AC to split supply - Page 1
Forum section: electronics / beginners

author: ranum
author_rank: Newbie
timestamp: May 06, 2024, 06:22:48 pm
content: Hello, I have been trying to find a solution for split supply. Hopefully can shed some light asking here.Requirement:1-1.5A nominal 1A+-5V+3.3V low mA as VrefAgaints:BatteriesWiring mains with center tap, was looking for wall plug with centre tap toroid. Dont really want to mess with mains, cause I dont really have experience with it.I am thinking to buy 240VA 12V AC 2A AC/AC wall plug or 240VA 12V AC 2A AC/AC. Then rectify it this way. Will this work?I also need 3.3V Vref, should I put LDO after 5V LDO or with VIN+?Problem is I dont think it will work. Simulation shows that at some point will cause LDO +5V dropped to 0V.Using half wave rectifier, the VIN+ and VIN- is steady with no down slope. While half wave is certainly doable, I'm more keen to have efficient full wave bridge. Is it possible to create split supply by using two wall plug and connecting the output in series12 ac/ac plug -> positive rail12 ac/ac plug -> negative rail

author: jwet
author_rank: Frequent Contributor
timestamp: May 07, 2024, 03:22:11 am
content: I saw that you got no replies so I'll wade in...You want to go from 240VAC line to +5 and 1.5A and -5 at 1.5A plus 3.3 at 20 mA.First, the 3.3 20 mA requirement will just be a linear from the +5v rail. LM1117, etc.If the + 5v and - 5v rails, are truly balanced, this is pretty simple? (Usually +5v is loaded more than -5v.) If they truly are loaded equally, you could use a 4.5 VAC output transformer- this will give you about 6v DC after half wave rectifier and filter. Rectify one wire from transformer to make a bipolar supply. Use a forward diode and big cap and a reverse diode with big cap and tie the other side of transformer to other side of secondary output- this will be common. You'll have a +-6v supply. Add your favorite pos and neg linear regulators and the then the little 3.3v linear.If the +5 and -5 are not substantially equal, it gets more complex.Below is a pic that I found online from EE Stack Exchange that should get the idea across, details will be different for your voltages, etcGood Luck

author: Andy Chee
author_rank: Super Contributor
timestamp: May 07, 2024, 08:21:20 am
content: Quote from: ranum on May 06, 2024, 06:22:48 pmAs the previous poster has commented, what you need is a slight variant of the full wave voltage doubler:https://sound-au.com/articles/rectifiers.htm#s5The center capacitor connection becomes reference ground.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 07, 2024, 01:12:27 pm
content: Quote from: Andy Chee on May 07, 2024, 08:21:20 amProbably better get the variant 6 from the same reference. The half wave rectifier version gets quite some ripple and is more a thing for low current.Compared to the 2 separate bridge rectifiers it saves half the diodes and there voltage drop. There is not problem with uneven loads.For the transformer 2x12 V is a little on the high side (it may make sense in areas where the mains voltage can drop quite a bit. Especially with low drop regulators 9 V AC could be enough and save quite a bit on the heat loss. 2 A current capacity is likely OK with an EI style transformer. For the toroid type I would look for a little higher current rating. The rule of thumb is 1.6 A AC for 1 A DC for a rectifier with filter cap, because of the not so good power factor with current flowing in pulses. The larger the caps and the low impedance the transformer (e.g. large and toroid) the factor can be higher (e.g. 1.. With small transformers also smaller (e.g. 1.4).Of cause the diodes should be larger than 1N4148 in the simulation.

author: ranum
author_rank: Newbie
timestamp: May 07, 2024, 05:14:34 pm
content: Thanks, I have scraped the idea of using ac instead using two 12v dc wall plug. I have them lying around and have tested it and it work as split supply. I suppose they are smps and I read I need zener diode parallel to the terminal to avoid issues like different startup/ one off and one on. Should I put two zener with rated nominal current or rated surge current = nominal? I suppose Vzener is 12V.

author: Andy Chee
author_rank: Super Contributor
timestamp: May 08, 2024, 11:09:20 am
content: Quote from: Kleinstein on May 07, 2024, 01:12:27 pmVariant 6 would indeed be better, but it would rule out the use of AC output wall adapters, which typically lack a centre tap.I interpreted the original poster wanting to stay away from mains wiring, hence the AC output wall warts.I suppose you could use two (properly phased) AC wall warts connected in series, the connection becoming the centre tap. I feel that’s asking for more trouble though!

author: S. Petrukhin
author_rank: Super Contributor
timestamp: May 08, 2024, 11:19:15 am
content: Forget about using capacitors to get the midpoint, this is a very compromise solution, completely unsuitable for static linear consumption.It can only be justified for harmonic signal amplifiers.Using a half-bridge rectifier is much more reliable for obtaining a midpoint if the need forces the use of a transformer or a current source without a midpoint.