expand...
Just paralleling the regulators doesn't sound like a good idea. You
might want to look at the adjustable 15A regulator on page 14 of
National's LM138 data sheet at:

http://cache.national.com/ds/LM/LM138.pdf

--
John Fields
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:4km701dufei64v35o5voj2ck7udqi5hfiu@4ax.com...
On Fri, 04 Feb 2005 13:01:52 -0600, "Jack P. Flash" <jack@flash.net
wrote:

Hi,
I am attempting to build a power supply that will supply 6 to 14v DC at
around 20amps. I'm going to parallel some LM338's for the regulator side
and now am looking for a transformer.
Is there any reason I couldn't use a transformer out of a battery
charger? I've got some large caps, 22,000uf 50v electorlytics and a
large bridge rectifier, 40amps at 50v I believe.

Any help?

Just paralleling the regulators doesn't sound like a good idea. You
might want to look at the adjustable 15A regulator on page 14 of
National's LM138 data sheet at:

http://cache.national.com/ds/LM/LM138.pdf

--
John Fields
Click to expand...
Uhh The 138 is a 5amp jobbie isnt it ??

--
Regards ..... Rheilly Phoull
 
On Fri, 04 Feb 2005 17:06:17 -0600, James <james@holbrooks.net> wrote:

John,
I had the same thought.. Parelleling the regulators is a bad idea.
However.. I ran across this application note on National's site
outlining the way to parallel the regulators and get them to load share
within 60mv.
http://www.national.com/an/LB/LB-51.pdf

Thoughts on that??
Click to expand...
---
Looks good!

--
John Fields
 
On Fri, 4 Feb 2005 18:55:49 -0500, "jsmith" <juddo@ix.netcom.com>
wrote:

So what do you need the capacitor for anyway???
Click to expand...
---
A charge reservoir between the full-wave bridge and the regulator.

--
John Fields
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:4k0a01910m9ith6vddrn27ups4kht6j8mo@4ax.com...
On Fri, 4 Feb 2005 18:55:49 -0500, "jsmith" <juddo@ix.netcom.com
wrote:

So what do you need the capacitor for anyway???

---
A charge reservoir between the full-wave bridge and the regulator.

--
John Fields
Click to expand...
Since when is "charge reservoir" necessary when charging a lead acid
battery??
 
On Tue, 5 Apr 2005 19:02:39 -0400, "jsmith" <juddo@ix.netcom.com>
wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:4k0a01910m9ith6vddrn27ups4kht6j8mo@4ax.com...
On Fri, 4 Feb 2005 18:55:49 -0500, "jsmith" <juddo@ix.netcom.com
wrote:

So what do you need the capacitor for anyway???

---
A charge reservoir between the full-wave bridge and the regulator.

--
John Fields

Since when is "charge reservoir" necessary when charging a lead acid
battery??
Click to expand...
---
If you don't use a charge reservoir, then reverse leakage current when
the battery conduction angle is less than 43° will cause osmotic
heating of the separators and the rectifiers will lose their
passivation.

Duh...

--
John Fields
Professional Circuit Designer
 
It may be hard to get 14 volts at 20 amps from
a battery charger transformer.

12 volt battery charger transformers are
typically 17 volts peak.

You can work out the filter cap size
using about 8,000uF per amp per volt.
So, if the supply is 17 and the load is
20 amps, and the voltage can only fall
a couple volts, the cap will be
(8000*20)/2 = 80,000uF

The other problem is heat from the
regulators. If you adjust the output
to 6 and the input averages around
15, and the load is 20 amps, the
heat dissipation will be
(15-6)*20 = 180 watts.

This will require a fairly large heat
sink, and possibly a fan to cool it off.

If the transformer has a center tap, you
can switch between using the entire winding
with a bridge rectifier or the two halves
with 2 diodes and the center tap at ground.
This will reduce the heat losses when you
need a lower voltage.

A multi-tapped transformer would be even better
since you can keep the input very close to the
output and minimize the heat loss.

-Bill
 
John,
I had the same thought.. Parelleling the regulators is a bad idea.
However.. I ran across this application note on National's site
outlining the way to parallel the regulators and get them to load share
within 60mv.
http://www.national.com/an/LB/LB-51.pdf

Thoughts on that??

Thanks
James

John Fields wrote:
On Fri, 04 Feb 2005 13:01:52 -0600, "Jack P. Flash" <jack@flash.net
wrote:


Hi,
I am attempting to build a power supply that will supply 6 to 14v DC at
around 20amps. I'm going to parallel some LM338's for the regulator side
and now am looking for a transformer.
Is there any reason I couldn't use a transformer out of a battery
charger? I've got some large caps, 22,000uf 50v electorlytics and a
large bridge rectifier, 40amps at 50v I believe.

Any help?


Just paralleling the regulators doesn't sound like a good idea. You
might want to look at the adjustable 15A regulator on page 14 of
National's LM138 data sheet at:

http://cache.national.com/ds/LM/LM138.pdf
Click to expand...
 
So what do you need the capacitor for anyway???


"Bill Bowden" <wrongaddress@att.net> wrote in message
news:1107551514.078312.139350@g14g2000cwa.googlegroups.com...
It may be hard to get 14 volts at 20 amps from
a battery charger transformer.

12 volt battery charger transformers are
typically 17 volts peak.

You can work out the filter cap size
using about 8,000uF per amp per volt.
So, if the supply is 17 and the load is
20 amps, and the voltage can only fall
a couple volts, the cap will be
(8000*20)/2 = 80,000uF

The other problem is heat from the
regulators. If you adjust the output
to 6 and the input averages around
15, and the load is 20 amps, the
heat dissipation will be
(15-6)*20 = 180 watts.

This will require a fairly large heat
sink, and possibly a fan to cool it off.

If the transformer has a center tap, you
can switch between using the entire winding
with a bridge rectifier or the two halves
with 2 diodes and the center tap at ground.
This will reduce the heat losses when you
need a lower voltage.

A multi-tapped transformer would be even better
since you can keep the input very close to the
output and minimize the heat loss.

-Bill
Click to expand...
 
On Sat, 5 Feb 2005 11:36:15 +0800, "Rheilly Phoull"
<Rheilly@bigpong.com> wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:4km701dufei64v35o5voj2ck7udqi5hfiu@4ax.com...
On Fri, 04 Feb 2005 13:01:52 -0600, "Jack P. Flash" <jack@flash.net
wrote:

Hi,
I am attempting to build a power supply that will supply 6 to 14v DC at
around 20amps. I'm going to parallel some LM338's for the regulator side
and now am looking for a transformer.
Is there any reason I couldn't use a transformer out of a battery
charger? I've got some large caps, 22,000uf 50v electorlytics and a
large bridge rectifier, 40amps at 50v I believe.

Any help?

Just paralleling the regulators doesn't sound like a good idea. You
might want to look at the adjustable 15A regulator on page 14 of
National's LM138 data sheet at:

http://cache.national.com/ds/LM/LM138.pdf

--
John Fields

Uhh The 138 is a 5amp jobbie isnt it ??
Click to expand...
---
Read the application notes on the data sheet or look up their LB-51...

--
John Fields
 
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