Computers Overview
Commodore PET
Sinclair ZX80
Sinclair ZX81
BBC Micro
Sinclair Spectrum
Memotech MTX
      Hardware Hacks
      Legacy (1980s)
              Games ROM
              I/O Ports
              System Bus
          Design Data
      PAL Reader
    User Groups
    Video Wall
Memotech CP/M
Atari ST
DEC 3000 AXP
Raspberry Pi




"MTX Plus+" Power Supply

Version 2

My MTXPlus+ backplane has connections for external power using two VCC planes and two ground planes. The VCC planes are reserved for +5VDC and +3.3VDC (for future use) and backplane pin 17C is reserved for +12VDC. Version 1 of the power board was designed to allow an ATX power supply to feed power to the backplane.

I have now designed and built a second version of the power board, although the ATX PSU should have been a good solution for providing power to MTXPlus+, I have replaced it for a number of reasons :

While I was using my MicroATX PSU for testing MTXPlus+, it was connected to the power board using a couple of cheap ATX PSU extension cables, as time went on, these cables became more & more temperamental and started causing power drop outs.
Even the "small" ATX PSU that I was using was designed to supply well in excess of what MTXPlus+ will ever need. I was using the same model of PSU (a HEC-200) that I had used in my FDX PSU replacement project, this PSU can supply 16A at 3.3V, 12A at 5V and 10A at 12Vs, even though small by PC standards, it is overkill for MTXPlus+. While fine for development and testing, the physical size is also far greater than it needs to be for the final design. 
Using a dedicated design for the PSU will allow the system to be assembled from components should MTXPlus+ ever make it to the "product" stage.

The power supply 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. I decided to use dedicated power wires when I had less clarity on what the MTXPlus+ power requirements were likely to be. Now that the power requirements and supply rating have been reduced, I may revisit this. The +5VDC, +3.3VDC (future) and ground connections on the backplane have 2 x DIN41612 connector pins allocated, each with a rating of 2A, so I could potentially feed power to the backplane through the backplane connector up to a maximum of 4A per rail.

The MTXPlus+ system bus was designed to support +12VDC, +5VDC and +3.3VDC, the latter voltage was provided to cater for future expansions that may have needed a 3.3V power supply. With MTXPlus+ essentially complete and no requirement for 3.3V power having come to light, the version 2 power board will only provide 12V and 5V. Should it be necessary to provide 3.3V at a later date, space has been left on the power board for the components to generate 3.3V from the 5V supply.

The input power to the board will now be supplied by low voltage DC "wall warts", providing the power feeds for the +12V and +5V rails, and suitable regulation to ensure stable voltage levels. Wall wart AC adapters are easy to find on eBay, but come in different flavours, from cheap, unstabilised adapters, providing pretty ropey DC voltages somewhere in the region of what they say on the case, to more expensive, regulated supplies that could provide a clean and stable supply for MTXPlus+.

A cheap unregulated wall wart will probably just contain a step down transformer, regulating diodes and an electrolytic capacitor across the output to provide some smoothing.

More expensive wall warts may include a switching regulator circuit to provide a stable output, but without testing them in advance it is not guaranteed that an off-the-shelf (or off eBay) supply would be suitable for providing power to MTXPlus+, or indeed, anything else ! For that reason, I decided to design the Version 2 board based on the assumption that the input power would not be clean and would require to be stabilised on the power board.

Home computers from the 1980s typically used power supplies based on the use of linear voltage regulators, such as the LM7805s that Lez had supplied us with in his kits of parts. As I had them available, power regulation on the Version 2 PCB was based on the use LM7805 and 7812 regulators. To maintain a stable output voltage, the LM78xx needs to have a minimum overhead of about 2.5V above the required output voltage. However, this excess input voltage is dissipated as heat, the bigger the overhead, the hotter the regulator will get, so, ideally, the input voltage should be little more than the minimum to provide stable regulation.


+5 VDC Supply

The schematic above shows the circuit for the stabilised +5V supply for MTXPlus+, if you are familiar with the Memotech MTX, you may find that it looks somewhat familiar! The circuit uses a LM7805 linear regulator for voltage regulation, but R1, with a value of 10 ohms, ensures that the majority of the current is switched through the TIP2955 power transistor and only a minimum supplied by the LM7805.

Depending on the manufacturer and model number, an LM7805 can normally supply approximately 1.0-1.5 amps, the MTX computer board has a 3.15A fuse in the +5VDC supply line, suggesting that the MTX power requirement was somewhere close to this, perhaps 2-3A, well in excess of the capacity of an LM7805. The use of a TIP2955 to provide current in excess of the capacity of an LM7805 seems to be a pretty common arrangement, so while the lower power requirement for MTXPlus+ may have meant that an LM7085 on its own may have been sufficient, I wanted to make sure that I was not going to be power limited.

Although I had a pretty good idea of how the TIP2955/LM7805 combination worked together and how the sense resistor R1 was used to set the bias between the TIP2955 and LM7805, I wanted to model the circuit to evaluate the effect of changing the value of R1 on the current split between the two.

The trend shows how the current through the TIP2955 and LM7805 varies as the value of the sense resistor, R1, is varied from ~0 Ohms to 10 Ohms and the corresponding voltage differential between the Emitter and Base of the transistor for a fixed load of 1.5A. When the sense resistor value is 0, the transmitter's base and emitter are at the same voltage, the transistor is switched off and all of the load is supplied by the LM7805. As the value of the sense resistor increases, a voltage differential is created between the transmitter's base and emitter.

The system behavior can be seen more clearly by zooming in on the range 0 to 1.5 ohms; when the sense resistor value is zero, the transistor's emitter and base are at the same potential and there is no appreciable current flow at the emitter/base or emitter/collector junctions. The transistor is essentially inactive in this "cut-off" region.

As the value of the sense resistor starts to increase, a voltage difference between the emitter and base develops and increases with the resistance.

Zooming in on the range below 1 Ohm shows that the currents begin to change when the value of the sense resistor starts to increase above ~0.27R and the corresponding voltage difference between the transistor's base and emitter (VQe - VQb) increases above ~410mV when the transistor urns on and starts to pass current from the collector.

The above trends show that a 1 Ohm sense resistor would limit the current from the LM7805 to around 0.7v, a 10 Ohm resistor would limit the LM7085 current to around 0.1A and a 3 Ohm resistor would set the LM7805 current to around 0.25A. In all cases, the TIP 2955 would pick up the rest if the load. A typical TIP 2955 has a maximum collector current rating of 15A - much more than MTXPlus+ requires and a level that would risk damaging other components and PCB traces under fault conditions, therefore, a fuse should be installed in the TIP supply line as Memotech did in the original MTX512 power circuit.


+12 VDC Supply

The +12VDC supply is only used for aspect ratio switching of the SCART connection, power for the RS232 serial ports and potentially, for a disk drive motor should a disk drive eventually be attached to the system. These power requriements are well within the capacity of an LM7812.


+3.3 VDC Supply

The 3.3VDC rail is included on the MTXPlus+ backplane to allow the flexibility to add modern 3.3V devices should it be necessary. The Version 2 power supply board will not supply +3.3VDC by default, but will have sufficient free space to allow a 3.3VDC regulator to be powered from the +5VDC supplty.


Backplane Power Supply


Schematic diagram of the +5VDC portion of the Version 2 board

Download the complete diagram as a PDF
Schematic diagram of the +12VDC portion of the Version 2 board

Since the quality of +12V power rail is less critical than the +5V line, the +12V line includes an option to bypass (or not install) the regulatory components and power the rail from a good quality external supply.
Component side of the Version 2 power board.

The +12VDC portion is on the top half of the board and the +5VDC portion on the lower half.

The solder side of the board :-


coming soon . . .

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.


  • The voltage simulations on this page were produced with LTSpice IV, "a high performance SPICE simulator, schematic capture and waveform viewer with enhancements and models for easing the simulation of switching regulators."


 LTSpice IV is available free from Linear Technology

References :

Sparkfun, Unregulated Power Supply Tutorial, A Dummiesí Guide to Working with Wall Warts, Hobby Electronics Power Supplies Part 1: Wall Warts



mailto: Webmaster

 Terms & Conditions