Digital's DECSYSTEM-20 - Part 6 - Power

DECSYSTEM-20 POWER (Part 6)

As all old mainframes so was DECSYSTEM-20 (PDP-10 / KL10) also power hungry. That was due to the development coal that speed was always desired over energy saving.

DECSYSTEM-20 had enough power to serve as a heater in a sauna

The total system power requirement was about 22 kW for the KL10-C processor. If you think that the electricity consumption of an ordinary small electric sauna is about 7.5 kW then you understand the extent of the cooling required in the computer room. All electronics power is converted into heat. In addition to the fact that computer rooms were well cooled the floors were raised so that it was easy to carry out cables under the pseudo floor.

Floors were raised to allow easy installation of cables.

Here are the front and rear views of the DECSYSTEM-20 cabinets with main power part names on them.

Front view (Cabinets 1, 2, and 3)

Rear view (Cabinets 3, 2, and 1)

Here is a block diagram of the KL10-C power distribution system and how the parts were situated in the three cabinets. The front end drawers had their own power supplies additional to this block diagram.

KL10-C Power Distribution System

The KL10-C Processor operated with 120 or 240 VAC, 60/50 Hz, 3-phase, 5-wire power. The primary power cable was connected to the front end bulkhead behind the computer. A basic KL10-C Processor required 21.6 KVA of input power.

The 863 Power Control controlled application of AC power to the entire power system via Dec Power Control Bus. AC power to the PDP-11/40, the DECwriter, and the Floppy Disk System was further controlled by the 861 Power Control which received its primary input from the 863. The 861 also controlled AC power to the fans in the CPU cabinet.

The H7420 Power Supplies in the CPU cabinet are identical to those in the I/O cabinet. The power supplies and regulators powering the PDP-11/40 and Floppy Disk system were internal to those devices and were not considered part of the Power System in the above picture.

DECSYSTEM-20 Switch Panel

The switch panel was located in the front end cabinet with the 863 Power Control. During normal operation, the system was turned on and off by means of a POWER ON switch on the Switch Panel. A separate switch removed power under emergency conditions.

DC power for the ECL logic in the CPU was furnished by a ferroresonant H760 Raw Power Supply, a Capacitor Assembly for energy storage, and an H761 Regulated Series Pass Assembly. With the capacitor assembly added, the power supply was capable of 20 ms of energy hold-up when power was removed.

Type H770 and H745 regulators are added to the I/O cabinet compared to the CPU cabinet. A H7420 Power Supply is used with type H744 (+5V), H745 (-15V), H754 (+20V, -5V), and H770 (+15V) DC Regulators.

DEC Production at the Mill Maynard MA ca. 1960

In H760 ferroresonant transformers were used to regulate changes in line voltage. The Capacitor Assembly was connected between the H760 Power Supply and the H761 Series Pass Assembly. Thirteen (13) 0.3 Farad capacitors were connected in parallel to achieve an overall capacitance of 3.9 Farads. A delay of 20 ms was thus obtained before the discharge voltage decays too low. During this period critical information could be stored in memory for reuse after power restoration.



[The next part of this text will cover the console.]


REFERENCES

/1/ decsystem20 POWER SYSTEM SYSTEM DESCRIPTION
      (EK-PWR20-SD-001_DEC20_PowerSysDescr_Apr76.pdf)

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