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13.8V high current power supply



 
 
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  #1  
Old April 29th 06, 04:14 PM posted to rec.radio.amateur.homebrew
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Default 13.8V high current power supply

Hi all,

You may recall me mentioning I was thinking about adapting a 24V PSU
for use at 13.8V for powering up mobile HF rigs and stuff. I had some
spare time yesterday and took this thing to bits for a better look
inside. It turns out the mains transformer has several secondary
tapping points. I discontected the DC-stab/reg side of the supply
circuitry and reconnected the secondary tapping points to lower voltage
tappings.
Now I measure 16.8V after the transformer's output has been rectified
and applied across the *big* electrolytic. This is under no-load
conditions. Now, is that 3V enough 'headroom' to use up in regulation
and stabilisation for 13.8V final output if I throw the old 28V
reg/stab away and build something from scratch? There are 6 pass
transistors plus a driver (2n3055) already mounted which I plan to use
in the adapted version, so basically I'll just junk the existing
control board and re-use everything else. Any observations/tips?
Thanks,
P.

  #2  
Old April 29th 06, 04:24 PM posted to rec.radio.amateur.homebrew
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Default 13.8V high current power supply

This is probably a stupid idea for some reason that's beyond me right
now, but how about losing the extra volts with a series of diode drops?
Provided the diodes were high forward current types with adequate heat
sinking. I know it wouldn't be as stable as a proper design, but would
it work well enough to run a mobile 100W rig?

  #3  
Old April 29th 06, 05:11 PM posted to rec.radio.amateur.homebrew
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Default 13.8V high current power supply

wrote in message
oups.com...
This is probably a stupid idea for some reason that's beyond me right
now, but how about losing the extra volts with a series of diode drops?
Provided the diodes were high forward current types with adequate heat
sinking. I know it wouldn't be as stable as a proper design, but would
it work well enough to run a mobile 100W rig?


I think you'll end up with lousy regulation and
a ripple problem.


  #4  
Old April 29th 06, 06:22 PM posted to rec.radio.amateur.homebrew
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Posts: n/a
Default 13.8V high current power supply


wrote in message
oups.com...
Hi all,

You may recall me mentioning I was thinking about adapting a 24V PSU
for use at 13.8V for powering up mobile HF rigs and stuff. I had some
spare time yesterday and took this thing to bits for a better look
inside. It turns out the mains transformer has several secondary
tapping points. I discontected the DC-stab/reg side of the supply
circuitry and reconnected the secondary tapping points to lower voltage
tappings.
Now I measure 16.8V after the transformer's output has been rectified
and applied across the *big* electrolytic. This is under no-load
conditions. Now, is that 3V enough 'headroom' to use up in regulation
and stabilisation for 13.8V final output if I throw the old 28V
reg/stab away and build something from scratch? There are 6 pass
transistors plus a driver (2n3055) already mounted which I plan to use
in the adapted version, so basically I'll just junk the existing
control board and re-use everything else. Any observations/tips?
Thanks,
P.

I would cobble together a simple, say lm317 regulator to drive your series
pass driver and see what you come up with under some small loads. You may
be ok with that big transformer. The power supply I built uses 4 2n3055 in
parralel like what you have. Have fun.. JTT


  #5  
Old April 29th 06, 08:43 PM posted to rec.radio.amateur.homebrew
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Posts: n/a
Default 13.8V high current power supply

wrote in message
oups.com...
Hi all,

You may recall me mentioning I was thinking about adapting a 24V PSU
for use at 13.8V for powering up mobile HF rigs and stuff. I had some
spare time yesterday and took this thing to bits for a better look
inside. It turns out the mains transformer has several secondary
tapping points. I discontected the DC-stab/reg side of the supply
circuitry and reconnected the secondary tapping points to lower voltage
tappings.
Now I measure 16.8V after the transformer's output has been rectified
and applied across the *big* electrolytic. This is under no-load
conditions. Now, is that 3V enough 'headroom' to use up in regulation
and stabilisation for 13.8V final output if I throw the old 28V
reg/stab away and build something from scratch? There are 6 pass
transistors plus a driver (2n3055) already mounted which I plan to use
in the adapted version, so basically I'll just junk the existing
control board and re-use everything else. Any observations/tips?
Thanks,
P.


Paul -

An ideal secondary voltage is about 5 more volts than you want to regulate.
Let's say that it is 13.8 volt finished.
Then and ideal secondary voltage would be 18 or 19 Volts AC on the
secondary.

Using 6 pass transistors (2N3055) - The 2N3055 is good for an intermittent
current or amp draw of 10 amps.
A heavier one that is often used for 13.8 VDC supplies is the 2N3771.
This one is good for 15 amps intermittent. Both of these are NPN type
transistors.
These are placed in a parallel circuit, if more that 10 amps surge, is
required.

Heat is the biggest enemy of our power supplies.
Keep them cool, use well sized heat sinks and they last a lot longer!!

You will find that the LM723 is the most used regulator IC for commercial
linear supplies -
it has been around since the 1970s !!

Here are some web site articles and references for both your design and
building.

David Metz, WA0AUQ Classic web article (November 1996)
Regulated Power Supply Construction
OR What is Inside your Astron?
http://www.seits.org/features/pwrsup.htm

Metz's schematic
http://www.seits.org/features/pwrsup.gif

Astron Power Supply schematics
See how the "best of breed" US commercial unit is built
http://www.repeater-builder.com/astr...ron-index.html

KBT Linear Regulated Supplies
Specification sheets on pass transistors, ICs, DIY, How to, FAQ
Index for multiple knowledge pages is gray bar on left column of main page
http://www.kbt-dc-supplies.com/index.php

Best of luck with the project !

w9gb


  #6  
Old April 29th 06, 11:15 PM posted to rec.radio.amateur.homebrew
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Default 13.8V high current power supply

James Thompson wrote:
I would cobble together a simple, say lm317 regulator to drive your series
pass driver and see what you come up with under some small loads. You may
be ok with that big transformer. The power supply I built uses 4 2n3055 in
parralel like what you have. Have fun.. JTT

===========================
There as an exellent design in RadCom Oct 2001 ,pages 34-35 by OZ1XB ,
with a LM317 , TIP32 (PNP) and 2xBUV20 (NPN) ,with the latter
replacable by 4x2N3055 or whatever you have .
Simple,rugged ,reliable design.
It incorporates sensing lines ,ensuring the full set voltage under all
load conditions at the consuming apparatus.

Frank GM0CSZ / KN6WH
  #7  
Old April 30th 06, 01:40 AM posted to rec.radio.amateur.homebrew
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Default 13.8V high current power supply

You're probably right! I did actually try to rip the heating element
out of a 2kW electric fire to use as just such a test load, but the
manufacturers had used some really devious sort of screw heads to keep
the two casings together. :-(
Failing some serious resistance wire, the only other 'to hand' option
is to hook up the HF mobile and key-up; but I don't have a sufficiently
gutsy dummy load or antenna to dump 100W into, I'm afraid. :-(

  #8  
Old April 30th 06, 11:37 AM posted to rec.radio.amateur.homebrew
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Default 13.8V high current power supply

My favorite high current 12V load is auto headlamps average sealed
beam is not less than 80W often as much as 120W so two are easily a
10-20A load. Some of the halogen lamps are higher power but get very
hot (caution).

the key thing is enough load to simulate a radio and parallel lamps
of sufficient power will do that safely.

==============================
In the absence of car bulbs , a good dummy load to test a power supply
unit is to take a length of galvanised fence wire of say 1mm dia or larger.

For a 13.8V-20A supply the load resistance needs to be 0.7 Ohms.

Send a current of 1 ampere through the wire and measure the voltage with
a DVM. Subsequently slide a crocodile clip along the wire until the
voltage measured is 700mV ,then you have the correct length.
Coil the wire and connect to a "chocbloc" connector.
Place coil in a plastic bucket filled with water.
Although tap water is somewhat conductive the overall resistance will
probably not decrease significantly. If it would you could make the wire
resistance somewhat higher (longer wire).

An alternative method would be using a roll of PVC insulated copper wire
of sufficient diameter determining a 0.7 Ohms resistance with the above
method. Again use as dummy load placed in a bucket filled with water .

Frank GM0CSZ / KN6WH
  #9  
Old May 1st 06, 10:22 AM posted to rec.radio.amateur.homebrew
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Default 13.8V high current power supply

Sounds like a good start

Don't forget the 6 x Emitter ballast resistors (each should drop 100mV
at full load), so maybe 10 milli ohms each (short length of electric
heater element ?).

May need a fan to keep the heatsink cool.

You probably also need to mount the driver transistor on a heatsink.

Regulation using an LM723 or similar. There should be plenty of
detailed designs out there - also see the Nat Semi application notes on
the subject.

See http://www.national.com/pf/LM/LM723.html

Regards,

Mark.

  #10  
Old May 2nd 06, 11:13 PM posted to rec.radio.amateur.homebrew
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Default 13.8V high current power supply


wrote in message
oups.com...
You're probably right! I did actually try to rip the heating element
out of a 2kW electric fire to use as just such a test load, but the
manufacturers had used some really devious sort of screw heads to keep
the two casings together. :-(
Failing some serious resistance wire, the only other 'to hand' option
is to hook up the HF mobile and key-up; but I don't have a sufficiently
gutsy dummy load or antenna to dump 100W into, I'm afraid. :-(



Paul,

Welcome to the world of design...



No need to make a load to measure the ripple on the caps. You can
calculate it. I did this long ago for a 5V 28 amp supply I re-wound from a
28V 5A supply.



You have to know that for a capacitor, I = C * Dv / dt.

When the 60 HZ wave drops, the diodes drop out of conduction and the filter
cap is now supplying *all* the load current...and the voltage drops in the
usual capacitive nature following that formula.

I don't recall if you said the current, but...if you will be drawing, say 20
amps - you have the "I". If you have a "C" you plan to use from the old
design, you have "C". Dt is the discharge time between the (full-wave)
peaks from the bridge rectifier. This is 8.333 ms. minus the conduction
time. I forget the typical conduction time I have seen, but I'll assume 5
ms max-load (the diodes don't conduct very long, typically.and.and the peak
current is pretty high to boot!) -- leaving 3ms for the discharge time.
Then Dv is the ripple, or more accurately, the sag. Also, consider that the
peak voltage will be lower when loaded due to the transformer winding
resistance and diode drop and anything else in there.



So, solving for Dv. Dv = (I* dt ) / C



If you wanted 20 amps and had 20,000 uF, You'd have (20*.005)/.02 or 3
volts of sag.



BUT! This assumes *normal* line voltage. If you DO measure as some of the
others suggest, remember that you'll be using whatever line voltage is at
the time. Consider when there is a brown out. Pick a low-line voltage, say
105 volts and use THAT voltage, Or the ratio 105/120 and adjust your output
voltage number accordingly when calculating things.



You have to make sure that *Everything* that you want to power from this
voltage has enough voltage at the low point; series pass & Driver & driver's
driver (perhaps the 723).



Finally, what I did to keep the required overhead to a minimum and, as a
result, the dissipation in the series pass transistors lower (and guarantee
poorer brown-out protection, unfortunately), I *added* a few extra, lighter
gauge wire turns to the *OUTPUT* winding to supply the 723 and driver stuff.
Add the same number of turns to each side of the secondary (put one and
measure the volts-per-turn to figure out how many) and put a diode from both
of them (pointing to) to another smaller filter cap. This requires, of
course two more diodes and filter cap, but they're smaller. You also have
to watch the ripple in the same way.



Hope this helps. Let me know @arrl.net



73, Steve, K9DCI








 




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