I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Jimmie

wrote:
I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Jimmie

Tons, but all of them start with 'maybe'.

The biggest one is 'maybe your power supply was designed for straight
transistors' -- if it seems to have hefty current drive to the pass
transistor bases, then that would be the case.

Otherwise, go ahead and put in your pass transistors and see what happens.

Keep in mind that those pass transistors may be missing for a reason,
and the reason may be that the power supply never worked well in the
first place -- so if what you have doesn't work, it may be the design or
execution of your circuit, not the transistors themselves.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Sun, 29 Jun 2008 09:32:47 -0700 (PDT), wrote:

I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Some of the Astron power supplies had a separate (higher voltage)
power supply for the control and driver stages and the input-output
voltage difference could be as low as Vce(sat) i.e. below 1 V, thus
the power dissipation could be minimized.

However with a darlington, the minimum voltage drop would be Vce(sat)
for the driver transistor and Vbe for the big transistor, thus
requiring a higher transformer secondary voltage and hence suffer a
large dissipation in the series pass transistors.

Note also that even if the darlington could handle 50 A, the power
dissipation for TO-3 packages are typically in the 100-150 W range so
a single transistor could tolerate only 3-5 V voltage drop at 30 A.
However, those power dissipation figures apply for 25 C case
temperature, so in practice, this could be achieved only if the
transistor and the heat sing is submerged into running cold tap
water:-).

So in practice, you would have to use several darlingtons in parallel
with large heat sinks and also use larger emitter resistors to balance
out the gain differences between darlingtons. This may require a
slightly higher transformer secondary voltage.

Paul OH3LWR

wrote in message
...
I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Good web page for your overall education and repair.
http://www.repeater-builder.com/astr...ron-index.html

Regulated Linear Power Supply Construction, or What's inside your AstronT?
by David Metz WAØAUQ

"Power Supply Analysis"
This is a PDF of an article from the December 2005 QST

Tim Wescott wrote:
wrote:
I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Jimmie

Tons, but all of them start with 'maybe'.

The biggest one is 'maybe your power supply was designed for straight
transistors' -- if it seems to have hefty current drive to the pass
transistor bases, then that would be the case.

Otherwise, go ahead and put in your pass transistors and see what happens.

Keep in mind that those pass transistors may be missing for a reason,
and the reason may be that the power supply never worked well in the
first place -- so if what you have doesn't work, it may be the design or
execution of your circuit, not the transistors themselves.
===========================================
If the design of what you have is 'debatable' and having NPN Darlintons
which require relative low drive current you might consider a very
simple circuit with a LM317 in a wrap-around circuit.
Two references :
1) Experimental Methods in RF Design , by W7ZOI ,et al ,page 1.15

2) Radcom (RSGB) October 2001 ,page34 : A simple rugged power supply
,by OZ1XB


Frank GM0CSZ / KN6WH

Highland Ham wrote:
Tim Wescott wrote:
wrote:
I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Jimmie

Tons, but all of them start with 'maybe'.

The biggest one is 'maybe your power supply was designed for straight
transistors' -- if it seems to have hefty current drive to the pass
transistor bases, then that would be the case.

Otherwise, go ahead and put in your pass transistors and see what
happens.

Keep in mind that those pass transistors may be missing for a reason,
and the reason may be that the power supply never worked well in the
first place -- so if what you have doesn't work, it may be the design
or execution of your circuit, not the transistors themselves.
===========================================
If the design of what you have is 'debatable' and having NPN Darlintons
which require relative low drive current you might consider a very
simple circuit with a LM317 in a wrap-around circuit.
Two references :
1) Experimental Methods in RF Design , by W7ZOI ,et al ,page 1.15

2) Radcom (RSGB) October 2001 ,page34 : A simple rugged power supply
,by OZ1XB


Frank GM0CSZ / KN6WH

Not to mention the ARRL handbook, which has several choices in their
power supply chapter.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott wrote:
wrote:
I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Jimmie

Tons, but all of them start with 'maybe'.

The biggest one is 'maybe your power supply was designed for straight
transistors' -- if it seems to have hefty current drive to the pass
transistor bases, then that would be the case.

Otherwise, go ahead and put in your pass transistors and see what happens.

Keep in mind that those pass transistors may be missing for a reason,
and the reason may be that the power supply never worked well in the
first place -- so if what you have doesn't work, it may be the design or
execution of your circuit, not the transistors themselves.

--

Tim I made a few repairs that were obvious and discovered the power
supply design was minimal at best. It does have what seems to be a
very heavy duty transformer probably 30 amps CCS. There is no
overcurrent/overvoltage protection. The heat sinks are small in
comparison to my Pyramid brand 20 amp power supply. The transformer is
center tapped but a paralelled pair of bridge rectifers are being used
with the (-) leads disconnected. The regulator doesnt have its own
power supply or even operate from higher voltage taps on the
transformer. Oh well, I bought it for the transformer and it looks
like I got a good deal on that and the box.

Jimmie

Highland Ham wrote:
Tim Wescott wrote:
wrote:
I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Jimmie

Tons, but all of them start with 'maybe'.

The biggest one is 'maybe your power supply was designed for straight
transistors' -- if it seems to have hefty current drive to the pass
transistor bases, then that would be the case.

Otherwise, go ahead and put in your pass transistors and see what happens.

Keep in mind that those pass transistors may be missing for a reason,
and the reason may be that the power supply never worked well in the
first place -- so if what you have doesn't work, it may be the design or
execution of your circuit, not the transistors themselves.
===========================================
If the design of what you have is 'debatable' and having NPN Darlintons
which require relative low drive current you might consider a very
simple circuit with a LM317 in a wrap-around circuit.
Two references :
1) Experimental Methods in RF Design , by W7ZOI ,et al ,page 1.15

2) Radcom (RSGB) October 2001 ,page34 : A simple rugged power supply
,by OZ1XB


Frank GM0CSZ / KN6WH

For what it worth almost any 3 legged regulator will work in this
circuit. The minimum voltage of the power supply will depend on the
voltage of the regulator. Basically the LM317 is a 1.25 volt
regulator.

Jimmie

wrote:

Highland Ham wrote:
Tim Wescott wrote:
wrote:
I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Jimmie
Tons, but all of them start with 'maybe'.

The biggest one is 'maybe your power supply was designed for straight
transistors' -- if it seems to have hefty current drive to the pass
transistor bases, then that would be the case.

Otherwise, go ahead and put in your pass transistors and see what happens.

Keep in mind that those pass transistors may be missing for a reason,
and the reason may be that the power supply never worked well in the
first place -- so if what you have doesn't work, it may be the design or
execution of your circuit, not the transistors themselves.
===========================================
If the design of what you have is 'debatable' and having NPN Darlintons
which require relative low drive current you might consider a very
simple circuit with a LM317 in a wrap-around circuit.
Two references :
1) Experimental Methods in RF Design , by W7ZOI ,et al ,page 1.15

2) Radcom (RSGB) October 2001 ,page34 : A simple rugged power supply
,by OZ1XB


Frank GM0CSZ / KN6WH

For what it worth almost any 3 legged regulator will work in this
circuit. The minimum voltage of the power supply will depend on the
voltage of the regulator. Basically the LM317 is a 1.25 volt
regulator.
===========================
The LM317 ,being basically a 1.25 v regulator can have a variable output
by means of a potmeter or any fixed output by means of 2 fixed resistors
,that's why it is often used as a universal device.

Frank GM0CSZ / KN6WH

On Jul 2, 4:04 am, Highland Ham
wrote:
wrote:

Highland Ham wrote:
Tim Wescott wrote:
wrote:
I have a 12volt home made power supply similar to Astron models. I was
told it could handle 30A. The pass transistors are missing and I have
some 50 amp NPN darlington transistors on hand I was considering using
for replacements. Can you think of any reason these could/should not
be used.

Jimmie
Tons, but all of them start with 'maybe'.

The biggest one is 'maybe your power supply was designed for straight
transistors' -- if it seems to have hefty current drive to the pass
transistor bases, then that would be the case.

Otherwise, go ahead and put in your pass transistors and see what happens.

Keep in mind that those pass transistors may be missing for a reason,
and the reason may be that the power supply never worked well in the
first place -- so if what you have doesn't work, it may be the design or
execution of your circuit, not the transistors themselves.
===========================================
If the design of what you have is 'debatable' and having NPN Darlintons
which require relative low drive current you might consider a very
simple circuit with a LM317 in a wrap-around circuit.
Two references :
1) Experimental Methods in RF Design , by W7ZOI ,et al ,page 1.15

2) Radcom (RSGB) October 2001 ,page34 : A simple rugged power supply
,by OZ1XB

Frank GM0CSZ / KN6WH

For what it worth almost any 3 legged regulator will work in this
circuit. The minimum voltage of the power supply will depend on the
voltage of the regulator. Basically the LM317 is a 1.25 volt
regulator.

===========================
The LM317 ,being basically a 1.25 v regulator can have a variable output
by means of a potmeter or any fixed output by means of 2 fixed resistors
,that's why it is often used as a universal device.

Frank GM0CSZ / KN6WH

Linear Technology has recently introduced an interesting new regulator
that should get the attention of experimenters wanting a variable
bench supply that goes easily down to zero volts. You can consider it
an amplified emitter follower with max offset of a very few
millivolts, along with a precision 10uA current source feeding the
input pin. So you hook a variable resistor between the input pin and
ground, and the output voltage is R*10uA. It's also acceptable to
drive the input pin with a voltage source, such as the output of a
DAC, if you want. Guess I should mention that it's the LT3080. The
design choices that went into it make it easy to parallel, and also
easy to add a "pre-regulator" so that the '3080 doesn't have to
dissipate all the power when delivering low output voltage from a high
unregulated input. (No, I don't work for them...just think it's a
cool part.)

Cheers,
Tom