IBM 5155 - Power Supply Unit (PSU)
Turning off the power supply's big red switch does not remove mains (house) voltage from all components within the power supply.
With that information, some people may decide to also power off the 5155 at the wall socket.
Wattage - Breakdown
The IBM 5155's power supply is rated at approximately 114 Watts. Expressed that way, the wattage is misleading. For example, it does not mean that 114 Watts of power can be drawn from the +5V line if no power is drawn from the other lines. Each of the output voltage lines has its own power rating, that must not be exceeded. Click here to see those ratings.
Minimum Off Time
From the IBM 5155/5160 Technical Reference: "When the supply is switched off for a minimum of 1.0 second, and then switched on, the 'power good' signal will be regenerated."
So, after turning off the power supply, wait at least 1 second if you are planning to turn it back on. Any shorter; the motherboard may not start up (due to lack of a POWER GOOD signal).
This power supply requires a sufficient load in order to start/operate.
In some cases of insufficient loading, the power supply starts (you see the fan turn) but the power supply stops after a second or so. In other cases, the fan won't even turn. Note however that both of those symptoms can also be produced by the power supply being overloaded by something.
Some loads that you think may be sufficient are not.
* An IDE drive (eighties/nineties vintage) is not enough of a load. My experiments with two 5155 power supplies show that approximately four to five IDE drives are required.
* Even some full height multi-platter MFM hard drives fail to enable this power supply to start.
* The 5155's CRT unit is not enough of a load.
* In most cases, the 5155 motherboard is enough of a load to start/operate the power supply. This is the case even if P8 is disconnected (but note that the motherboard usually requires both P8/P9 to operate).
I have heard of one case where the motherboard alone was not enough of a load.
From the above, it appears that it is the +5V line that needs the start-up loading, with the other lines (-5V, +12V, -12V) either having no effect or having little effect.
IBM did not provide circuit diagrams of the PSU in the 5155 manuals.
If the SAMS company produced a Computerfacts document for the 5155, then I would expect a circuit diagram of the PSU to be in that.
Hugo Holden's detailed description of the PSU workings is at here. (PDF file download)
A hand-drawn set of circuit diagrams by ???? appears to be at here. (PDF file download)