The idea is to use a PC power supply to feed the ATLAS-210 .A typical PC power supply provides at least four DC voltage (+5 , +12 , -5 , -12 ) .ATLAS 210 is only asking for 13.8 volts with up to 16 amp (peak value) . Several descriptions appeared in various magazine like ELEKTOR (6/2003 issue) and MEGAHERTZ MAGAZINE (march 2005 issue) .We prefered to follow the indications given by F1LVT in the latter magazine (please refer to march 2005 issue page 24 to 29 ).
Nowadays it is still very easy to get surplus PC power supply for almost nothing as an old PC is a waste that is even costly to get rid of .
The first thing is to select a power supply giving a low level of HF signal in the 3.5 mhz band and rated at least at 200 watt minimum .
DO NOT TRY TO DO THIS MOD !!! IF YOU ARE NOT AWARE OF THE SHOCK RISK !!!
You cut all wires coming out of the supply keeping only the various 12 volts wires which are soldered together and also 1 wire giving 5 volts .
In order to boost the ouput voltage from 12 volt to 13.8 volt locate the Pulse Width Modulator which drives the switching transistors .It is the brain of the supply ! Generally it is a small digital circuit (reference TL 494 or something else ending with 494) having 16 pins .To get full details about the TL494 circuit see the following link TL494 datasheet ) .There is various circuit equivalent to the TL494 like HA17339, and the KIA494, KA7500, IR3M02, and MB3759 . To increase the output voltage you only have to put a resistor between pin 1 and 7 .The value has to be adjusted in order to get the 13.8 volts at the output .(the value is between 10 and 20 Kohms ) .It should be noted that for some power supply ,to have a good regulation of the output , it is necesary to load the 5 volt circuit with a resistor in order to have some current anywhere between 0.5 and 1 amp .
To get a reading of the output voltage with a good sensivity it is a good practice to measure it with a meter in serie with a zener diode of 10 or 12 volts and a resistor so to have a scale starting at 10 or 12 volt and ending at 15 volts .It is also a good idea to put an AMP meter (0 to 20 amp on the ouput .You put everything in a box .You are all set !
|Get full diagramm of standard switching power supply
|Four power supply where done according to this design and they operate without any problem on 2 ATLAS-210 and two VHF / UHF transceivers .No problems of QRN or QRM due to the power supply on any band even on 3.5 mhz .Keep on going .|
|On some recovered power supply you can be bothered by some small birdies , which are harmonics of the frequency used by the power supply .This can be observed mainly on the low bands (3.5 mhz) .Generally on that band the noise signal is quite high and this is not a problem .However if it is not the case and if the signal of your fellow contact is not very strong and if his signal is right on the harmonic this can be annoying .That is why it is a good idea to put a QRN SHIFTER .THis is a quite simple add on .It consists of a simple resistor which is put in parallel of the existng resistor R16 (see the following diagram) .THis resistor in conjunction with C11 capacitor defines the working frequency of the power supply .If you add through a switch a resistor in the 47 k range you will be able to shift the frequency to clear the frequency that you are listening either on 3.5 or 7 mhz band .|
|Detailed view of the regulator circuit of a typical switching power supply|
is not the only way to get 13.8 volt from a regular PC power supply .As
you have seen there are various description of modification on the
internet . Please find some we have discovered , this list is not
limited !and will grow in the future ||Another schematic for a PC power supply|
The mod to output 13.8 volts
Another schematic with the modification
|Nowadays (as of april 2010) it becomes easy to get for free more modern PC power supply beeing able to deliver more power with better regulation using more sophisticated .Pulse width modulator than the standard TL494 or its various equivalents .For example the LC-B400 ATX is such a power supply .Three of them were recovered in goof shape|
on what was written on the cabinet they are able to deliver 12
volts with 17 amps , 3.3 v with 28 amp and 5 volts under 40 amps
.Surprisingly it is very easy to modify these power supply to make them
being able to deliver 20 amps under 13.8 volts as needed for your ATLAS
principle used to increase the output of the power supply is to alter
the voltage taken by th Pulse Width Modulaor to keep voltage constant
for whatever power .This is possible by inserting a small resitor
in the feed back loop going from the 5 volt ouput to the Pulse
Width Modulator . |
Open the power supply and remove the circuit board to find the trace going from the 5 volt output to the pulse with modulator labeled 2003 IFFASK...... (the exact label varying from power supply to other power supply of almost the same type )
LC-B400 ATX Schematic before modification
|Cut the trace as indicated on the schematic .Insert a 1/2 watt 47 ohm resistor .That is all ..Now the power supply deliver 13.7 volts on the 12 volts output .The regulation is very good the voltage output being only reduced by 0.2 volt when the current vary from 100 milliamp to 20 amp (full load of the ATLAS 210 ) .The modifications were done on the 3 power supply with the same good results .It should be noted that the circuit board was not exactly the same on one of the supply .If the link between the 5 volt output to the 3.3 volt through 2 silicon diodes in serie and a 0.7 ohm resistor does not exist it should be implemented to obtain the desired result|
|LC-B400 ATX Schematic after modification|