Re: Modify Astron Power Supply

On Wed, 07 May 2008 08:08:44 +0300, Paul Keinanen <keinanen@xxxxxx> wrote:

On Sun, 04 May 2008 23:51:08 -0700, geek <geek@xxxxxxxxxxxxxxxxxxxxx>
wrote:

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


"Toby" <josdek2@xxxxxxx> wrote in message news:i8vTj.791$b%1.295@xxxxxxxxxxx
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/astron/pix/astron-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/astron/pix/astron-rs20-1988-09-23.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!!
.

Relevant Pages

  • Re: Astron power supplies...
    ... The crowbar is there to NOT blow up your equipment. ... As already explained earlier in this thread,the crowbar should also be fitted 'upstream the pass transistors (but 'downstream' the fuse, while sensing the output voltage. ... If the Astron voltage regulator would be suspect I would recommend a very simple circuit with a 7812 voltage regulator and a PNP transistor,its output 'steering' the pass transistors. ...
    (rec.radio.amateur.homebrew)
  • Re: Gottlieb schematics and the chimpanzees that drew them
    ... ground braid to the lamp socket. ... but the lamp continued not to work. ... than the one called for in the schematics. ... they transposed the 7000 and 8000 point transistors but I didn't ...
    (rec.games.pinball)
  • Re: Avalanche Pulse Generator: Chassified
    ... Not having any suitable PWM chips on hand, I had bad luck with an LM393 ... At least it doesn't burn transistors on ... Noise BTW seems to be okay with the 0.1 after the regulator. ... What's more, adding an open line to the Pulse Line jack stretches the pulse, ...
    (sci.electronics.design)
  • Re: Avalanche Pulse Generator: Chassified
    ... Not having any suitable PWM chips on hand, I had bad luck with an LM393 ... At least it doesn't burn transistors on ... Noise BTW seems to be okay with the 0.1 after the regulator. ... What's more, adding an open line to the Pulse Line jack stretches the pulse, ...
    (sci.electronics.design)
  • Re: Basic power supply design question
    ... I don't mind overdesigning the regulator and putting in a beffier ... And, as a side note, I already have a HY1802, which is ok, and a Power ... Also, as another side note on the audio amp subject, one of the ... a fast blow fuse will save the transistors. ...
    (sci.electronics.design)