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ATX POWER SUPPLIES FOR D.C FEEDS

 
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ATX POWER SUPPLIES FOR D.C FEEDS
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Black_Hawk
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Post ATX POWER SUPPLIES FOR D.C FEEDS Reply with quote
Greetz,

There are some ATX Computer power supplies that can be used to provide a very stable and well filtered source of Direct Current at 12 to 13volts which can be used for Powering up Various transmitters such as the PCS VERSIONS...However some minor modifications have to be made to the power supply it self by opening up the unit.

A)The POWER OK SIGNAL LINE ..WHICH IS A GRAY WIRE COMEING FROM THE 20 PIN ATX CONNECTOR THAT GOES TO THE MOTHER BOARD HAS TO BE ATTACHED TO EITHER THE 12 OR 5 V LINE.

The power ok signal(voltage)purpose is to allow the motherboard to initiate the boot process and to continue to function in the absence of unstable or improper power levels.
The PSU is designed to complete a series of Power On Self Tests (POST) before the motherboard starts to powerup. These tests determine if all the voltages are up to design specification and stable before sending a signal to the motherboard. If the POST completes successfully, a POWER_GOOD or POWER_OK signal is sent to the processor over the POWER_GOOD line. This signal must be continuously present for the system to operate and, if withdrawn (due to a brownout, for instance), will generate a system RESET. The system will remain in a continuous RESET mode until the signal is restored. Since the processor initializes in the RESET mode, the system will not start until sensing the PWR_OK signal.

So to transform this "TEORY' into a pratical working unit,the Gray wire is Located and we now "Trick' the power supply in cycleing thru the Power on self test,which makes it think that it is feeding a Mother Board//CPU.

Stated another way a voltage on the PWR_OK line indicates that the PS has completed a successful POST and that the output voltages are stable and within design specification.

I usually use a LED (light emitting diode) as an indicator that the PSU is on, rather than tie it to one of the +5V or +12V lines, attach the PWR_OK line to the anode (+) side of the LED and place a 220 ohm resistor on the cathode (-) leg before grounding it. The cathode leg is normally shorter on new LEDs -- if the legs have been clipped, the cathode will be on the same side as a flat spot on the LED base..

HERE IS THE SCHEMATA LEGEND OF THE 20 PIN ATX MOTHER BOARD CONNECTOR..A VOLTMETER SHOULD BE ALLOW ONE TO VERIFY THE FACTS AND PROCEEDURE.
========================================

PIN 1= +3.3V =Orange
2=+3.3V =Orange
3=GND= Black
4=+5V =Red
5=GND= Black
6=+5V =Red
7=GND = Black

8=PWR=OK =Gray

9=+5VSB =Purple
10=+12V =Yellow
11=+3.3V =Orange
12=-12V= Blue
13=GND =Black
14=PS_ON =Green
15=GND =Black
16=GND = Black
17=GND =Black
18=-5V =White
19=+5V= Red
20=+5V =Red

The Highlight here is the Gray wire..
======================================

Now lets slap some more "Teory" around some more..Power supplies in today's computers are known as 'SWITCHMODE 'or Switching Mode
power supplies and require a load to continue to operate after being switched on (the term switching mode actually applies to the technique of A/C to D/C
conversion and not to the power up action).
=========================================

Wiring coming off an industry standard circuit board(AT FORM FACTOR) will be

ORANGE=+3.3 V
YELLOW=+12 V
BLUE=-12 V
RED=+5 V
WHITE=-5 V
BLACK=GND
GREENPOWER=ON
GRAYPOWER=OK PURPLE=+5 V= STANDBY

=========================================
So here is the second trick or modification to the PSU..A resistor is then connected across the 5 volt lines or feed tap of the power supply .This load is provided by a 10 watt, 10 ohm wire wound load resistor ..This provides the load for the Power supply to accomplish the Load required to keep it truned on..

BEAR THESE FACTS IN MIND THAT MOST PLL's also go thru a post operative check itself(PLL ERROR VOLTAGE LOCK)..SO EVEN THOUGH EVENTUALLY THE PLL OR AMPLIFIER WILL MORE THAN LIKELY BE A LOAD FOR THE PSU..THE CYCLICAL POWERUP FUNCTIONS DOES NOT ALLOW THE OUTPUT SENSEING CCTS. OF THE PSU TO SEE A CONSTANT LOAD CURRENT PULL UNTIL THE DEVICE HAS SETTLED DOWN TO IT's SPECIFIED OPERATEING STATS.


The 12 volt load feed is connected between the YELLOW AND BLACK WIRES COMEING OFF THE PERIPHERAL PLUGS.(Hard drive ..floppy..cd rom etc..etc.)

The 10ohm load resisitor is connected across the RED/BLACK LEADS AND THE RESISTOR IS HEAT SINKED TO ANY METAL SURFACE OF THE POWER SUPPLY TO ACT AS A HEAT SINK..AND ALSO A EXTRA FAN IS INSTALLED TO ALLOW FORCED AIR CIRCUALTION..BECUSE IN THIS CONVERSION PROCESS(AS IS WITH ALL CONVERSION PROCESSES) THERE IS ALWAYS A TRADE OFF ..IN THIS CASE THE TRADE OFF IS "HEAT" DISSIPATED IN THE LOAD RESISITOR.

NOW EVEN MORE HEAT WILL HAVE TO BE DEALT WITH IF ONE IS USEING A 300 TO 475 WATT PSU...THEN WE HAVE TO CHANGE THE VALUES OF THE LOAD RESISITOR.

ANOMALY PRACTICALS..SO OK WE HAVE HAVE DONE THE CONVERSION..AND M,EASURED THE OPUTPUT VOLTAGE AND IT IS FOUND TO BE 11.6 VOLTS AS OPPOSED TO BEING 12 T0 12.8 RANGE UNDER CONSATNT LOAD....SO WHAT THE FRIGGIN HECK HAPPENED?

OHMS LAW AND KICRCHOFF's LAWS KICKS IN TO HELP OUT.

Power (watts) is calculated as Pt = V2 / R. Thus, increases in the power level
may be accomplished by increasing the voltage or by reducing the resistance.

TAKE NOTE The ATX switching mode power supplies require a static loadto function and for many, a 10 ohm 10 watt resistor on the +5 V output is sufficient, but voltage levels on the +12 V
line may fall in the range of 11.5 to 11.75 volts....However we are aiming for 12 to 12.8 volts minimum..Ok here is how we apply the mothod to this here anomaly.

A 10 ohm load across 5.15 volts consumes
about 2.65 watts of energy. Pt = V2 / R = 5.152 / 10 ~ 2.65 watts. By virtue of
being a regulated power supply, we cannot easily change the voltage levels
the ATX power supply design guide specifies a +/- 5% variation for the 3.3, 5
and 12 volt outputs and the internal circuitry is designed to maintain output
within those specifications. Consequently, to increase the perceived load, the
most easily controlled variable is resistance. If we replace the 10 ohm resistor
with a 2 ohm resistor, what change can we expect in the load? As before, Pt = V2
/ R = 5.152 / 2 ~ 13.26 watts. This substitution has certainly increased the
load, but it has also introduced another potentially destructive situation. The
load resistor must now dissipate this increased energy and it does so in the
form of heat. We can deal with heat to some extent, but when the load exceeds
the rating of the resistor, our best efforts may not save a 10 watt resistor
carrying a continuous 13 watt load. WOW AMAZEING AIN'T IT FOLKS...WELL I SHALL DETAIL HOW TO "RAMP UP" THE VOLTAGE ON THE 12 VOLT LINE. IN PART 2.

I AM OUTTA MY MIND,,HENCE I SHALL SEE YOU ALL WHEN MY MIND RETURNS..HAHAHAHA!

YOU ALL NOSTALGIC HAITIAN POSSE SQUAD..SOME OF YOU MEN HAVE ****** BIG TIME WITH ME!!THIS HERE STUFF I AM WRITEING ABOUT IS LATER YEARS TECHNICAL NOSTALGIA
Respects..Norm B
Wed Jul 21, 2004 8:25 am View user's profile Send private message
Black_Hawk
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Post WATTAGE VERSUS AMPS OUTPUT Reply with quote
By the way here are some details that i offer as a practical guideline if anyone decides to do the conversion.

FOR A 250-275 WATT COMPUTER SWITCHING PSU FROM EXPERREINCE AND DEPENDING ON THE MANUFACTURER ONE CAN EXPECT A LOAD CURRENT DELIVERY OF APROX 8-10AMPS AT 12 VOLTS

A 300 -325 WATT PSU 10-13AMPS AT 12V DC.

A 350-400W 12-15A AT 12 VDC

A 475 W 25-30A AT 12 VOLTS.

SO PAY ATATENTION TO THE HEAT DISSIPATION FOLKS..THINGS DO GET VERY HOT EVEN WITH A 300W PSU.

IT IS YOUR RESPONSIBILITY TO OBSERVE PROPER ELECTRICAL SAFE GUARDS WHENEVER ONE IS WORKING ON ANY SUCH PSU..ESPECIALLY WITH THE POWER ON(AC MAINS CONNECTED)THERE ARE SOME FILTER CAPS THAT ARE PULSED AT SOME VERY HIGH VOLTAGES THAT CAN ZAP YOU..HARD..EVEN THOUGH THE UNIT IS OFF..THEY RETAIN A CHARGE DUE TO ELECTROLYTIC ACTION ETC.ETC.

SO PLEASE OBSERVE ALL SAFETY PRCAUTIONS..HOWEVER FOR THE SERIOUS LPFM OR COMMUNITY RADIO DEVOTEE OR HARDWARE GEEK..THIS IS ONE WAY OF OVERCOMEING THE POWER FACTOR REQUIREMENTS AT A VERY MODEST COST.

IF ONE IS CAPABLE ENOUGH TECHNICALLY ONE CAN ALSO (DEPENDING ON THE MANIFACTURER)ALSO MODIFY SUCH PSU's to deliver a higher voltage 24v at 5 amps to supply a BLF 278 AMP TO GIVE AT LEAST 150 WATSS OUT..BUT ONE HAS TO BE FAMILAR WITH THE CCT. DETAILS..AND ALSO CAUTION IS REQUESTED ONCE AGAIN..SO PLEASE PROCEED WITH DUE CARE..




O
Wed Jul 21, 2004 8:59 am View user's profile Send private message
Black_Hawk
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Post IN A DAY OR SO I SHALL POST SOME JPEGS Reply with quote
On the YAHOO Pirate Radio site,a few units that i have modified for use overseas..As i said it is one way for those that are Technically sound and wanna take advantage of the low cost and wide availability of some very versatile PC PSU's ...

As i said before one can also armed with a bit of cct. component recognition and device specs..also modify (some units are more congenial) units to provide 18-24 volts,obtained from other parts of the cct. and draw at least 5 amps comfortably to run a BLF278 AT AROUND 130-150 WATTS OUT..WITH A FILTER OF COURSE,AT A LOWER VOLTAGE(LESS THAN 48VOLTS) The exciter driveing the amp has to be able to supply drive power a bit More than the specs call for at 48-52volts

And nothing else can be fed from that power supply because it would then be come overloaded..

So i shall add one more topic area in regrds to the conversion process.

Respects..Norm B
Sat Jul 24, 2004 10:24 am View user's profile Send private message
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