The normal reason for this is the unsatisfying problem of a weak power supply in the apple system. This problem is related to all computers throughout the entire series....this is the result from the fact that in those days the market did not offer many chips for switching-power-supply-control and the power supplies from those days where rather weak all together or like later with the arrival of the IBM PC the switching-power-supplies became rather "bulky" and wasted much space and needed large cases.
 

Years later the switching-power-supplies started to "shrink" dramatically and the power raised at the same time dramatically....

The first power supplies for the IBM-systems provided a summary of 120 Watt the next generation offered 150 Watt and few years later the supplies commonly offered 250 to 350 Watt (summary-) power. Nowadays in the same cases power supplies with 450 Watt to 550 Watt are usual. In the same time the cases of those power supplies started to shrink in size and became smaller and smaller......

Some "slimline"-systems have really tiny little switching-power-supplies in this days.....

So i for example started few years ago to collect the cases of old power supplies from Apple II clones just to get an amount of cases for power-supplies that fit in the Apple II series computers..... I really didn't bother if those supplies were working or not ... i just wanted the cases and the electronic inside the cases went to the recycling yard.

At the same time i started - when visiting a electronic-shop - to keep an open eye for PC-power supplies from small slimcase-systems and if some were offered with correct dimensions ( i.e. it would fit in one of those apple-power-supply-cases  ) i bought that supply and time by time
i collected a cheap little amount of "fashionable" fitting supplies ( usually paying some $15 to $25 for that used supply.... sometimes you could get them by a visit at the recycling yard for $5 bucks per unit....

Thereafter at home again i warmed up my soldering iron and did a small "heart-transplantation" by moving the interior electronics from the PC-supply into the case of the apple-power-supply-case from those collected Apple-Taiwan-clone computers.

Note: In some cases a ATX-power supply might not offer the -5 Volt power rail !

You may be able to recognize the difference first at the largest plug used for the mainboard:

The newer supplies have the larger 24 pin plug while the older ones have the 20 pin plug. In a period between the time when the new standard
became common to the market and the former time when old standard was common many companies used a "interim"-solution with the old plug and a additional "attach"-plug that permitted to serve as well at old mainboards as at the newer mainboards. But beware not to confuse that additional "attach"-plug ( with 1 black cable, 1 orange cable, 1 red cable and 1 yellow cable ) with the additional 4 pin plug for additional +12 Volt also common at that days ( with 2 yellow wires and 2 black wires ) !

So either pay attention to this fact while performing a purchase to ensure that the supply that you buy  has the -5 Volt present

or

in the other case you might be forced to add a tiny PCB  ( like displayed below ) to the construction to solve that issue:


In case that you are forced to use this - 5 Volt Adapter please pay attention :
The heatsink must either be mounted with isolation material

or you must  avoid contact of that sink with any other electrical part of the construction

because then there are -12 Volt present at that heatsink !

here is a picture ( as example ) of such a PC-system slim-line-power-supply from LITEON with 200 Watt:

In this pictures both power supplies are just set beside each other to compare  the dimensions of both power supplies...... the LITEON-power supply is 3 mm less high and the measure turns out with the same wideness. The LITEON-power supply turns out to be 46mm less in the length than the Apple power supply and this permits to mount inside an additional fan. The second  advantage of the power supplies usually used in the Taiwan-clones is that  the engineers kept enough spacing in the case to make a good venting of  the air possible. And dragging less power from the LITEON-power supply ( normally i only drag about 140 Watt from that supply with a full loaded  Apple II system with 3 or 4 external floppydrives or with 2 floppies and a harddisk-system and also a 1 MB RAM card inside too and usually also a processorcard included ) that LITEON-powersupply won´t get that hot like the one from apple.... because only 65% to 75% of the maximum from the limit is pulled out of the source.

The only tricky thing is the fact - that 2 cables must be connected internal - to make sure that the LITEON-power supply gets switched "on" and "off" by the power switch on the backside and not by a "Power-button" like in the PC. The green cable from pin 14 an a black cable from pin 15
( connected to ground ) must be shortened ( i.e. connected ) internally.
 

Next the green cable from pin 14 and the black cable from pin 15 are shortened and soldered together and isolated carefully with shrinking material.

Next step is to solder the switch between the one power line of the plug-connector and the line to the PCB. The the remaining connection
between the power plug-connector and the PCB is soldered.

Finally connect for testing purpose a very old outdated hard disk with the power lines +12 V, +5 V and Ground and switch the power on and measure with multimeter the  voltages at the plug.

If the voltages including also the - 5 V and -12 Volt are all O.K. - enjoy the new power source for your apple II series computer
- otherwise check and review all connections.
 

The picture below shows a modified powersupply:

the warm air out of the case and keeps the Apple cool inside..... so this is a kind of replacement of the Kensington Powersaver too...

well as 4 channel supplies :
If you are not able to make a small PCB for use of drawing -5 Volt from the - 12 Volt rail like explained in the graphic in the top section of the

page, then I'd recommend to limit your search to the 4 channel power supplies and reject the purchase of a 3 channel power supply.

And next recommendation would be to make a brief calculation of your requirements to a power supply.

In general the most supplies used by Apple themselves have been calculated to rather poor use of expansion cards in the system.
This means that if you have more than 4 to 5 cards in your system your power supply may probably get rather hot and that causes faster

ageing and sooner risk of failure.

The largest risk will be RAM-expansion cards like the RAMworks II, RAM works III and similar cards with full population of 1MB or more.

Also to that category belong the RAMcards from Cirtech even in the IIGS some RAM cards stress the power supply of the IIGS that far that the system reaches a instable status.

The next but a bit less risk will be card with alternate Processors or accelerators like the cards with Z80B or C processor with additional

64 kB additional own RAM "on board" or  some 68000 Processor cards with additional 256 kB of additional "on board" RAM.

Don't forget some of the Printer interface cards with large amount of RAM on the card used as printer buffer !

The following table might give a imagination about the use of terms like "heavy load" or similar terms...

Bear in mind that power supplies should not be used at full limit -  but rather more only up to 75% of the total ability to avoid to high temperatures !

In general you don't need to pay attention to the amount of Ampere at the minus voltage rails.... nearly all supplies offer more than 0,5 ampere and that will serve all needs at the minus voltage rails. Only of you use a 3 channel supply it's recommended to ensure that 1 Ampere is delivered and that will ensure that you can draw the -5 volt rail from the -12 volt rail and at both rails there will be 0,5 ampere to serve plenty enough the demands to that negative voltage rails. ( there is only one exception from this rule: if you use additional large amount of old 4116 RAM chips - but that is a very uncommon configuration )

The 2 power rails you must focus your attention are the positive power rails. Fist take a look at the +12 Volt power rail:

The highest demand results from drives either floppy drives like the Disk II or similar drives or from harddisk drives.
The original Apple power supply only offers 1,5 Ampere and every floppy drive demands average 0,6 to 0,7 ampere while in action and a harddisk drive will demand 0,6 to 1,2 ampere depending to the kind of construction.
That's the reason that it's recommended to use external power supplies for additional external floppy drives or harddisks.
In former days therefor the engineers that offered 5,25 floppy drives with Schugart bus ( the 34 wire cable ) like used at the TEAC drives
often sold that drives as external "floppy station" with integrated own power supply and that's also valid in general the external harddisk
drive solutions also had own power supply.
In the very moment you add any kind of harddisk solution to internal use of the system you must ensure that your supply delivers at least 2 ampere and better even at least 3 ampere !
If you have a "full loaded" daisy chain with external 3,5 inch and 5,25 inch Apple drives then you also must have at least 2 Ampere at least or even
better 3 ampere at the + 12 volt rail !

Bear also in mind that some soundcards with small "onboard" amplifier or measurement cards like AD or DA cards also request +12 Volt
and sometimes even small amount of - 12 Volt !
 

Now lets examine the requirements at the + 5 volt rail:

The regular Apple power supply offers 4 to 5 ampere. That sounds much... but it isn't that much if you examine the details....

At Apple II or II+ the mainboard only itself demands nearly 3 to 3,5 Ampere ....

At Apple IIe the mainboard only itself demands nearly  2,7 Ampere ....
At Apple IIGS the mainboard only itself demands nearly  3,2 Ampere ....

Simple expansion cards demand average  0,3  to 0,4 ampere ( less than 25 chips )

medium expansion cards  and small RAM cards ( like Saturn 128 kB ) demand 0,4 to 0,6 ampere

large expansion cards ( like Z80B w. 64 kB RAM "onboard" ) demand 0,6 to 0,9 ampere

very large expansion cards ( like PC-transporter, some 68000 cards w. RAM  or large RAM cards like RAMworks full populated ) demand from 1,5 ampere up to 2,5 ampere !

And while performing your math adding up the current required don't forget to add for each disk drive average 0,5 ampere per drive without external power supply and a harddisk internal will demand to add additional 0,8 ampere !

After adding all demands together you should not be very frightened by the amount resulting from your math at the one hand and the weakness of the power supply that shall deliver the demanded current at the other hand......

and above the rule of performing the math to recognize your needs there is another rule to keep in mind:
You should take a view to the power supply itself.... and it's cooling....

if it is passive without fan

or if it's active with fan....

if it's passive the power supply should not be forced to exceed 60%  of it's total strength otherwise it will start getting hot when working

long periods of time forcing it to age much faster heading to get instabile and then heading to fail

if it is active the power supply should not exceed more than 85 % to 90 % of it's total strength...

After that explanation now let's view some present solutions beside spotting at the cheaper solution of a used ATX or PC power supply:
The now mentioned solutions are far more expensive than the previously discussed solutions. Recognize that the so called "3 channel

solutions" are cheaper than the "4 channel solutions" but that 3 channel solutions require the add of a PCB like mentioned above  to
pick a portion away from the - 12 Volt to make the the missing - 5 Volt from that rail. Such a PCB and the requested regulation IC and parts

should be available for amount less than 7,00 to 8,00 Euro ( less than 10 bucks ).

Another point relevant to the price of the solution is that some supplies have additional certification for use in medical constructions....

that demands additional certification and additional parts in the PCB's to trigger alarm in case of failure and less tolerance in the output

voltages. This are reasons that cause the higher price. For the decision of use in our purpose that's not relevant.

remember finally also the dimensions of some of the original power supply : 246mm x 87mm x 55 mm ( length x width x height )
and compare that measurements with the measurements of the devices listed.

I'll list the devices in order of increasing prices that would fit in the original case....: