13.8 VOLT DC (High Current) SUPPLY

Inspired by the articles (referenced below), this is my build of a 13.8 volt DC high current (up to 20 A ) power supply using re-cycled computer switched mode power supply units (SMPS).
An obvious use is to power a (Mobile 12V supply type) Ham Radio Transceiver, which can draw 13~15 amp on transmit.
For the physical construct I used a cut-down computer enclosure and fitted re-calibrated voltage and current meters, plus a 12v cooling fan (also from donor PC)
Overall dimensions are 15cm (H) 25cm (W) 19cm (D).



Obtain 2 (or more) power supply units from disgarded PC tower type computers.
These are self contained in a metal enclosure.
and deliver +12v -12V +5V -5V +3.3V (to power the PC)

LINK TO "TYPICAL" SMPS CIRCUIT

The +5V supply (highest current capability) is the one we are interested in.
The circuit boards should be removed from their containment.
Essentially the "trick" is to reconfigure things to deliver 13.8V DC.
One idea is to adjust the voltage output (up) from 5V to 13.8V by changing circuitry components.
However, this may seem as trying to acheive a little too much.
The other idea is to "float" one unit on top of another in order to acheive the o/p voltage of 13.8 v and still have the required current capability.
I chose the latter approach.
Electrically 2 boards are "series'ed" together (see block diagram) which would then give a +10V regulated DC supply.
The "upper" board floats on the other, care must be taken to ensure no part of this assembly connects to the grounded chassis.
The earthy side of the upper board connects to the (nominal) +5V o/p of the lower board. Shown in yellow on diag.
The lower boards earthy connection is bonded to the metal enclosure.



By increasing the value of one resistor at a voltage sensing point (pin 1 TL494) they can be "tricked-up" to give 6.9V output each (i.e. 2 x 6.9 = 13.8V)
Most boards that I have investigated use the TL494 chip to do all the switching/voltage regulation/control etc.
Data on this device is freely available.
Pin compatible equivalents I have found are: SL494, IR3MO2, KA7500B



As you can see from the circuit pin 1 is a voltage sensing point and by simply setting the correct ratio of the voltage divider chain at pin 1 we can force up the o/p enough to meet our needs.
The worst thing that could happen if there was some kind of circuit failure;
and raw 240 volt mains would appear at the DC output teminals. (and therby damage any connected equipment!)
To prevent this there is some additional circuitry in the form of overvoltage protection.
Pin 4 of the TL494 (labelled deadtime control) seems to be a "shutdown" pin, which if sourced by a +ve voltage inhibits the chip.
Pin 4 would have to be isolated from all existing circuit connections.
Although there is some existing on-board overvoltage protection circuitry, I chose to include a small auxiliary circuit rather than modify the existing circuit values. I have used a variation of the familiar 'crow-bar' circuit to protect against overvoltage on the 13.8V output.
A small step down trandformer/rectifier/filter supplies DC to the SCR -
if an overvoltage occurs the 15 volt zener will heavily conduct, trigger and latch the SCR and apply a signal through steering diodes to the pin 4 of the TL494's and hence shut them down.
A red LED will indicate that this has occured.
If so; switch off and investigate !



I have tested my example for extended periods into an oil cooled resistive load.
15 amp continuous current and no problems!

Another useful circuit which uses the TL494 is Radio 'B' battery circuit (DC/DC converter)

References:

1) Using PC (switched mode) Power supply as 12 volt DC supply by VK6APH
Amateur Radio Action Mag Nov 1998 p14

2) Converting computer PS's - an easy way by Keith Alder VK2AXN
Amateur Radio (Australia) Jan 2000 p11

3) You-tube video of a similar conversion (from USA) SMPS as ham radio DC supply