Getting +5, -5, +12 V from a single channel lab power supply - Page 1
Forum section: electronics / beginners

author: VSV_electron
author_rank: Regular Contributor
timestamp: December 26, 2022, 04:30:04 am
content: Hello!I''ve got a generic 30V/5A (or is it 10A, don't remember) lab power supply.What is the right way to "split" the single channel 30V DC into three voltages +5V, -5V and +12V that I need for powering up the 8080 MPU?Besides the 3 power suplies need to be connected/disconnected from the 8080 in a certain order which is -5V first and then +5V and +12V and the reverse for ther disconnection.This of course further complicates the whole thing but first I need to figure out how to get the 3 voltages.What are my options other that getting seriously down to building a complete dedicated power supply? I've got the schematic but boy... it is complicated. I would happily start the 8080 power supply project if I knew I could get the right mains to the secondary transformer which I doubt. Other than that it's manageable.

author: Ian.M
author_rank: Super Contributor
timestamp: December 26, 2022, 08:44:20 am
content: Personally I wouldn't try to 'split' a single supply as that's going to be nothing but trouble. Buck converter modules that can be hacked to access their enable pin are readily available off Ebay, Amazon and the usual far east eBazaar sites, so that sorts out +5V and +12V with a logic level /ON|OFF (/ENABLE) pin. Add a small 8 bit MCU (with a 78L05 feeding it) to manage sequencing and drive a Zener regulated charge pump for the -5V rail, with four ADC inputs to monitor the rails and Vin. Power to the whole thing would be any 15V to 20V ex-laptop PSU, or even your bench PSU.Another approach would be two power P-MOSFETs to gate 12V direct from your PSU, 5V regulated down from it, and a 1W isolated DC-DC converter for the -5V rail. Then you've got to add power sequencing - possibly built round a LM339 quad comparator if you don't want a modern MCU in there.

author: pqass
author_rank: Super Contributor
timestamp: December 26, 2022, 04:54:56 pm
content: WRT which rail to bring up first.... maybe you just have to keep the CPU in reset until all rails are within spec."PIN 99, POC*, This signal must start out low when the system powers up, and remain low for at least 10 milliseconds after power is stable. POC* must be active only at power-on. POC* must also assert RESET* and SLAVE CLR*."http://www.s100computers.com/S100%20Bus%20Pins.htmYou'll likely only need a modest current requirement for -5V and +12V unlike yester-year; just for the 8080 and 8224. Are you planning to use old memory chips too?For comparison, I've got a bread-boarded 8085 with 16550 UART+MAX232, 8255 PPI, 32K SRAM, 8digit-16seg display, keypad interface, HD44780 LCD, plus 6 other support chips; all fed from an Arduino +5V (acting as a ROM). It draws about 400mA.ADDITIONALLY,wow the 8080 really sucked: "Pin 11: The −5 V power supply. This must be the first power source connected and the last disconnected, otherwise the processor will be damaged. "https://en.wikipedia.org/wiki/Intel_8080Apparently, the solution in the Altair is to use larger bulk capacitance and lower value discharge resistors on the unregulated positive rail (slower to ramp up and faster to ramp down) than on the unregulated negative rail. See first answer:https://retrocomputing.stackexchange.com/questions/18387/how-did-the-altair-ensure-that-the-%E2%88%925-v-supply-was-the-first-connected-and-lastAnd the 8080 got it's own -5V zener shunt supply (see attachement).https://wiki.theretrowagon.com/w/images/a/a8/IMSAI_Schematics.pdf

author: Terry Bites
author_rank: Super Contributor
timestamp: December 28, 2022, 08:07:56 am
content: Rail spitteing is a very bad idea- you ened up with two "grounds". If you're not careful they will contest and the new common point will die.You have to be sure that your PSU negative is not (ever) referenced to local ground.In addition you will have likely get magic smoke the moment you attach a scope ground to your new common.I'd avoid rail spitting.Make or buy 3 DC-DC converters as suggested. It will be effcient, low cost and simple to build.Power supply sequencing is fairly simple. Use a dedicated ic like an LM3881 or make your own counter based one. Many years ago I made a shabby one with an RC ramp fed into a bargaraph ic.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2022, 11:38:23 am
content: This is a lab power supply, so should be floating, so it's possible to split it, but that doesn't mean it's a good idea.How much current does your circuit require on each power rail?The main issue I can see is the power dissipation.Power the circuit which uses the most power directly from the power supply and the other rails from regulators. Sequencing can be implemented using timer relays.

author: jpanhalt
author_rank: Super Contributor
timestamp: December 28, 2022, 12:23:47 pm
content: Multi-voltage wall warts used to be available and cheap. Years ago, I got a Motorola 744203030001 from a surplus dealer (e.g., MPJA or Jameco). It was about $12 new and was made by Ault, Inc in Korea with an alternate # of 5712-305-4C3E . They are still available on eBay (used) for the same plus shipping, for example:https://www.ebay.com/itm/224663234411Output is regulated to +5V @0.5A and ±12V @ 0.13A. I used it for well over 10 years until about 2013 as my only "bench" supply for low current. Converting that output to give ±5 volts is simple with either a voltage inverter or negative voltage regulator. I would ignore any price over $15 that is asked for one, if you decide to go that direction.

author: Infraviolet
author_rank: Super Contributor
timestamp: December 28, 2022, 11:34:20 pm
content: For prototyping work the quickest option is definitely multiple wall warts, two 5V ones (USB ones and a little adapter to convert each USB port/plug to header pins) and one 12V one (usually barrel jack connector of some sort). Get properly made higher current (2A is the usual which common wall warts can do) with proper voltage regulation, and ensure they are each using a transformer internally (some of the very cheap and low current ones do horrible things with capacitative voltage dividers or other dangerous ugly tricks which aren't fully isolated from the mains earth). Each of them ends up with a positive voltage output and a ground output, and these grounds are all floating relative to each other and to the mains supply. Decouple the output of each with a lot of large capacitors to smooth out the switching noise. These decoupled voltages can then be carefully connected up so that one of the 5V and the 12V share a Gnd, and then the other 5V has it's positive rail tied to that shared Gnd, hence it's own Gnd becomes -5V.