Hi all and thanks again for all the responses.

Well I had another 10 minutes to spare again today so took a different
secondary tapping to try to get more voltage headroom. Following that
tweak, I now have 25.5VDC available across the large electrolytics.
This is obviously plenty enough for 13.8VDC regulated; but is it now
*too* much to drop efficiently?
Thanks,
p.

wrote:
Hi all and thanks again for all the responses.

Well I had another 10 minutes to spare again today so took a different
secondary tapping to try to get more voltage headroom. Following that
tweak, I now have 25.5VDC available across the large electrolytics.
This is obviously plenty enough for 13.8VDC regulated; but is it now
*too* much to drop efficiently?
Thanks,
p.

It's more than ideal but there is no such thing as "too much" if you've
got an adequate supply of heat sinks and circulating air :-).

Is 25.5VDC measured at your "full load" current or at "no load"? If
that's what it is like under a typical load then you'll get about 50%
efficiency from a linear supply.

A switching step-down converter is possible but if that were an option
you wouldn't be where you are today.

The no-load voltage is irrelevant for power dissipation because with no
current there's no heat.

Tim.

Yes sorry, Tim. I should have mentioned that the 25.5VDC is the
open-circuit voltage across the caps.
I assume this new drop is do-able because this PSU in its original
configuration provided 24VDC regulated from 40VDC unreg across the
caps. That seemed like a hell of a drop without generating heaps of
heat, but I freely admit my recollection of linear PSU design (like a
lot of other electronics-related stuff) is very, very hazy.

wrote:
Yes sorry, Tim. I should have mentioned that the 25.5VDC is the
open-circuit voltage across the caps.
I assume this new drop is do-able because this PSU in its original
configuration provided 24VDC regulated from 40VDC unreg across the
caps. That seemed like a hell of a drop without generating heaps of
heat, but I freely admit my recollection of linear PSU design (like a
lot of other electronics-related stuff) is very, very hazy.

A MUCH more relevant number would be the average DC voltage and minimum
DC voltage across the caps under pretty much full load.

A good dummy load for an unregulated supply is auto bulbs/headlamps in
series/parallel as appropriate.

The average DC voltage under full load is very relevant to overall heat
dissipation.

The minimum DC voltage (that is, put it on a scope and observe the
bottom of the ripple) under full load is very relevant to making sure
you have enough headroom for your regulator.

If you don't have a scope you can guess what the ripple is from
average DC and rectifier type (half or full wave). Guess won't be far
off from reality unless you've got abysmal ESR caps or really really
sucky transformer.

As to headroom: Typically for a high-current supply on a 723 chip,
there would be a "secondary" supply at higher DC voltage that helped
you develop drive to the pass transistor. Without this you need several
extra volts headroom.

So far you haven't given us an awful lot to go on :-).

Tim.

Paul,

If you didn't understand where I was going, Tim's is a good summary of my
original post.

73, Steve, K9DCI



"Tim Shoppa" wrote in message
oups.com...
wrote:
Yes sorry, Tim. I should have mentioned that the 25.5VDC is the
open-circuit voltage across the caps.
I assume this new drop is do-able because this PSU in its original
configuration provided 24VDC regulated from 40VDC unreg across the
caps. That seemed like a hell of a drop without generating heaps of
heat, but I freely admit my recollection of linear PSU design (like a
lot of other electronics-related stuff) is very, very hazy.

A MUCH more relevant number would be the average DC voltage and minimum
DC voltage across the caps under pretty much full load.

A good dummy load for an unregulated supply is auto bulbs/headlamps in
series/parallel as appropriate.

The average DC voltage under full load is very relevant to overall heat
dissipation.

The minimum DC voltage (that is, put it on a scope and observe the
bottom of the ripple) under full load is very relevant to making sure
you have enough headroom for your regulator.

If you don't have a scope you can guess what the ripple is from
average DC and rectifier type (half or full wave). Guess won't be far
off from reality unless you've got abysmal ESR caps or really really
sucky transformer.

As to headroom: Typically for a high-current supply on a 723 chip,
there would be a "secondary" supply at higher DC voltage that helped
you develop drive to the pass transistor. Without this you need several
extra volts headroom.

So far you haven't given us an awful lot to go on :-).

Tim.

Okay Tim, thanks. I'm not short of test equipment; only the savvy to
use much of it. ;-)
Seriously, in this instance I can easily measure the ripple nadir under
full load - if I can source some headlamps from someplace. This may
take a few days so I'll report back in due course.

wrote:
Well I had another 10 minutes to spare again today so took a different
secondary tapping to try to get more voltage headroom. Following that
tweak, I now have 25.5VDC available across the large electrolytics.
This is obviously plenty enough for 13.8VDC regulated; but is it now
*too* much to drop efficiently?
==============================
25.5 V seems a normal voltage level for a regulated 13.8 V PSU , no way
too high.
Suggest you check the working voltage of the large elctrolytic
capacitors. They should be 40 V or higher.
I am currently repairing a Yaesu PSU ,output 13.8 V -20A max, having
40V electrol. caps in output of 25A bridge rectifier.
FYI the unit has a soft start facility being a 10 Ohms resistor in the
230V -AC supply circuit ; this to limit bridge rectifier inrush current,
charging the electrolytic caps from zero.
As soon as DC output (13.8V) is there, a relay will be energised ,its
contact shorting the above 10 Ohms resistor .

Frank GM0CSZ / KN6WH

Interesting. There are all sorts of protections one can build into PSUs
of course and it's a job to know where to stop with some of them. I'm
just wondering why there's a need to limit this inrush current. What is
it you're seeking to protect here?

One is that to properly fuse it, you can't have an excessive in-russ, or
you'll need a really big fuse and it might be too big to protect the smoke
hinside.
Another is that you reqally don't want to pound either the diodes or the
filter caps if you don't have to.


73, Steve, K9DCI

wrote in message
ups.com...
Interesting. There are all sorts of protections one can build into PSUs
of course and it's a job to know where to stop with some of them. I'm
just wondering why there's a need to limit this inrush current. What is
it you're seeking to protect here?

wrote:

Hi all and thanks again for all the responses.

Well I had another 10 minutes to spare again today so took a different
secondary tapping to try to get more voltage headroom. Following that
tweak, I now have 25.5VDC available across the large electrolytics.
This is obviously plenty enough for 13.8VDC regulated; but is it now
*too* much to drop efficiently?
Thanks,
p.

I find that about 19 to 20 Volts off-load is about ideal for 13.8 Volts
output - if your unregulated voltage is much higher than that, you make
a /lot/ of heat! If you're using a suitably rated transformer (so that the
secondary voltage doesn't dip too much under load), 19 Volts gives ample
headroom.

A brief word of advice - if you're going to use a 723 type regulator
(usually a good choice), be careful to RF-bypass the op-amp (inside the
723) and provide an over-voltage trip of some kind. It is sometimes
possible for the 723 to suffer from "RF brain damage" and completely lose
the plot if you get enough RF in the wrong places! The best over-voltage
trip is really brutal - a high current thyristor to ground after a fuse in
the unregulated DC rail driven by a simple transistor and zener comparator
- if you get it right, you can get a large "thump" out of the blowing fuse!

Bob

--
Everything gets easier with practice, except getting up in the morning!