Universal AC to AC 9V power supply? - Page 1
Forum section: electronics / projects

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 25, 2015, 02:59:39 pm
content: Hey all, I was hoping you might have some ideas for a power supply. I have a friend who is working on a piece of traveling audio gear. The objective is to make this system power compatible all over the world. That's not too bad with modern switching power supplies but those all seem to be DC output. For what ever reason one of the devices needs 9V AC power. So the question is how to get a 9V AC output that auto detects/switches based on input voltage.The one idea I've had, which is Rube Goldberg at best, is universal AC to 12V DC power supply -> 12V to 110 AC inverter -> 9V AC to AC wall wart.A manually switched system so long as it will protect from over voltage is OK. Thanks!

author: tom66
author_rank: Super Contributor
timestamp: March 25, 2015, 03:50:33 pm
content: Can the widget accept a rough AC source, say some kind of square-wave? If so, it would be a simple enough task to use a 12V wallwart and a pair of transistors to drive a 2:1 mains transformer to get +/-12Vpk (about 8.5VAC)

author: mariush
author_rank: Super Contributor
timestamp: March 25, 2015, 03:56:21 pm
content: something very simple would be using a relay and a microcontroller or something to switch between parallel or series on the primary side of a transformer.A capacitive dropper would be simple enough to provide a few mA to power the micro (or comparator or whatever) and relay.By default set transformer primary windings in series for 220v - if you're in 110v country then the output will be 4.5v ac instead of 9v. If it's under 9v, set the relay to change the primary windings to parallel, to get 9v on the output.

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 25, 2015, 03:57:28 pm
content: I doubt it can accept a square wave and I'm not sure they would want to build any circuit. I got asked because the people dealing with this are even less EE and I am.

author: BlueBill
author_rank: Regular Contributor
timestamp: March 25, 2015, 04:32:19 pm
content: How much current?

author: mikerj
author_rank: Super Contributor
timestamp: March 25, 2015, 06:18:10 pm
content: Does it actually need an AC source? If the device simply rectifies, smooths and regulates the incoming supply then a DC source should work fine. If it's doing something tricky like using a charge pump to get negative voltages then this obviously won't work.

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 25, 2015, 06:23:15 pm
content: I'm not sure about the current requirements (I wasn't told). Since we are talking 9V AC wall wart I'm guessing no more than 500mA. As for the need for AC, I'm just assuming that some thing needs it and that is that. This is what happens when you are twice removed from the actual problem.Thanks for the suggestions thus far!

author: firewalker
author_rank: Super Contributor
timestamp: March 25, 2015, 06:34:22 pm
content: Every consumer device (adsl routers, alarm clocks, telephones etc) I have seen that uses low voltage AC as power input, had a bridge rectifier and a filter capacitor internally. If that is the case a DC power adapter could be used.Alexander.

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 25, 2015, 08:02:43 pm
content: Quote from: firewalker on March 25, 2015, 06:34:22 pmHow would you go about doing that? If I'm not mistaken you are saying I can get the AC power out of the transformer directly. I'm not sure about that first because this probably (assumption here) should be UL compliant or something to that affect. This is traveling stage/presentation equipment. Also, if I'm not mistaken what you describe would limit me to one voltage ratio. That is what they currently have. The input in this case could be US 120V @ 60hz or 240V @ 50hz.

author: firewalker
author_rank: Super Contributor
timestamp: March 25, 2015, 09:00:30 pm
content: I am saying that a device with power input of 9 volts AC could potentially (you have to know the inner workings of the device) be powered by a DC power supply. E.g a universal switching power supply.If you don't want to "play" around with it, disregard my option.Alexander.

author: Seekonk
author_rank: Super Contributor
timestamp: March 25, 2015, 09:22:19 pm
content: I run 120V AC electronics like DVD & VCR players along with electronic ballast CFL lamps off 140V DC. Most 9V DC electronic wall warts will work on 100-250V AC 50/60 Hz, problem solved. I only have one device that will only run on 9V AC and that is a sound reinforcement headphone amplifier. Trying one on your friends device will not do any harm. It is the best solution.

author: mushroom
author_rank: Regular Contributor
timestamp: March 26, 2015, 12:11:47 am
content: Quote from: tom66 on March 25, 2015, 03:50:33 pmcould generate a sine with a PWM generator "between" DC and transformer... Or even by direct digital synthesis... A few years ago, I made such a thing from an old UPS. The UPS provides 8 bit sine. I just had to replace the small battery with a car battery. It works fine. (used to power 230V laptop PSU in a car). Could be adapted to output 9V AC by just rewinding the transformer.A small switching PSU, a microcontroller to generate a "PWM sine" or a "DDS sine", power transistors, a transformer, and maybe some very basic filtering ?

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2015, 09:00:08 pm
content: Quote from: Robomeds on March 25, 2015, 03:57:28 pmWhy not?It's possible it might accept a square wave.Quite often, a voltage doubler circuit is used to provide a bipolar power supply, say +/- 12V in this case if it's being run off 9VAC. If this is the case, a 12VDC adaptor and an h-bridge driven by a suitable oscillator would do.Someone else (can't remember if it was here or another forum) had this problem with running an old modem off a battery and it worked perfectly using a square wave.The only way to find out is to look at the piece of equipment in question.

author: amyk
author_rank: Super Contributor
timestamp: March 26, 2015, 09:04:23 pm
content: Quote from: Seekonk on March 25, 2015, 09:22:19 pmI wouldn't be quite so confident, in the rare case that there's a transformer or other part in there which would burn out if given a DC input. It would help to ask and post exactly what this device is...

author: electr_peter
author_rank: Unknown
timestamp: March 26, 2015, 09:18:41 pm
content: Quote from: Seekonk on March 25, 2015, 09:22:19 pmMost AC mains -> 9V DC converters will work from rectified AC mains or DC directly (because of SMPS design). However,DO NOTtry this if you do not know what you are doing.Generating regulated sine wave output is pretty difficult. I would seriously question why it is needed before acquiring or designing such equipment. Regular DC is fine, there are good cheap universal AC->DC 9V adapters.Also, small sized power adapters are considered to be AC->DC type. By getting AC->AC in the mix, you are asking for trouble - somebody will surely think it is DC (or vice versa) and blow their equipment. It is like keeping sweet and salty candies in one box - everybody will assume all candies are sweet (or salty) if they do not test the taste before eating.

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 27, 2015, 04:45:32 am
content: So I have a bit more information. The application is guitar pedal boards for bands that will travel over seas. I suspect the odds of getting a band member who likes his particular brand of pedal to change over to a DC powered model is basically nil. The requested current is at least 3A. Again I really have no idea what the pedals expect to see from an AC wave. I'm curious if a modified square wave into a 110V to 9V transformer would effectively filter the AC wave enough. I guess the best thing to do would be ask them to try it. They might even have an inverter and the 12V supply should also be easy to come by. They did say moving jumpers was OK. I did see some universal transformers. You have to jump them to change voltages and the road crew would have to remember or there would have to be an over voltage protection circuit.

author: ajb
author_rank: Super Contributor
timestamp: March 27, 2015, 05:59:35 am
content: If it's a guitar pedal then I'd bet it's doing dual half wave rectification to produce +/-9V rails internally. 3A at 9V is only 27W, a small 220-110V step down transformer when needed would do that just fine, as long as they have somebody who knows when it's needed and when they can plug straight into the wall.

author: Zero999
author_rank: Super Contributor
timestamp: March 27, 2015, 10:01:53 am
content: Quote from: ajb on March 27, 2015, 05:59:35 amThat's what I said, except 9VAC is 12.7V peak so it's probably nearer 12V.3A also sounds like a lot for a guitar effects pedal.

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 27, 2015, 02:37:06 pm
content: I think the intent is quite a few pedals would be driven off the one power supply.

author: Seekonk
author_rank: Super Contributor
timestamp: March 27, 2015, 03:17:37 pm
content: Quote from: Robomeds on March 27, 2015, 02:37:06 pmThat sounds like a good source of induced ground loop noise. How come there is no problem with the rest of the amps power wise? I still think there is better than a 95% chance these will run fine on a DC electronic wall wart.

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 27, 2015, 07:00:33 pm
content: No idea about power for other things. Since I don't know much about the bigger picture I'm just going for the question I was asked.