lowering a.c. voltage output for power supply - Page 1
Forum section: electronics / projects

author: mgvguy
author_rank: Newbie
timestamp: February 11, 2020, 07:00:12 pm
content: I have a device that requires 9v a.c. and rated for 2.1 amps. I thought I could modify the output of either of the following:1.) a power transformer that outputs 24 v, 50 va2.) a power supply walwart - output 15v a.c., 1.5 ampsIs there a simple way to do this - any schematics/component values possible?Thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: February 11, 2020, 07:30:51 pm
content: Quote from: mgvguy on February 11, 2020, 07:00:12 pmNo!If it *NEEDS* AC, then there is no easy way to change the voltage. Even if the transformers have a dual voltage primary, going from the 120V connections to the 240V connections while keeping the supply at 120V would halve the output voltage, and as neither of them is 18V to start with, you cant get 9V from them so it wouldn't help.

author: KaneTW
author_rank: Frequent Contributor
timestamp: February 11, 2020, 07:54:09 pm
content: Yeah, there's a simple way. Go on Amazon and buy a 9V AC wallwart.If you have a transformer with an appropriate winding ratio lying around, sure, you could do that. If you don't, just get the wallwart.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: February 11, 2020, 07:55:58 pm
content: Modifying transformers isn't particularly easy, especially at wall-wart sizes. Even if you did decrease the voltage of option 2, you wouldn't have enough current capability.For option 1 you would probably have to find a way to convert the unit to accept DC. That could be easy or it could be hard - completely depends on the circuit within. Or you could get a DC power amplifier to output 9VAC lol

author: mariush
author_rank: Super Contributor
timestamp: February 11, 2020, 07:58:59 pm
content: I would recommend actually buying a transformer, it's not expensive.9v 2.1A is around 20VA, so anything higher than that would work.Ex.27VA 9v/18vhttps://www.digikey.com/product-detail/en/hammond-manufacturing/266K18/HM2140-ND/235804425VA 9v/18vhttps://www.digikey.com/product-detail/en/triad-magnetics/VPT18-1390/237-1323-ND/2090061Get the toroidal transformer, put it in a plastic project box, install a IEC C7 connector on the box and the cable with barrel jack connector or whatever your device needs on the other side and you're done.If you want to modify one of the transformers, probably the easiest would be to get the 15v AC 1.5A one and unwind the secondary winding, measure the length and then cut a bit of the wire to match the 9v ... you'd get 9v 1.5A ... hope that your device doesn't actually need 2.1A and is happy with 1.5aTo increase current, you'd have to mess with the diameter of the primary and secondary wires, calculating magnetic flux and other crap.With the 24v transformer, maybe you could unspool the 24v secondary winding and make 3 x 8 secondary windings and parallel them and hope that your device will be happy with 8v ACOr maybe make 2 x 9v secondary windings and parallel them.

author: Gregg
author_rank: Super Contributor
timestamp: February 11, 2020, 09:06:25 pm
content: In thepast(carefully avoiding mentioning how old I am) a lot of devices used wall wart style transformers with AC output; things like dial up modems, phone answering machines etc. A lot of these wall warts have ended up in thrift stores and are available inexpensively; many are even UL listed. A trip to some thrift stores is also entertaining; take a magnifying glass along to read the tiny specs on some of these devices if your vision isn’t the up to the task. Many of the old school wall warts have small fuses inside that can be changed if they have no output. Also be aware that open circuit voltage on a simple transformer rated for 9V and 2 plus amps will be much higher because the transformer voltage is specified at the specified current.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 12, 2020, 12:57:02 am
content: Check if the device actually needs AC or if DC will also work.

author: David Hess
author_rank: Super Contributor
timestamp: February 13, 2020, 12:42:09 am
content: The current is a little low but if you have a 24 volt AC output and a 15 volt AC output, then wiring them in series and anti-phase will produce 9 volts AC.

author: sam[PS]
author_rank: Contributor
timestamp: February 28, 2020, 04:54:29 am
content: Quote from: David Hess on February 13, 2020, 12:42:09 amThat's assuming there is no capacitive effect there to screw your phase alignement. i.e. probably not a good idea in real world.

author: sam[PS]
author_rank: Contributor
timestamp: February 28, 2020, 04:55:46 am
content: Quote from: NiHaoMike on February 12, 2020, 12:57:02 am+1

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2020, 08:56:24 pm
content: Quote from: sam on February 28, 2020, 04:54:29 amThe power line frequency is low enough that I have never seen it not work even with massively different transformer sizes. The phase shift is just not that great.It is not uncommon to do with only one transformer. A low voltage floating winding can be placed in series or anti-series to buck or boost the voltage of the main winding which would otherwise require multiple taps.