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"MTX Plus+" Power Supply

Superseded by Version 2.0

 

My MTXPlus backplane has connections for external power, two VCC planes and two grounds. I shall be using an ATX power supply to feed power to the backplane. The easiest method of connecting power from an ATX PSU would have been to remove the PSU multi-way plug and solder the appropriate wires directly to the backplane. This has a number of disadvantages :

  • It would render the power supply useless for other purposes
  • A switch would need to be installed in a convenient location to turn on the PSU
  • High current supplies would directly feed the backplane, with the potential to damage the bus in fault conditions

For these reasons, I shall be building another card to provide an interface between the PSU and the backplane. The power module does not actually need to connect to the backplane itself, but in my case, as there are plenty of spare bus slots, it is a convenient location for the card. Although plugged into the backplane, the only signals on the power module actually connected to the bus are the bus voltage lines and ground, this will allow the bus voltage status to be displayed on the power module, but the power lines themselves will be wired directly from the module to the backplane - not through the bus interface connector.

 

Backplane Power Supply

The primary reason for creating the power module was to provide a location to install fuses in the power feeds to the backplane.

An ATX PSU is capable of delivering very high currents, typically around 30 amps on the 5v/3v rails, in the event of a short circuit in the system, a current of this magnitude is pretty much guaranteed to damage the PCB and/or the backplane power line traces.

The first boards to be designed and built will be the CPU and Video boards, these boards only require a +5V DC supply, it is possible that later boards may also need +12V and +3.3V supplies. Provision  has been made in the bus design to cater for this and, as the PSU has these voltages available, it makes sense to connect them now. As described on the system bus page, it would have been possible to use one of the backplane ground planes to distribute a third voltage level, but instead, pin 17c has been reserved for the +12V supply.

The images show the pin-outs for 20-way ATX and 24-way ATX-2 connectors.

(If you are not familiar with ATX PSUs and how they are powered up, you can find more details on my FDX PSU replacement page.)

The ATX PSU multi-way connector will be plugged into an ATX socket with flying leads connecting to screw terminals on the input side of the power board.

The power module includes a switch to turn on the ATX PSU and LEDs to indicate the PSU main and standby power output status. Other LEDs indicate the status of the +5V, +3.3V and +12V lines from the backplane.

ATX Power supplies are switched mode supplies and require a minimum load to allow them to be switched on and to allow them to properly regulate their output voltages. The PSU may have load resistors wired internally, to cater for PSUs that don't, the power module schematic has a power resistor installed that will provide a minimum load on the 5v line.

The schematic shows a 10 ohm / 10 watt resistor connected between one of the PSU's 5v outputs and ground. This resistor can be enabled or disabled using the jumper. Some PSUs need the load to be on the 12v line, therefore a load resistor will be installed on the 12v line too.

The power resistor draws current from the supply and dissipates the energy in the form of heat, so the resistor must be designed to handle the heat generated and are typically constructed by encasing a wire-wound resistor in a block of ceramic material.

Load Resistor Power Calculations

A load resistor operating at its rated power will get quite hot, larger power resistors are often mounted on heatsinks to dissipates the heat, but smaller ceramic resistors such as this should be able to dissipate the heat be convection. To aid this, the resistor should not be mounted flush with the board - leaving an air gap will aid natural cooling.

The power drawn by the resistor can be calculated from the equations :

 

  P = V2 / R ( P = I2.R ) ( P = V.I )
Where : P = power (Watts)    
  V = voltage (Volts)    
  I = current (Amps)    
  R = resistance (Ohms)    
       
Example calculations :
10 Ohm resistor on 5v line P = 52 / 10 = 2.5W
10 Ohm resistor on 12v line P = 122 / 10 = 14W
47 Ohm resistor on 12v line P = 122 / 47 = 3W

It can be seen that the same resistor installed on the 12v line would draw significantly more power than the resistor was rated for, to provide adequate load on the 12v line, a 10W resistor would need to have a value of around 50 Ohms, the closest standard resistor value is 47 Ohms.

To reduce the risk of injury to people or damage to adjacent components, it is desirable that the resistor runs as cool as possible, using a resistor of a larger wattage than the calculated requirement will result in it running cooler.

The assembled power board, the only thing missing are the fuses.

I am also considering adding some capacitors to the power lines, but as the boards will have their own power capacitors and I am using a regulated ATX PSU, I am undecided at this point.

The solder side of the board :-

 

Wire

Voltage

XSA

Strands

Rating

Red

+5V, +3.3V

0.5mm2

16 / 0.2

11A

Yellow

+12V

0.5mm2

16 / 0.2

3A

Black

Ground

0.75mm2

24 / 0.2

4.5A

A close up on the rear of the backplane showing a pair of the power and ground plane connection points, there are four of these pairs on each of the planes. On this one, the +5V supply will be connected to a couple of these points. Similarly, ground connections will be made on the other side of the board - you can see the insulating space between this power plane and the ground connection on the other side.

+3.3V and ground will be connected to the bottom plane in the same way, and the +12V line will connect directly to the backplane on pin 17c on any slots where it is required.

 

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