H720 Power Supply

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The H720 Power Supply was a power supply produced by DEC, principally for the BA11-C and BA11-E Mounting Boxes, the two earliest earliest standard system unit backplane mounting boxes for the PDP-11 line.

They were also produced in a rack door-mounted configuration, for use with some larger peripheral device controllers. For the mounting box application, it was a separate unit, mounted transversely at the back of the box.

There were six different variants, -A through -F; three sets of two, of which the earlier in each set (e.g. -A) was configured for 110V, and the latter (e.g. -B) was configured for 220V. (The input transformer had two input windings, which could be connected in series or parallel, depending on the desired voltage.)

The three sets were the earlier mounting box version (-A and -B), the door-mounted version (-C and -D), and the later mounting box version (-E and -F).

The later mounting box versions of the H720 differed in:

  • Use of a circuit breaker rather than a fuse on the main AC input;
  • It had a pair of connections to the standard DEC Remote Switching Control Bus, so it could turn other units off and on with its power switch;
  • The power switch on the front of mounting box uses these, instead of a switch in the main AC input;
  • A temperature sensor to power down the unit in case of over-heating;
  • Improved +5V output capability.

It supplied regulated -15V (10A), and +5V (16A, -A/-B; 22A -E/-F), and also i) a 60Hz signal for a line time clock, and ii) ACLO and DCLO power status signals.

It contained an input transformer, and a PCB which produced all the outputs. The actual power supply at both regulated voltages was a switching supply, for improved EI conversion.

The H720 was 8" high, 16-1/2" wide, and 6" deep, and weighed 30 lbs.