Thanks Jim for posting the flaws in that design !

Here is a better schematic http://www.wa8dbw.ifip.com/rs-70a.gif

for an Astron 70a linear ps.

clarke


Jim wrote:

On Mon, 20 Oct 2003 13:57:46 GMT, in
, clarke wrote:

Hi, check this link and see what you need to do,
one or two pass trans. will not do the job for you,
easier to use 6-10 pass trans to share the load.

http://www.mitedu.freeserve.co.uk/Ci...er/1230psu.htm

Is your transformer able to supply necessary power to this supply ?

I'm not sure anyone has ever built the PSU shown at the above
URL, and if so, I'd guess they haven't run it at the full 30-amp
output for more than a few seconds.

The basic design demonstrates a generally acceptable approach to
designing a power suppy, but some of the components chosen just
don't seem up to meeting the design criteria of 24 volts in, 12
volts out at 30 amps.

The ballast resistors are rated at 1 watt, but will be expected
to dissipate roughly 2.5 watts at the full 30-amp output.

The text analyzing the regulator performance has me baffled. It
correctly describes a current gain of about 35 for the pass
transistors, which at a glance suggests the regulator has to
handle a bit under 1 amp. The web page author says 871 ma is the
current into the regulator input and 866 ma is the current out,
for a net current of 5 ma and a cool regulator. Looks to me like
that poor thing is going to be asked to dissipate about 10 watts
and will run about 110 degrees C even if mounted on an *infinite*
heat sink. I'd redesign to include a transistor driver.

I'd say the designer of this supply never ran it at full output
for more than a few seconds.

--
Jim Higgins, KB3PU

Help for prospective and new hams, see:
http://groups.yahoo.com/group/HAM-ELMER
Elmers also welcome

Dave Platt wrote:

In this particular application - pass transistors in a linear
regulator - I don't see that MOSFETs would have any particular
advantage.
. . .

Someone mentioned the 2 to 4 amps of base current you'd have to supply
for bipolar pass transistors.

Good point. The base current wouldn't be wasted (it'd flow into the
load), but the need for a fairly high-current driver would certainly
complicate the circuitry and require the use of a higher-voltage
transformer and unregulated supply, compared to a MOSFET solution.

You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.

Might be even more advantageous to use N-channel VMOS, and regulate on
the negative side of the load. N-channel power MOSFETs are
significantly less expensive than the closest P-channel equivalents,
and there seems to be quite a bit more choice in terms of size and
rating. If I recall properly, the charge-carrier mobility in an N
channel (electrons) is a good deal higher than in a P channel (holes),
which means that N-channel MOSFETs can have smaller channel areas than
equivalent P-channel devices, hence more dice per wafer, hence lower
prices.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:

In this particular application - pass transistors in a linear
regulator - I don't see that MOSFETs would have any particular
advantage.
. . .

Someone mentioned the 2 to 4 amps of base current you'd have to supply
for bipolar pass transistors.

Good point. The base current wouldn't be wasted (it'd flow into the
load), but the need for a fairly high-current driver would certainly
complicate the circuitry and require the use of a higher-voltage
transformer and unregulated supply, compared to a MOSFET solution.

You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.

Might be even more advantageous to use N-channel VMOS, and regulate on
the negative side of the load. N-channel power MOSFETs are
significantly less expensive than the closest P-channel equivalents,
and there seems to be quite a bit more choice in terms of size and
rating. If I recall properly, the charge-carrier mobility in an N
channel (electrons) is a good deal higher than in a P channel (holes),
which means that N-channel MOSFETs can have smaller channel areas than
equivalent P-channel devices, hence more dice per wafer, hence lower
prices.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:
Dave Platt wrote:

In this particular application - pass transistors in a linear
regulator - I don't see that MOSFETs would have any particular
advantage.
. . .


Someone mentioned the 2 to 4 amps of base current you'd have to supply
for bipolar pass transistors.


Good point. The base current wouldn't be wasted (it'd flow into the
load), but the need for a fairly high-current driver would certainly
complicate the circuitry and require the use of a higher-voltage
transformer and unregulated supply, compared to a MOSFET solution.


You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.


Might be even more advantageous to use N-channel VMOS, and regulate on
the negative side of the load. N-channel power MOSFETs are
significantly less expensive than the closest P-channel equivalents,
and there seems to be quite a bit more choice in terms of size and
rating. If I recall properly, the charge-carrier mobility in an N
channel (electrons) is a good deal higher than in a P channel (holes),
which means that N-channel MOSFETs can have smaller channel areas than
equivalent P-channel devices, hence more dice per wafer, hence lower
prices.


I built a cheap ps using a bunch of 2n3055's as pass transistors. (think
I had 5 or 6
of them in parallel). Each had a 0.1 ohm resistor in the emitter lead
to balance
out the current (or one might try to hog it all if the transistors
aren't well
matched). They well darlington connected to another 2n3055 which acted
as a driver (and helps increase the gain at high collector current).
Yet another
transistor was used to drive the driver (TO-5 si, probably a 2n3053 or
equal)
from the regulator IC. A large heat sink was used and I mounted surplus
computer power supply fans on it to add cooling.

I used a heavy transformer and the input voltage to the pass regulator was
about 15-16 volts for 13.6 out. Key here is that with a large
transformer and
enough filter C the voltage won't droop on high current (guess that means
it's well regulated without the regulator!) , and we don't ask the
transistors to disapate too much power.

Course' such a supply is HEAVY, a switcher is much lighter, but a good
switcher with low noise is hard to come by! (Not to mention problems
of rf feed back!).

Dave Platt wrote:
Dave Platt wrote:

In this particular application - pass transistors in a linear
regulator - I don't see that MOSFETs would have any particular
advantage.
. . .


Someone mentioned the 2 to 4 amps of base current you'd have to supply
for bipolar pass transistors.


Good point. The base current wouldn't be wasted (it'd flow into the
load), but the need for a fairly high-current driver would certainly
complicate the circuitry and require the use of a higher-voltage
transformer and unregulated supply, compared to a MOSFET solution.


You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.


Might be even more advantageous to use N-channel VMOS, and regulate on
the negative side of the load. N-channel power MOSFETs are
significantly less expensive than the closest P-channel equivalents,
and there seems to be quite a bit more choice in terms of size and
rating. If I recall properly, the charge-carrier mobility in an N
channel (electrons) is a good deal higher than in a P channel (holes),
which means that N-channel MOSFETs can have smaller channel areas than
equivalent P-channel devices, hence more dice per wafer, hence lower
prices.


I built a cheap ps using a bunch of 2n3055's as pass transistors. (think
I had 5 or 6
of them in parallel). Each had a 0.1 ohm resistor in the emitter lead
to balance
out the current (or one might try to hog it all if the transistors
aren't well
matched). They well darlington connected to another 2n3055 which acted
as a driver (and helps increase the gain at high collector current).
Yet another
transistor was used to drive the driver (TO-5 si, probably a 2n3053 or
equal)
from the regulator IC. A large heat sink was used and I mounted surplus
computer power supply fans on it to add cooling.

I used a heavy transformer and the input voltage to the pass regulator was
about 15-16 volts for 13.6 out. Key here is that with a large
transformer and
enough filter C the voltage won't droop on high current (guess that means
it's well regulated without the regulator!) , and we don't ask the
transistors to disapate too much power.

Course' such a supply is HEAVY, a switcher is much lighter, but a good
switcher with low noise is hard to come by! (Not to mention problems
of rf feed back!).