pc power suplly

8/13/2019 PC Power Suplly

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By August Hoecker, [email protected]

OVERVIEWIf you are an amateur radio operator, Short Wave Listener(SWL) or involved in any number of other areas ofcommunications, sooner or later you will have a need for a highcurrent, well-regulated 1 to 1! volt "# power supply$ It may be

that you have removed your all band, 1%%-watt &Super 'and'anger& rig from the car for the winter and need a power source to handle that transmit load$ rperhaps you have your station all setup with emergency battery bac up and re*uire a muchshorter recharge cycle to get your batteries ready for the ne+t blac out$ Whatever your reason,if you have a use for such a supply ranging in output current from ! to over % amps this ideamay be the one for you$ If you have priced such a supply, you will find the cost is in the range%% to .%% (/S )$ "iscussed here will be a way to cut that cost by a factor of 1%0

HOW O WE !E" "HERE#"eveloping a power supply providing 1! "# or less, at the currents discussed is more of asearch and find mission than it is a high tech one$ If you are one of the newer members of ouramateur radio family or new to needing a high current power or charging system, this relativelystraight forward pro2ect should not stretch your technical s ills and more importantly it will notstretch your budget either0 3he tas at hand is to loo through your &2un bo+& or visit your local&4adio 5oodies& store or perhaps better yet, a computer store that sells old e*uipment andused parts$ 6ou will be loo ing for used but wor ing 7ersonal #omputer power supplies$ 3heones I found were purchased at a &5ood Will& store, still mounted in three scrapped out &83#hassis& for ! (/S ) each9see 7hoto-1$ In this case I not only obtained the three wantedsupplies, that were all ali e but I also ac*uired the cases, power switches, 8# power cords, etc$3he cost was low enough to allow the purchase of two additional units to eep as spares or forother service$ 6ou can also use the old 7# cabinet for mounting multiple supplies if you desire$

ABO$" %OM&$"ER &OWER '$&&(IE'It may be worth a few minutes of your time, for those of you who may not have had anye+posure to a typical 7# power supply, to grasp a general understanding of their inner wor ingswithout us going into any great detail$ 3he power supplies I am discussing are enclosed in ametal case about ! + : + ; inches (1. + 1! + 1< cm) containing their own cooling fan, circuitbrea er, power switch (case mounted or on short wire leads to the case) and a standardcomputer . pin I=# power connector$ 3he power ratings of these supplies vary greatly from aslow as 1.! watts to over .%% watts$ 3his wattage rating e+presses the total power output of thesupply$ 6ou will be searching for supplies no smaller than %% watts with bigger being betterwith only one caveat discussed a bit later$

3he 7# power supplies described will normally provide four output voltages which are > 1"#, -1 "#, >! "# and -! "#$ 3he two minus voltages are usually rated at about 1 ampor less each and are of no practical value for our use here and you may 2ust disregard them$3he plus 1 "# will have an output current rating of anywhere from ; to 1 amps "#$ 3he !"# output rating will range between % and % amps depending on the output wattage ratingof the supply$

3he units used for my supply were manufactured in 3aiwan by &?7I &, with a power output ratingof !% watts which turns out to be a very conservative rating for this particular supply$ 3he >1"# is rated at 1% amps with the > ! "# rating at ! amps0 3hese two power outputs loaded

to their full rating will add up to an output power availability of ! watts, very near its postedrating$ 8s shown, we have >! "# and >1 "# and several options and a few restrictions fortheir use in assembling this new supply to suit our needs$
mailto:[email protected]:[email protected]


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ne important point with reference to these computer power supplies is that they are &switching&power supplies not the more conventional &linear& type supplies so many of us may be morefamiliar with$ 3here are several distinct advantages of switching supplies over the older lineartypes$ @ainly they are more efficient and run cooler$ Aowever, there are also a few things to becautious about or have nowledge of when using these supplies$ 3he first item comes under the%A$"IO) flag$ Switching power supplies ta e the 8# line voltage (1%% to % 8#, sometimesre*uiring a 2umper to be removed for the higher line voltages) rectify it and charge high valuecapacitors to a high voltage in the range of 250 to 400 VDC $ 3hese high primary voltages do

present a dangerous lethal shoc haBard and caution must be used if you decide to remove theprotective cover over the supply and po e around inside$ If you do decide to do so alwaysdischarge the large high voltage capacitors within$ A*ways++

ne disadvantage of switching type supplies, in particular the older units, is their re*uirementfor a minimum load on their output terminals$ 3his will be in the range of to amps on the >!volt output and from % to amps on the 1 volt output$ @any supplies re*uire this minimum loadon the >! volts only$ 3his may present a problem depending on the type of service you intend toplace your supply into$ ne *uic , simple fi+ is to place a 1 or ohm, !-watt load resistoracross the >! volt output$ 8lthough you are wasting from 1 $! to ! watts of power, this willsettle the supply down and allow it to provide a stable, well regulated output on both the >! and>1 volt outputs$ 3his is also a good techni*ue to use when first testing your new supplies$ 8nadditional load resistor may be necessary on the >1 volt output, if so re*uired, depending onthe supply in *uestion$ 3his is the caveat mentioned earlier, but will usually not present a bigproblem$ 3he two 5= receivers in my rac draw enough power alone, on the 1 -volt bus, toprovide this load$ I felt however it was necessary that you be made aware of this factor whendealing with switchers$ Without this load present at & ower on & the output voltages will notcome up$

&$""I)! "HE '$&&(- "O WOR 8t this point you have three options for the use of the supply outputs$ Cirst, if your load is at orless than the 1 volt ma+imum rating you may connect your e*uipment to the blac and yellowleads supplying the voltage to the four pin @ole+-type connectors coming from the supply$ Corhigher power e*uipment, parallel several or all of the blac leads together and do the same withthe yellow leads to minimiBe your voltage drops$ #onnect your load to these new 2unctions$ 8llwires of the same color are the same voltage$ 8ll blac wires are common ground or referencepoint for all voltages$ @ore on this &ground& connection a bit later$ 3here is a long cable comingfrom the supply containing about twelve wires$ 3his cable formally connected to themotherboard in the 7#$ 3his may be used as a source of your power$ 3he color codes remainthe same$

6our second means of utiliBing this low cost power is to connect the >! "# outputs of three

separate supplies together in series$ 3his will provide you with >1! "# at ! amps on up tothe ma+imum rating of the supplies you have ac*uired$ If this >1! volts is beyond the wor ingvoltage range of your e*uipment, once again you have two options$ ne simple solution is toplace one or two !% amp stud diodes, with a heat sin , in series with the >1! "# output$ 3hiswill reduce the output voltage to 1 $. or 1.$: "# respectively$ It will also provide isolationbetween the supplies and any standby batteries you may have in your system$ 1.$: to 1.$1 "# ad2ustment as well$ nceagain &$se %aution & when you have the supply uncovered$ In particular while 8# primary

power is applied$ Locate the >! "# ad2ustment pot and set each supply output to from to $!volts which will provide a combined output between 1 and 1.$! "# or as re*uired for yourneeds$


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6our last option is to use both the first and second choices above$ =+cept for the commonground connections (blac leads) each of the supply voltages are independent and isolatedfrom each other$ 8s an e+ample, I power my two 5= @aster transmitters using the high currentseries connected ! volt supplies$ I power each receiver independently with the 1 volt outputs oftwo of the supplies and use the third 1 volt output for other monitor and scanning receiverswith lots of 1 volt power to spare$

A /ew c*osing t0oug0ts : 8lthough there may be a few variations by certain manufacturers, the

standard color coding of these 7# power supplies is as followsD (See L-1)

"a1*e (23

%O(OR VO("A!E %OMME)"'

RE 45 V % A** re6 wires common to eac0 ot0er

-E((OW 437 V % A** ye**ow wires common to eac0 ot0er

WHI"E 25 V % )ot $se6

B($E 237 V % )ot $se6

B(A% %ommon !r6. A** 1*ack wire6 common to eac0 ot0er

ORA)!E 45 'igna* Wire9 &ower !oo6 (ea6, connect to 45 V %

*Note: @ost 7# 7ower Supplies have an 48E5= wire that provides the &&ower !oo6 &signalbac to the supply$ 3ie this wire to the >! "# leads$ Eot all supplies have this feature,however, the supply will provide no output voltages without seeing a positive going ! v signal onthis line$

Switching power supplies have several uni*ue properties$ 8mong these is their ability to &7owerShare& between supplies$ 3his means that although the >! volts may be rated at ! amps, it willbe capable of supplying a larger output current that if the >1 volt supply is lightly loaded$ 6oucannot push things too far due to the limits of the power components within the supply$ 3here is

a fair amount of leeway in most supplies however$If a switching supply is overloaded or short-circuited, it will shut down$ Its out voltage will drop toBero$ "o not attempt to improve the filtering of the output power by adding an additional, largefilter capacitor across the output$ When the supply first begins to come up it will see that largecap as a short circuit and shut the supply down$ 8dding an additional capacitor will not onlycreate a problem you will find it is not necessary0

It is suggested, if you are using more than one supply, (the three in series for e+ample) connectall primary main power input circuits together so they are all powered on at the same time$ 8lso,although the switching power supplies suggested here are *uite efficient while operating theywill draw a large surge for the first several cycles of primary power after you turn on the 8#power$ 'e sure your 8# main supplying the primary power is not overloaded with other heavycurrent e*uipment$ 8fter they are switched on however, the three supplies in series will drawabout 11%% watts ma+imum under full load$


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ne last caution$ @any power supplies connect all of the &blac & wires, "# common, to earth(. rd wire ground)$ If this is so with the supplies you have on hand, you must open up the supplyand cut the trace or traces that tie this "# common point to ground so the outputs are allfloating$ 3his will EL6 have to be done to the second and third supplies$ 3he first supply willhave its 'lac "# #ommon lead connected to ground in most installations and need not betouched$

If you have considered simply connecting the three 1 "# outputs together in parallel you

must diode isolate the positive lead in each supply before connecting them together$ 8lthoughthis will wor fine and provide higher outputcurrents, the output voltage will drop to 11$."#$ 3his can be reduced somewhat through

the use of barrier diodes, but the output will now beappro+imately 11$: volts$ If the >1 "# isad2ustable in your supplies this will not present aproblem simply read2ust the pot to compensate

for the series diode voltage drop$

8s an alternative to using standard 7# powersupplies there are many open frame, highcurrent, >! and >1 "# supplies available

(see 7hoto- ) at very reasonable prices on the usedand surplus mar et$ In fact there are several listed now in the antenneX %*assi/ie6 %*earingHouse (##A) listing under power supplies$ 4emember please & $'E %A$"IO) & when you haveyour fingers inside these switching supplies$ 8lthough their output voltages are very tame thereis a &Tiger in The Cage with Lethal Teeth &02:;2

' er ca artico*u* 6e mai sus sa /ie uti*.

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