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Author Topic: HB PS questions  (Read 2556 times)

Posts: 6

« on: December 30, 2000, 10:38:15 AM »

  First, I am not very knowledgable with electronics, I just enjoy building and learning. I have built a power supply to run my QRP station and have made some mistakes, I would like some advice.
  The station and accessories require about 1.3A; hence my first mistake was using an LM317 (1.5A)regulator, it is getting hot enough to malfunction on long excahnges into a dummy load inspite of a nice heat sink. I tried running one of those small cpu processor fans to cool it. It solved the heat problem but had to add filtering to the output to kill the noise created in the reciever, also the high rpm is causing an annoying vibration and noise.

* What regulator is commonly available that I may sub?

  I believe the fused output will protect the rig from high current but am concerned that if the regulator should fail, it could provide a higher voltage to the rig than the rig can handle and damage it.

* How can I add over-voltage protection?

tnx es 73 de KB8X

Posts: 12771

« Reply #1 on: January 02, 2001, 12:54:33 PM »

There are some 3 and 5 amp regulator chips in TO-3 cases
which should handle your power level just fine.  Another
option would be to use the LM317 to drive a pass transistor, such
as a 2N3055, which would handle most of the current.  Look in the
ARRL handbook or similar reference for a circuit with the transistor
"wrapped around" the regulator:  this allows some of the protective
circuitry of the regulator to keep working.

Make sure you use a tiny bit of heat sink compound between the
regulator and the heat sink:  just enough to fill any gaps.

You can also reduce the heat dissipated in the regulator by
lowering the input voltage.  If the rig is drawing 1 amp at 13 volts,
and you are feeding 20 volts to the regulator, it will dissipate
(20 volts - 13 volts) * 1 amp = 7 watts.  Reducing the input to
16 volts will reduce the regulator power to 3 watts. (You have to
maintain at least 2.5 volts difference between input and desired
output with most regulator chips, though there are newer ones
which will operate with only half a volt difference.)

Note that adding a resistor in series with the input of the regulator
will shift which element dissipates the heat, but doesn't change the
total amount generated.

For over-voltage protection, the standard approach is to use an
SCR "crowbar" to blow a fuse when the output voltage exceeds
a certain level.  (When the voltage on the SCR gate get high
enough, the SCR conducts and shorts the output side of the fuse
to ground.)  However, there is a finite time required for the SCR to
trigger, and it might be a good idea to add a large 16 volt zener
across the output to keep the voltage from damaging the equipment
before the SCR triggers.

Good luck! - Dale WB6BYU

« Reply #2 on: January 02, 2001, 10:57:23 PM »

Hi, check out the above link,
lots of small projects including power supply
info and detailed instructions on what
the circuit will do and how it works.



Posts: 6

« Reply #3 on: January 04, 2001, 09:30:20 PM »

Thanks Dale for the response. I found an older (‘97) ARRL handbook and studied a power supply project that uses several of the techniques you describe. It uses an LM338K regulator which  can handle the power requirements. Also found schematics showing how to use even a  small current regulator (LM723) in higher current applications using a “pass” transistor as you described. If I can find the LM338K, I plan to use it and experiment later using a pass transistor. I already have a heat sink for the type of case the LM338K is packaged in.
Interesting is the fact you mention lowering the input voltage; I am using a 25V 2A power transformer and did not consider the extra dissipation required. Thanks for pointing that out. Also in the handbook project the technique of the “crowbar” over-voltage protection is shown and I see how it works.
I have always wanted to have a station I built to the extent that my modest skill level would allow. I am not able to design or understand circuits very well as yet, but am surprised at what has been achieved to this point. Most of the station is built from kits or partial kits, including the Kent paddles.

Thanks again for taking the time to reply and helping me to gain several steps toward the goal!
Vy 73 de KB8X

Posts: 6

« Reply #4 on: January 04, 2001, 09:36:11 PM »

Anonymous :-),
   Thanks for the informative URL. I have book-marked it for future reference. And low, the weekend draweth nigh, and Debco Electronic Supply only a few blocks from work; I may finish this project this very weekend.

Tnx om es 73 de KB8X

Posts: 12771

« Reply #5 on: January 05, 2001, 11:53:59 AM »

Power supplies are a good place to start learning, since DC is
usually better-behaved and more predictable than RF.

If your transformer has a center tap, you may be able to change
the rectifier from a bridge to a full-wave (two diodes) and reduce
the input voltage to the regulator.  First measure the actual
voltage at the input to the regulator:  with a 25V transformer it
probably is around 30V with no load.  Then, if the transformer has
a center tap, use this to drive the regulator instead of the output
terminal of the bridge rectifier.  This will cut the output voltage
about in half.  If this is too low to maintain regulation under load,
try a Low Drop-Out (LDO) regulator, which typically only needs
0.5 volts of "headroom".  (You may find a 5V LDO regulator with
adequate current rating: it can be used in the same circuit as the
LM317 by varying the voltage adjust resistors.)

Oh, yes... make sure you have a bypass capacitor (0.1uf or so)
bypassing both the input and output terminals of the regulator
to ground.  Otherwise, it can oscillate at RF.  (I've seen diagrams
for an 80 transmitter using an LM317 as the output device!)

Good luck! - Dale WB6BYU

Posts: 12771

« Reply #6 on: January 05, 2001, 12:16:22 PM »

I guess the real lesson to learn here is that you have to consider
the power dissipation of a regulator, not just the current rating.
In fact, this is more often the limiting factor for both regulators and
pass transistors.  (With the latter  you will also have to consider the
amount of drive current available, the decrease in gain at high
currents, and the junction temperature. However, at this point in
your learning process, just be conservative with the ratings of
transistors and you should be fine.  And when you do get bit by
one of these issues, you will eventually figure out why it doesn't
work, and, as a result, you will remember the lesson far better.)

Learning through mistakes is one of the best ways to make the
lesson stick...  as long as the mistakes are not TOO costly!
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