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

[Paul Anderson]

The H744 was 25 amp, but then they came out with the H7440  28amp? And the
H7441 at 32 amp.

A lot of units were upgraded in the field.

Paul

On Mon, Sep 17, 2018 at 5:49 PM, Eric Smith via cctalk <
cctalk at classiccmp.org> wrote:

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