Replacing a multi voltage linear supply with multiple switching mains adapters - Page 1
Forum section: electronics / projects

author: wilfred
author_rank: Super Contributor
timestamp: March 24, 2018, 12:37:04 pm
content: /.

author: fourtytwo42
author_rank: Super Contributor
timestamp: March 24, 2018, 04:37:59 pm
content: Well -5V is obligitory for the memory for a start. I would not go down the route of multiple switchers as unfortunate rail sequencing may result possibly damaging your logic.The better solution is to use more obtainable transformers and suffer a slightly higher regulator dissipation (bigger heatsink or add a fan).15-0-15 is very common for your 12V rails and 9-0-9 is fairly common for the 5V railsOnly other thing to watch is existing electrolytic voltage but given the small change I cannot imagine that would be a problem.

author: Ian.M
author_rank: Super Contributor
timestamp: March 25, 2018, 12:04:36 am
content: Start with 12V in. Buck to 5V and (also from 12V) inverting buck to -5V. Design a rail supervisor that monitors the outputs and gates MOSFETs in each rail to manage the sequencing. Run it from the +12V supply.For the rail supervisor, you need three window comparators, with a Zener or other absolute reference, monitoring +12V, +5V and -5V to ensure they are in their acceptable tolerance bands. Then you need logic to switch the MOSFETs: P-MOS for the 12V rail and N-MOS for the other two. the -5V MOSFET needs some level shifting - a 6.2V Zener in series with its gate drive from the logic + a gate-source pulldown resistor should do nicely. The logic will be 4000 series so it can run on 12V. If all rails are in spec, turn on the -5V MOSFET and N ms later turn on the +5V and +12V MOSFETs. If any rail goes out of spec or at loss of input power (you'll need enough bulk capacitance to keep the superviser running for a short time + a diode to isolate it), immediately turn off the +12V and +5V MOSFETs and then the -5V one.However a better idea (unless you are restoring legacy hardware) would be to hack the design to use 4164 chips in place of the 4116 chips so you can run everything on +5V. With some minor mods they'll fit in the same socket, and don't need +12V and -5V, so no power sequencing issues. Seehttp://picmania.garcia-cuervo.net/recursos/redpicdatasheets/memorias_ram/4116_to_4164.htm.You can use small DC-DC converter modules if the +12V and -5V are also needed by video output or serial port circuits - without the RAM issue, there wont be any sequencing problems.Its also possible to use their full capacity and vastly cut down the number of DRAMs required, but that would need a redesign of the address decode logic and some software or hardware assistance to generate the A7 refresh address line as the Z80 only provides a 7 bit refresh address, and if you are going to do a board redesign anyway, simply drop in a large enough byte wide SRAM.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 25, 2018, 12:12:45 am
content: Well if you're talking about power supply sequencing, or rather switching power to the load when all rails are good, then a relatively simple series of comparators AND'd together, controlling relays would work. With 3 power rails that's only 3 comparators, so an LM339 with its open drain outputs driving a transistor, driving a couple of relays (or P_Channel MOSFETs). Sounds like it would be easiest just to replace the existing components though. As bulky as mains frequency transformers are they still have advantages including simplicity, low noise etc..As for using two 5V supplies to make a bipolar power supply (+5V and -5V) assuming that the power supplies are isolated (floating) then yes it cane be done. Almost all offline switchers are floating (but best to check!) but then as you've noticed you run into the trouble of sequencing, possible noise issues etc..edit:Apologies Ian, wrote this reply before seeing yours.

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 25, 2018, 01:04:32 am
content: I also have a bunch of those SMPS, but never tried since for whatever reasons they say it's not good.But what I did do was find two, 100W stereo's, so I have 2 nice transformers for free, that I am building swapable PSU circuits for. And I have some microwave oven controller PCB transformers, only 12V@0.1A, but sure.I'd say try and find transformer's for free, and save them from the landfills.

author: Ian.M
author_rank: Super Contributor
timestamp: March 25, 2018, 01:08:12 am
content: Another option would be to use a (floating) 5V PSU to supply the -5V rail directly + two cheap boost converter modules to supply the +5V rail and +12V rails (set the converters to 10V and 17V respectively as their output is relative to -5V).Add some diodes (normally reverse biassed) so the +12V rail cant go below the +5V rail, and the +5V rail cant go below 0V, and a Schottky diode in series with the -5V rail so when the power fails, the -5V stays up (on its downstream bulk capacitance) long enough for the other two to collapse, and you should be free of power sequencing issues.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 25, 2018, 01:39:07 am
content: Quote from: wilfred on March 25, 2018, 01:14:49 amWhilst I'm sure you've sorted the problem or at least have an idea about what to do, sometimes it can be worth exploring things that *aren't* in your skillset, purely for that reason. If you're using standard mains transformers and linear regs then a power supply sequencer isn't required, and certainly over-kill. But designing one anyway - to see if you can - might be a worth while exercise. And perhaps there are conditions where parts of the load can be damaged due to a failed regulator, then a "power good" indicator, or at least a power monitoring system could be knocked up on stripboard very cheaply.I'm not pushing the "make a power monitor!" thing, just saying, I saw this and thought it might be a good opportunity to give it a go, if you learn something on the way, its worth it.

author: Synthtech
author_rank: Regular Contributor
timestamp: March 25, 2018, 06:35:29 am
content: If you are in Australia I can direct you to a source of 18-0-18 + 9v transformers. I use them often.

author: C
author_rank: Super Contributor
timestamp: March 25, 2018, 07:58:24 am
content: The TRS80 model I used a DC power switch.TRS80 model III & model IV used switchers.Technical manuals can be found on web.

author: abyrvalg
author_rank: Frequent Contributor
timestamp: March 25, 2018, 09:30:21 am
content: Just an idea: old AT/ATXv1 power supplies have all these voltages.