Apple IIc Plus
Power Supply mod.
Jorge Chamorro Bieling, 12 de Febrero, 2006
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The Apple IIc Plus was designed for and sold exclusively in the U.S. market (?).
The Apple IIc Plus power supply is rated for 120Vac, and it should't be plugged into 240Vac mains.
There's not any readily available 240V power supply that you can buy to replace it.
I have recently bought an Apple IIc Plus.
I live in Spain and the mains voltage here is 240v.
I hate these heavy, bulky and ugly transformers..

I've been told in comp.sys.apple2 that I should use a transformer,
but I would like it much more if I could modify the IIc+ power supply instead.

Here is the (simplified) schematic diagram of the input of the Apple IIc Plus SMPS:



And the figures that we get when it's plugged to 120V mains (forget by now of Z1) are :

Vacin=120V ac
C1=100microFarads, 200V electrolitic
LOAD = P = 15 Watts (max) (the Apple IIc+)
Vab=Vacin*sqrt(2)=170V
The average current (I) through the LOAD is I=P/Vab  => 15W/170V == 88mA

When (if) plugged to 240V mains, the figures are :

Vacin=240V ac
C1=100microFarads, 200V electrolitic
LOAD = P = 15 Watts (max) (the Apple IIc+)
Vab=Vacin*sqrt(2)=340V
The average current (I) through the load is I=P/Vab  => 15W/340V == 44mA

The first thing to note is that even though the capacitor C1 is rated for 200V, the voltage Vab across it is now 340V, so that almost for sure it's going to explode.

The second thing to note is that the load is connected to 340V instead of 170V, that is, Vab is twice as much. Will the load explode too ?

The third interesting thing to note is that the current trough the load is 50% when it's plugged to 240V mains.
This is the normal switching-mode power supply's behavior, it "looks like" a negative resistance: as the the input voltage grows, the input current diminishes.

So What ?

If we insert a zener (Z1) as shown in blue on the drawing, as the voltage drop across it must equal the zener voltage, we can use it to reduce Vab to the required value. The math is as easy as Vab=340-Vzener.

For example, if we put a 140V zener, we get Vab==340V-140V==200V.

We will want to increase Vab as much as possible, though, (as much as C1 and the load can withstand), because this has two positive effects:

1.- It reduces the average load current because the load current is inversely proportional to the input voltage. For example if Vab=200V the load current will be 15W/200V=75mA instead of 15W/170V=88mA.
2.- The bigger Vab is, the smaller Vzener will need to be.

Both 1 and 2 determine the wasted power that gets lost, dissipated in the zener as heat.

The zener voltage times the current passing through it equal the power dissipated in the zener. So for example 75mA of current passing through a 140V zener equals 140V*75mA==10,500mW==10,5W of power, useless power, lost power that is dissipated in the form of heat and that we want to reduce to a minimun.

I went shopping for the zeners. All they got was 5 Watt ones, so I took 7 zeners each 20V, 5W.
That is in fact a good thing, as each one is going to dissipate 20v*75mA==1,500mW==1,5W, and (being 5W parts) they are quite safely overrated.
All I have to do is to connect them in series in order to build a 140V "zener" (7*20V).

I tested a single 20V, 5W zener under a current of 90mA to see how hot it gets. This current translates into 90mA*20V= =1,800mW==1,8W of dissipated (heat) power, that's a bit more than the 1,5W we expect to have in this application, and although it gets quite hot to the touch, it doesn't burn.

Then I prepared everything else for the job at hand.
I soldered all the zeners in a small pcb that fits inside the power supply's case. There happens to be a very handy jumper wire JW2 in the power supply's pcb that when removed leaves two empty pads at the exact points where this "pcb zener" has to be inserted in the circuit. I also replaced the 200V capacitor (C1) with a 400V one, because we will later want to try to increase Vab as much as possible (more on this later). That's how it looks like :



It works as expected : see it here (mp4 video, may require QuickTime).


THERE'S MORE TO COME...
13/2/2006