"AT" type computer power supplies are often used by model railroaders to provide 12 and 5 Volt accessory power for their layouts. These supplies are reasonably inexpensive due to their age and have been removed from old personal computers.
The AT type are switching type power supplies that do not use a heavy and expensive - iron core transformer. This makes them reasonably small and light weight. A cooling fan is a typical feature of this type of supply.
The supply used as example for this page is rated at 130 Watts and can deliver up to 15 Amps at 5 Volts and also 4 Amps at 12 Volts.
The supply also has low current negative 5 Volt and 12 Volt outputs but these are not usually of interest to modelers. The PLUS and MINUS 12 Volt supplies could be used for a switch machine circuit such as at the following link. (Track Routing Control Circuits For Stall-Motor Switch Machines)
The following diagram show the outputs available from the example supply. Computer power supplies have standard output configurations largely dependant on the power available from the supply.
Operation of the supply itself is not within the scope of this page.
Although the output connectors have multiple supply and ground wires they are connected together at their respective positions on the circuit board.
These power supplies typically have over current protection for their outputs but it would be good practice to split the supplied circuits into groups and then protect these groups with 1/2 or 1 Amp fuses.
Ageneral requirement of AT type supplies is a minimum load for the supply to stay in operation. In this example a minimum load of 0.8 Amps on the 5 Volt supply was needed to keep the supply operating.
If the minimum load was not maintained this supply would shut down and would then have to be cycled OFF and then back ON to reset it.
(When the load was reduced to 0.7 Amps or lower the voltage at the 5 Volt output began to rise. This could damage computer logic devices such as those in the TTL family so the power supply shuts itself down.)
The following diagram shows some methods of providing a minimum load on the supply.
It should be noted that some supplies may also need a minimum load on the 12 Volt output as well. Individual supplies will have to be tested to determine this need.
The 5 Volt outputs of AT computer supplies is not often used for layout accessories but has a considerable current capacity.
This output could be used to power 6 and 12 volt lamps directly with varying degrees of dimming. Bridge rectifiers could also be used to reduce the voltage for lower voltage lamps or dimming higher voltage lamps.
The following diagram shows how this would work for three levels of voltage reduction. The rectifiers should be rated for the current load of the lights.
The advantage in using bridge rectifiers over individual diodes is their larger cases which would allow for cooler operation. Also a 10 or 25 Amp bridge would be considerably less expensive than individual diodes of the same capacity.
NOTE: Layout lighting could also be used to provide the minimum load needed by the power supply.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.