Apple I Power supply pinout

In post #1, 'joeventura' wrote:

" ... Explain like I'm 12 "

Uncle Bernie comments:

A 12 year old should not wire up anything to line voltages. It's dangerous (and sometimes lethal) even for adults.

If you can't read the schematics page with the transformers in the Apple-1 manual, better don't try this hookup yourself and consult a professional who is trained in doing it right and in a safe manner.

The only thing which is somewhat confusing in said schematic is the numbering in the motherboard power connector. These are pin numbers. Pin #1 is closest to the heat sink. Pin #6 is the last one.

Pins #1 and #2 go to the 8V transformer secondary winding.

Pins #3 and #4 go to the 24V transformer secondary winding ends, not to their center tap, which on the transformer should have a different color of the wire insulation.

The center tap goes to either pin #5 or pin #6. These pins are connected on the motherboard. One wire connection to either #5 or #6 is enough.

Looks weird, but I guess the rationale behind this pinout was that if the power connector is inserted backwards (turned 180 degrees), the secondary winding of the 8V transformer would go to pins #5 and #6 and be short circuited, and the fuse would blow.

Leave the hookup of the primary sides of the transformers to a professional. The circuit in the Apple-1 manual is unsafe and the sole fuse produces a fire hazard ... with this circuit it is possible to overload the smaller transformer in a way that the fuse does not blow but the small transformer overheats and fries. If you use a sole fuse with a low enough current rating to protect the small transformer, the large transformer can't deliver enough current to make the 5V/3A which is the max rating of the LM323K regulator on the motherboard.

Now, if you use two appropriately calculated fuses on each primary winding, these would protect the transformer, but if one fuse blows, the other transformer still is online and the voltages it sources to the motherboard are still there, but the voltages sourced from the transformer with the blown fuse are gone, and this condition can damage ICs on the motherboard, which are quite pricey.

There were threads on Applefritter on how to do it right and conforming to current UL rules for safety, but it boils down to many fuses and an interlock circuit which turns off all secondaries if one fuse blows.

So you can see that all this is not trivial and should be left to professionals.

- Uncle Bernie

If you refer to the manual supplied with the Apple I, on page 11 it shows a schematic marked "10V CT" for P-8380, and "28V CT" for P-8667. Note 14 goes further, saying:

T1 equivalent transformer shall be 8 to 10 volt secondary, with 3 amp minimum current rating. T2 equivalent transformer shall be 28 volt secondary, center-tapped, with 1 amp minimum current rating.

In practice, these voltage levels are much higher than the circuit needs (10 V AC peak is about 14 V, with two diode drops from the rectifier, gives about 12.7 V DC into the linear regulator LM323. LM323's Vin is from 7.5 V to 30 V, with output power limited over 12.5 V. So the Apple design burns so much heat in its regulators that they run into power limiting). This is why using 8 VAC secondary for T1 is preferred to keep everything cooler.

do you already choose  built a power cable?