[time-nuts] Re: HP Z3801A project update

[Bob kb8tq]

Hi

Stacking switchers is generally not a good idea. Even stacking them with 
non-switchers is not a recommended way to go. Can it be done? Sure, if
you know a lot about exactly what’s inside the switcher ….

Best approach: 

Convert all the this and that to some voltage > 20 V and then do the “voting” 
from there. Diodes are one way, there are others. Let each supply do it’s thing
with a nice normal ground on the low side of the supply. 

Also keep in mind that the 3801 supply is far from perfect. They have a tendency
to blow input fuses for no apparent reason. 

Bob

> On Jan 19, 2022, at 5:00 PM, ed breya <eb at telight.com> wrote:
> 
> Since back in October, I've been working on getting the Z3801A in a 5065A carcass project closer to completion. I mostly finished the clock control board and other details, and moved on to the power system. I decided to completely redo a lot of the mechanical arrangement, wiring, and the line powered section with nicer layout and all the facilities for battery backup.
> 
> A couple weeks ago, I had a major setback. I was really on a roll, having wrapped up the external battery control section, and ready to start on the internal battery interface and charging system. I had already built in the facilities for external DC input years ago, so it was mostly ready to go, once the controls were set up. I was doing some checkout on external powering, when the DC-DC converter in the booster crapped out - specifically, during hand-off tests between AC powered and external DC powered modes. I had incorporated all sorts of protections against faults, but apparently not enough, or I missed something that could happen in certain conditions. After inspection and rethinking, I still can't determine for sure how the converter could have blown out, but I have some theories.
> 
> First, I should mention that this Z3801A is of the "BTS" type, which runs on 20-30 VDC. This is handy for battery backing operation. The internal battery system is two 12 V, 12 A-hr SLAs in series. When backing up, it is used directly, without any extra conversion loss, just the forward drop of the blocking diode.
> 
> The AC power system uses the original 5065A transformer and choke, plus a bunch of other stuff to make everything work. The main output is called "Vs," and is the central point for all the power, running from about 21-30 VDC, depending on line voltage and battery modes and conditions. Everything goes into Vs. Its blocking diode is the rectifier bridge back at the transformer output.
> 
> The external source was intended to be 12 V nominal (11-15 range), like a car battery. For this mode, I had stacked a DC-DC converter output on top of the supply voltage, with all sorts of diodes to prevent (so I thought) damage under all conditions. The regulated output of the converter was isolated 12 V, and the input could run on anything from 9 to 36 V. It was really nice, simply adding 12 V to the external supply voltage, giving around 23-27 V, less the blocking diode drop.
> 
> Anyway, it ruined my day when it crapped out. I had a pretty good deal going, with exactly three of these identical DC-DC converters - two deployed, and one spare. The other converter runs from Vs, to make the regulated 12 V wrt chassis ground, that runs all the peripheral circuitry. The common line for all the power side circuitry is separate from the chassis (but clamped by diodes), to better control any ground loops.
> 
> So, pondering the situation, I could have replaced the converter, and scrutinized everything thoroughly and added better protection. Or, I could have just dropped the feature for now, to be figured out some time in the future (likely never). I chose a third option, to build a fresh boost converter into it, where I know and control exactly what's going on inside, rather than guessing the limits of the not all that well specified last DC-DC converter module.
> 
> I'm happy to report that I finally managed to squeeze everything into the very limited space, and it's up and running fine so far. The new deal also opens up other options for improving and simplifying operation.
> 
> The converter is based on the LT1070, made civilized and protected by various circuitry. I'll have more to say next time about what's in there, and details of how all the stuff plays together.
> 
> Ed
> 
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