Ed thats quite a story and good to read. But a bit hard to follow to actually answer your question that appears not to need a answer now. The diode switch makes sense. But I think you need another diode to stop the converter from seeing the large filter cap. A thought at least. Switching converters can become unstable when a large capacitor is added. My 2 cents. The other thing I have done is use an actual relay for isolation and a holdup cap after the relay. Pretty simple and very safe. Relay dropouts about 250 ms or less. Regards Paul
On Wed, Jan 19, 2022 at 5:02 PM ed breya <[email protected]> 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 > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send > an email to [email protected] > To unsubscribe, go to and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
