: The diagram looks strange to me, with a direct connection from the
: higher voltage driver supply (e.g. pin 12 of the 723 or collector of
: Q2) to the base of the series pass transistors.

The pass transistor pre-drive transistor is a simple current amplifier.

: With a higher separate driver power supply the driver current should
: flow through the Q2 driver transistor, through the small resistors
: into the base connection of the series pass transistor. Thus in the
: diagram the direct connection from the driver supply to the series
: pass transistor base should be removed.

Some of the available circuit diagrams have mistakes...

:I modded one of those significantly. Too long ago to remember the exact
:thing I did, but DO put that crowbar SCR anode on the collector of the
:pass transistors. If it trips as is, you kiss the current meter and
:expensive current sense resistor goodbye.

: When using crowbars, there should be a some kind of fuse on the
: feeding side, preferably immediately after the filter capacitor.

No, you want the entire supply to remove itself from operation if
the crow-bar circuit fires.

: If the crowbar is before the series pass transistors, it is a good
: idea to put a large reverse biased diode across the series transistors
: (CR7 in the diagram), which would discharge any large capacitors on
: the regulated side through the crowbar. Without the diode, the
: collector side would be at 0 V after the crowbar tripped, but the
: emitters could still be at +12 V due to any large capacitors on the
: regulated side.

An expensive part... the crow-bar is best left at the tail end of the
supply (where it is now).

cheers,
s.

Amen Paul,

The Astron 25 amp variable supply is pretty much the parts for the higher
rated supply in the same case. The no free lunch rule applies. If you
modify your specific supply for varilable control... it will quickly
become to hot at lower voltage - higher current values.

cheers,
s.

: Paul Keinanen wrote:

:I have an Astron RS-20A, 20amp 12v fixed DC supply. I've heard that people
:have modified this model to become a variable voltage supply but have not
:found any schematics etc for this mod. Can anyone point me in the right
:direction? Thanks!


: While the output voltage of practically any regulated linear power
: supply can be varied by changing one or two resistors, one should
: remember that the power dissipation in the series pass transistors
: will increase significantly, if voltages much below the nominal
: voltage is required, thus you may have to increase the number of
: series pass transistors or at least increase the heatsink surface 2-4
: times the original.

: Also you may have to use some foldback current limitation system to
: keep the series pass transistor dissipation at a reasonable level e.g.
: when driving car head lights or other nearly short circuit loads.

: When fully understanding these issues, the commercially available
: devices can be quite interesting building blocks, when creating power
: supplies for your own needs.

: Paul OH3LWR

On Wed, 07 May 2008 08:08:44 +0300, Paul Keinanen wrote:

On Sun, 04 May 2008 23:51:08 -0700, geek
wrote:

On Mon, 05 May 2008 02:04:09 -0500, Ralph Mowery wrote:


"Toby" wrote in message news:i8vTj.791$b%1.295@trndny04...
I have an Astron RS-20A, 20amp 12v fixed DC supply. I've heard that people
have modified this model to become a variable voltage supply but have not
found any schematics etc for this mod. Can anyone point me in the right
direction? Thanks!


Here is the schematic for a 35, it should ve similar to the 20 except for
an extra pass transistor or two.

http://www.repeater-builder.com/astr...tron-vs35m.jpg

The diagram looks strange to me, with a direct connection from the
higher voltage driver supply (e.g. pin 12 of the 723 or collector of
Q2) to the base of the series pass transistors.

With a higher separate driver power supply the driver current should
flow through the Q2 driver transistor, through the small resistors
into the base connection of the series pass transistor. Thus in the
diagram the direct connection from the driver supply to the series
pass transistor base should be removed.



You're right about the schematic in the link above. There is an error in the
drawing. The wire between the collector of Q2 and the bases of the pass
transistors Q101 and Q102 should not be there. That connection would put full
rectifier output on the bases of the pass transistors, turning them fully on,
resulting in the PS output going way high with no regulation. Maybe even
destroying the pass transistors.

However, the link is to a schematic for a RS-35, not an RS-20. The OP has an
RS-20, and the proper schematic link should be
http://www.repeater-builder.com/astr...1988-09-23.gif.


I modded one of those significantly. Too long ago to remember the exact
thing I did, but DO put that crowbar SCR anode on the collector of the
pass transistors. If it trips as is, you kiss the current meter and
expensive current sense resistor goodbye.

Remember that the purpose of the crowbar is to protect the load from power
supply overvoltage, not to protect the power supply. The crowbar should be
connected to the regulator output, not the input. If connected to the input,
the reservoir capacitor(s) on the output are still charged when the crowbar
fires, and the overvoltage condition is applied to the load until the capacitors
drain. The load is unprotected.

Remember also that most of the Astron supplies have current foldback limiting.
That means that on a shorted output condition, the regulator should reduce the
available current into the short to a significantly lower level, protecting the
pass elements and regulator.

When using crowbars, there should be a some kind of fuse on the
feeding side, preferably immediately after the filter capacitor.

If the crowbar is before the series pass transistors, it is a good
idea to put a large reverse biased diode across the series transistors
(CR7 in the diagram), which would discharge any large capacitors on
the regulated side through the crowbar. Without the diode, the
collector side would be at 0 V after the crowbar tripped, but the
emitters could still be at +12 V due to any large capacitors on the
regulated side.

Paul OH3LWR

==============

Dave M

Never take a laxative and a sleeping pill at the same time!!

On Thu, 29 May 2008 18:57:47 -0400, Dave M
wrote:

Remember that the purpose of the crowbar is to protect the load from power
supply overvoltage, not to protect the power supply. The crowbar should be
connected to the regulator output, not the input. If connected to the input,
the reservoir capacitor(s) on the output are still charged when the crowbar
fires, and the overvoltage condition is applied to the load until the capacitors
drain. The load is unprotected.

Just put a reverse biased diode across the series pass transistors.
When the crowbar at the main capacitor fires, the output capacitor
will be discharged through that diode and crowbar, protecting the
load.


Remember also that most of the Astron supplies have current foldback limiting.
That means that on a shorted output condition, the regulator should reduce the
available current into the short to a significantly lower level, protecting the
pass elements and regulator.

Foldback limiting requires that the differential amplifier (in the
723), driver and series pass transistors are working properly, thus,
the crowbar+foldback limiting works properly only if the 723 reference
voltage is erratic. In all other failure modes, such as dead 723,
shorts in driver or series pass transistors, it is quite unlikely that
the foldback would work as expected, if the crowbar fires due to
output overvoltage.

Paul OH3LWR

In article ,
Skipp Isaham wrote:

: and if your really smart, you will dump the fuse, and replace it with
: a Resettable Circuit Breaker, because Murphy's Law states the Crowbar
: will do it's thing, ALWAYS at the worst possible moment, and the
: appropriate replacement fuse will be a 5 miles drive away, at the
: closest. You mileage may vary.....

The auto reset circuit breaker helps to start the fire, which takes out
the site/building. Not a good idea...

cheers,
s.

Nobody said ANYTHING about an "auto reset" circuit breaker, or anything
else "auto reset". Don't be reading in your own prejudices.....