How to properly invert a voltage (from +9V to -9V)? - Page 1
Forum section: electronics / projects

author: caius
author_rank: Regular Contributor
timestamp: July 01, 2025, 08:58:00 am
content: Hi all,I'm currently working a replacement PSU that must outputs +5 V, +9V and -9V.My plan is to use a common AC/DC converter board from 100-220 VAC to +9VDC then for +5V generation from +9V I will use a cheap 7805, the problem is how to make -9V from +9V.I looked at different designs (with NE555, OP-AMPs, etc...) but none has really convinced me.Any ideas about?Thanks in advance, I love this community!P.S.7909 is not good for my purpose since it can accept as input only negative voltages.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 01, 2025, 09:26:48 am
content: You need to specify what current and regulation you need.Charge pumps are common for low currents. The charge pump uart max202 etc are widespread and can be used for low mA loads.

author: tooki
author_rank: Super Contributor
timestamp: July 01, 2025, 09:29:31 am
content: Quote from: caius on July 01, 2025, 08:58:00 amIf you’re buying a premade AC/DC power supply anyway, why not just buy one that already has +5, +9, and -9V outputs? (Or the more common +5, +12, -12V units, if your circuit is OK with it.)

author: Buriedcode
author_rank: Super Contributor
timestamp: July 01, 2025, 09:30:17 am
content: What kind of current? Sounds like you've encountered the classic charge pump inverter (using a 555 works, but very crude) - but they're generally for lower currents < 30mA.You can get ready made DC-DC converter modules that have a bipolar isolated output - say a 5V in, and +/-12V out. They are almost always quite noisy, so it is recommended to have a fairly hefty filter on their outputs. They are also no regulated, so the +/-12V could be used with a 7809/7909 for regulation and better filtering. These are often 0.5W, 1W and 2W power, so for your +/-9V thats about 100mA on each rail max.For something more complicated but possibly cheaper if you're already designing a PCB, is a buck-boost inverting converter, and many buck converters can be configured to generate a negative output. That wouldn't be as "symmetrical" as generating the +/-9V from the same circuit, but it'll work.It's a common problem that has many solutions, just a question of time, hassle, cost and electrical requirements (current draw on the rails, acceptable voltage ripple, radiated EMI etc..).

author: tooki
author_rank: Super Contributor
timestamp: July 01, 2025, 09:42:02 am
content: Quote from: Buriedcode on July 01, 2025, 09:30:17 amYou can get both unregulated and regulated versions.In either case, post-regulating with a linear regulator is an option. I’d go with something better than a 7809/7909, though.

author: caius
author_rank: Regular Contributor
timestamp: July 01, 2025, 12:20:12 pm
content: Thanks for all replies.I tought about a premade AC/DC power supply with +5, +9 and -9 outputs but can't find it.As for current needs this PSU will power an old gaming console (so motherboard, CD-ROM, FDD unit and other external devices like PCMCIA cards), I don't exactly the current drawing but for sure it's not few mA.P.S.Ok, I may use a more common +5, +12, -12V AC/DC board and then regulate +12V and -12V to +9V and -9V with a 7809 and 7909.

author: squadchannel
author_rank: Frequent Contributor
timestamp: July 01, 2025, 01:01:35 pm
content: if it were me, I would use an ATX power supply (Flex-ATX/SFX/Pico-ATX depending on space) and an LDO.if you don't know the current, you should test it with a bench power supply.

author: caius
author_rank: Regular Contributor
timestamp: July 01, 2025, 03:18:56 pm
content: Quote from: squadchannel on July 01, 2025, 01:01:35 pmI need 2A on the +5V rail and 1.1A on the +/-9V onesThe picoPSU solution is good ( I use it on other projects) but I would need also the AC/DC CONVERTER 12V like this :https://www.digikey.ca/en/products/detail/mean-well-usa-inc/EPS-65S-12/7703210This would increase the cost, I want a cheaper (and reliable) solution.

author: tooki
author_rank: Super Contributor
timestamp: July 01, 2025, 03:57:03 pm
content: Quote from: caius on July 01, 2025, 12:20:12 pmWhat console? I’m sure somebody somewhere has measured the actual current requirements.

author: mariush
author_rank: Super Contributor
timestamp: July 01, 2025, 05:18:40 pm
content: A generic MC34063 can produce -9v easily. It can do around 400mA with 12v input, which is what I would suggest you'd use.Make it 12v-20v input, use a buck (step-down) to produce 9v and 5v , they're cheap, like 20-30 cents each chip (see for example AP6220x , AP6330x) and use the 34063 to make a negative voltage.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 01, 2025, 05:54:38 pm
content: +/-12 V is far more common, just post regulate.

author: Andy Chee
author_rank: Super Contributor
timestamp: July 02, 2025, 03:53:15 am
content: Quote from: caius on July 01, 2025, 12:20:12 pmWhich gaming console?It is likely that a replacement PSU solution has already been developed for your old console. The retro computing scene is bigger than you might imagine.Or in other words, this problem has already been solved by others.

author: caius
author_rank: Regular Contributor
timestamp: July 02, 2025, 08:59:49 am
content: The gaming console is the Fujitsu FM-TOWNS Marty.And yes, someone else made a PSU replacement but he did not release the project.Also, the design use three power modules, I want something cheaper.

author: mariush
author_rank: Super Contributor
timestamp: July 02, 2025, 11:34:57 am
content: It will be 5v heavy ... the -9v will most likely be used only to make -5v for the ISA slots or for an audio amplifier.The chipset will work on 5v, 386 cpus at that time were still working on 5v, the ram of that time was running on 5v or maybe 3.3vThere's no hard drive, and the optical drive probably used a 5v motor...If you have access to an actual power supply, you could probably get some hints about maximum output current from the rating of the rectifying diodes on the secondary - see for examplehttps://ibb.co/Pgj8XpX- diodes D10, D8, D9

author: tooki
author_rank: Super Contributor
timestamp: July 02, 2025, 04:52:27 pm
content: Quote from: mariush on July 02, 2025, 11:34:57 amThe switch to 3.3V happened during the Pentium era, IIRC.