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!

"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

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

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.

Cheers,
__
Gregg

On May 5, 2:51*am, geek wrote:
On Mon, 05 May 2008 02:04:09 -0500, Ralph Mowery wrote:

"Toby" wrote in messagenews: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

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.

Cheers,
__
Gregg

Ive modified a couple of these(similar) power supplies by doubling the
the number of pass transisors. In the process I also double the value
of the emitter
resistors. If they were .05 I make them .1. Seemed like a reasonable
thing to do. Any opinions?

Geez, I never thought about the SCR but you are right. While I have
added crowbar circuits and placed them before the pass transistors I
hadnt noticed that this connection on these power supplies needed to
be relocated.

Jimmie

On Tue, 06 May 2008 16:25:50 -0700, jimmie68 wrote:

On May 5, 2:51*am, geek wrote:
On Mon, 05 May 2008 02:04:09 -0500, Ralph Mowery wrote:

"Toby" wrote in messagenews: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

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.

Cheers,
__
Gregg

Ive modified a couple of these(similar) power supplies by doubling the
the number of pass transisors. In the process I also double the value
of the emitter
resistors. If they were .05 I make them .1. Seemed like a reasonable
thing to do. Any opinions?

Yeah, go 0.1 ohms. The 0.05 ohm resistors are useless for practical
purposes.


Geez, I never thought about the SCR but you are right. While I have
added crowbar circuits and placed them before the pass transistors I
hadnt noticed that this connection on these power supplies needed to
be relocated.

Jimmie

And there should be a fuse in there to blow between the filter cap and
crowbar.

The supply was actually 3rd world designed and made I swear.

Cheers,
__
Gregg

In article ain,
geek wrote:

And there should be a fuse in there to blow between the filter cap and
crowbar.

The supply was actually 3rd world designed and made I swear.

Cheers,
__
Gregg

Better yet, put in a Resettable Circuit Breaker, as you can never find
the right fuse, and they ALWAYS blow at the worst possible moment.....

--
Bruce in alaska
add path after fast to reply

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.

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.

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

After reading all these posts, the modification is likely over my head. I
think I'll sell it and find a nice used variable supply. Thanks for all in
the input.
"Paul Keinanen" wrote in message
...
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.

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.

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

: 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.

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!!