Modify Astron Power Supply
 

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!

Modify Astron Power Supply
 

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

Modify Astron Power Supply
 

"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

Modify Astron Power Supply
 

On May 4, 10:27*pm, "Toby" 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!

If you will install a 2500 ohm pot between pin 5 and 6 on the
regulator chip ,that will give you variable output.. I agree with the
use of the crowbar circuit, saves damaging your equipment and damaging
the power supply too.. GL W4PQW

Modify Astron Power Supply
 

On Mon, 05 May 2008 03:27:10 GMT, "Toby" 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

Modify Astron Power Supply
 

OK Thanks everyone for your input. Might post when I have it done.....Toby
"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!

Modify Astron Power Supply
 

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

Modify Astron Power Supply
 

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

Modify Astron Power Supply
 

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

Modify Astron Power Supply
 

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