Transformers and Power Supplies

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Goals and Objectives

There are two parts to this article:

Part 1: The fast facts about transformers

Part 2: The power supply circuit as a whole

The aim of this article is to demystify power supplies as typically found in vintage gaming consoles. The emphasis here is on Linear supplies, which use simple components to generate the +5 (and sometimes +12 & -5) volts used by the digital circuits.

Part 1: The Transformer

  • Fast facts:
    • Transformers are simple devices
    • Transformers are imprecise devices
    • Most transformers are functionally similar
    • Finding a substitute for a missing transformer is usually trivial

The box that plugs into the wall is the transformer. Sometimes it is actually an entire self-contained power supply, but for most consoles & computers made during the 70's through the 90's, it's a simple voltage conversion device known as a transformer, a power adapter, (incorrectly) a power supply, or a wall wart.

A transformer takes line voltage (100v - 240v, depending on the country / system) and changes it to something more suitable for the intended device.

Most transformers are unregulated. They are rated for a certain output voltage but may actually operate in a range above or below the stated value. They are subject to change based on line voltage and output load fluctuations.

For example, a North American Sega Genesis transformer converts 120v to 9v. If you measure the voltage on the DC plug (the end that would normally be plugged in to the console), it may read as high as 16v. If you were to plug it in and then read it, it might read 9, 9.5, 8.5, etc. Transformers are not precise devices.

Once the power is sent into the device, it is then filtered and passes through a regulator. This is a device that takes the power from the transformer and provides a reasonably constant, accurate voltage output. Typically this is 5v, as many older devices are based around "TTL" logic, which operates within narrow voltage tolerances around 5v.

Remember this: Most voltage regulators can work with a wide range of input voltages.

OK, so what's the point of all this? The point is that your console probably doesn't care what brand of transformer is plugged in. If you opened the cases up on 10 different transformers, they would all look remarkably similar on the inside.

If the power transformer for your console is both missing and hard to find, you can probably find or make a substitute for a lot less than the cost of an original replacement. Systems like the Turbo Duo, Sega CDX, or Emerson Arcadia always seem to be missing their original supply, and OEM replacements are impossible to find or incredibly expensive.

When looking for a substitute transformer, you need to know 4 things about the original:

  1. Is it AC or DC?
  2. What is the polarity?
  3. What is the voltage?
  4. What is the amperage?

Your substitute supply should match these characteristics like so:

1 - Is it AC or DC?
If the original is DC, only a DC transformer will do. If it is AC, you can use either.
2 - What is the polarity?
If the DC plug on the substitute fits your console, then the polarity will need to be the same. Tip positive on the old transformer means that you need tip positive on the new supply.
If you are going to use the existing DC plug but the polarity is wrong, you can cut and reverse the transformer wires.
If you have a new DC plug, the wires from the transformer will need to soldered to the appropriate terminals.
3 - What is the voltage?
Voltage should be reasonably close, but depending on the rest of the power supply circuit, can probably be more. For 7805-based systems, you can probably push 12-14 volts on the input.
4 - What is the amperage?
Amperage needs to be about the same or more than the original. You can never have too much amperage.

What are the exceptions?

Systems like the Colecovision, Commodore 64, and original Neo Geo AES use transformers that are fully regulated internally. A substitute supply will need to also be regulated.

Some supplies, especially on older machines, may produce multiple voltages, such as the Astrocade, Colecovision, and Channel F. Again, a replacement will need to produce the same voltages as the original, although sometimes with hardware alterations you can eliminate this need. A supply with multiple voltage outputs usually produces +5, -5, and +12 volts, or it provides unregulated voltages that will end up becoming +5, -5, and +12.

Part 2: The power supply circuit as a whole

Major Components

You will see most, if not all of these components in a typical 5V power supply section.

Transformer
Reduces line voltage from high (110v-220v typically) to low (6v-15v typically)
Bridge Rectifier
Converts AC to DC power by channeling the AC current through diodes. The output of this stage will not be a clean DC waveform, but for +5v systems, the output will now be positive voltage.
Big Filter Cap
Does the heavy lifting to smooth out voltage ripple from the rectifier output and keep the output voltage steady.
Power Switch
Ba-Chomp, Ba Chewy Chomp.
Voltage Regulator
This mostly-in-one component can accept a wide range of input voltages, and outputs a reasonably steady +5v (or -5v, or +12v, or other, depending on the part).
Other assorted capacitors shown in diagram
More filtering and stabilizing of the voltage as it makes its way through the circuit.


Transformers

Transformers seem to be source of confusion and paranoia, and not without good reason: Console manufacturers have used dozens of different transformers over the years. Frequently the transformer that originally came with the system has disappeared. Mismatching transformers with consoles can either work fine, or be an instant kiss of death.

There are two types of transformers:

  • AC
  • DC

The difference is:

  • AC transformers output AC power
  • DC transformers output DC power


Rectifiers

+5V Schematic Diagram

Lets examine a typical AC to DC power supply stage as found in an older game console.

AC-to-DC-Supply---DC-Input.png

From Left to Right

Transformer - Bridge Rectifier - Big Filter Cap - Circular Plug

These are all contained within, or part of the wall transformer.

Line voltage enters the transformer, is reduced, is rectified into DC, which is then filtered and sent to the circular plug.

Jack - Big Filter Cap - Power Switch - Voltage Regulator - Output

DC power enters the circuit via the jack, is filtered, sent through the power switch, enters the voltage regulator, which outputs a clean +5v source to power the console.

The most commonly used regulator is the 7805.


What about AC power supplies?

So I plugged and NES power supply into my Genesis and now it doesn't work...

Let's have a look at a console that designed for an AC power supply

AC-to-DC-Supply---AC-Input.png

See how the rectifier bridge is not part of the power supply, but instead is part of the device? What happens is that the input voltage is never rectified. Several components, including the regulator, are polarized and cannot function when voltage and ground are reversed. These parts can be damaged very quickly by an AC supply.


DC power supplies on AC devices

Generally this is a safe option, but not always. What happens is the addition of a redundant rectifier stage.

AC-to-DC-Supply---DC-on-AC.png

Technically, the additional set of diodes drops about .6v from the input of the regulator. Increasing the parts count increases the number of points of failure. In practice, neither of these will probably be an issue.


Specific Cases

  • Super Nintendo: 3rd party power supplies add noise to the sound, video, or both
    • When Nintendo began revising SNES hardware, they did away with a large filter cap. My guess is that cheap 3rd party supplies combined with the missing filter cap allow excess AC ripple and induce noise within the console. I will test this theory as soon as I can locate a console that has these symptoms.