Looking for two DEC H445 power bricks for PDP 11/40 project

Eric Smith spacewar at gmail.com
Mon Sep 17 17:49:39 CDT 2018

On Mon, Sep 17, 2018 at 4:17 PM, dwight via cctalk <cctalk at classiccmp.org>

> I had a problem with brick power supplies a number of years back. I found
> an issue that caused them to fail. I had about ten of them on the same
> power switch. You'd think this would not be an issue but it is.
> You see it works like this, each one had a transformer in it. When you
> disconnect the power, with a switch, each of the transformers often has
> energy left in the cores. Normally for just one supply, this isn't an
> issue. When you have a bunch of these, only one supply absorbs all of the
> energy. When it does, it will blow some part of that supply up. On the ones
> I had, it'd take of the negative rail.
> I put a MOV on the power rail and didn't have any more issues with power
> cycling.


The DEC regulator modules under discussion (H744, H745, H754, etc) probably
don't have that particular problem. They are switchers, and run on 20-30VAC
input rather than directly on mains voltage. The H742 or H7420 bulk supply
which the regulator modules plug into has a large power transformer from
mains to the intermediate AC, and supports up to five regulator modules, It
also has a control module which includes one or two built-in linear
regulators (low-power compared to the plug-in switching regulators).

The PDP-11/40 has one H742 with five regulator modules. The PDP-11/70 has
two H7420s with three or four H744 regulator modules each.

Some of the regulator modules are rated for up to 150W output. The most
common, the H744, is rated for 125W (5V 25A). However, DEC designed
somewhat conservatively and didn't normally operate the regulators near the
maximum rated current. I don't think the H742 or H7420 can handle much more
than 500W total, hence the 11/70 needing two of them.

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