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