Suggestions for converting 12V DC to 9 V AC 60 Hz

[EofN]

Greetings,

As stated in the title, I'm looking for ideas on how to convert a 12V DC source into a 9V AC / 60Hz (approx) / 2 amp max, output.

I'm wondering if this can be done with discreet components, or if there is an off the shelf IC that could be my starting point.

Many thanks for your time in reading and / or responding to my first post here!

[PGPG]

Give some more info:
 Do AC have to be sinus or can be square?
 How precise 60Hz have to be?
 Why you need AC (will not it be then converted back to DC)?

[EofN]

The waveform does not have to be a perfect sinewave, but I would want to low pass filter any square components to make it round - ish. It is for powering vintage audio gear that requires AC.

[EofN]

The waveform could be between 55 - 65 Hz, not precise.

[EofN]

My initial design idea would be a simple oscillator, driving a simple transistor amp, with no regard to distortion, and possibly a low pass filter on the output. I don't want to end up with a massive heatsink, or use any expensive components or anything crazy like that.

[EofN]

I've searched all over the internet, but have only been able to find standard transformers that go from 120V AC to 9V 2A. They cost about $20 if I buy them in bulk. I noticed that there are a glut of high amperage 12V DC power supplies, and thought it could be possible to use them with an add-on adapter.

[Zero999]

The waveform does not have to be a perfect sinewave, but I would want to low pass filter any square components to make it round - ish. It is for powering vintage audio gear that requires AC.

I very much doubt the wave shape matters.

Have you looked at the schematic? It probably just converts it to DC anyway. If it needs AC then it'll be to drive a voltage doubler.

A 12VAC square wave has similar peak voltage as a 9V sine wave, just use an H-bridge and an oscillator.

Here's a self-oscillating MOSFET H-bridge, complete with inrush limiting and inductive clamping, to protect the MOSFETs.

^{Astable inverter MOSFET bridge.asc}

I've searched all over the internet, but have only been able to find standard transformers that go from 120V AC to 9V 2A. They cost about $20 if I buy them in bulk. I noticed that there are a glut of high amperage 12V DC power supplies, and thought it could be possible to use them with an add-on adapter.

That's not hugely expensive.
The voltage probably isn't critical. It's fine to go with 10V and more amps is no problem. I found this in Digikey.
https://www.digikey.com/en/products/detail/hammond-manufacturing/187D10/2360624

[David Hess]

9 volts AC is about 25 volts peak-to-peak, so the 12 volt source needs to be boosted as part of the process.  This could take the form of a 60 Hz transformer on the output, but boosting the voltage earlier is probably better.

1. Boost 12 volts DC to +/-14 (or whatever) volts DC.
2. Convert the higher bipolar DC voltage to AC with an integrated class-D switching amplifier intended for audio.

A bipolar supply is required to avoid using an AC coupling capacitor on the output, but maybe this is acceptable?

[CaptDon]

9 volts A.C. is kind of standard for many synthesizer keyboards. You should be able to find many power packs on Ebay for under $20. Going from 120VAC to 12VDC and then to 9VAC is too much crap and you won't do it cheaper than just buying a 120VAC/9VAC power brick. Your inverter circuit would need a bunch of filtering and so forth to keep from leaking switching noise into the audio chain. As for an off-the-shelf bare transformer, 9VAC is kind of an oddball. 6.3VAC and 12.6VAC are far more common. The primary is always wound next to the core so you may be able to find a 12.6VAC transformer and remove some winding turns off of the secondary to get what you want. A 9VAC 2Amp rated transformer will actually probably read around 10 to 10.5VAC open circuit.

[bdunham7]

The waveform does not have to be a perfect sinewave, but I would want to low pass filter any square components to make it round - ish. It is for powering vintage audio gear that requires AC.

Just because the device is configured to use 9VAC doesn't mean it actually needs it.  If it's PSU uses a voltage doubler configuration for a bipolar supply internally, then AC you must have.  However, if is just using a full-wave bridge to create a single DC supply, then it may well work just fine on 12V of either polarity.  What is the vintage audio gear? Does it happen to be a Thorens turntable?

[EofN]

Thanks for the suggestions, however,

- Anything other than 9V AC is completely out of the question.

- power supplies that supply 9V AC at 2A are extremely rare and cannot be obtained cheaply. Please post a link if you know of one, I would be very happy to find one.

[CaptDon]

Forget the voltage doubler stuff. A lot of this audio gear hooks one side of the 9VAC to chassis and uses the other side with a pair of diodes to obtain + and - 11 to 12 volts filtered D.C. which then 'may' be regulated back to anything from +/- 5vdc up to +/-9vdc used to power the audio opamp chain in bipolar fashion.

[themadhippy]

but have only been able to find standard transformers that go from 120V AC to 9V 2A. They cost about $20 if I buy them in bulk.

Is a 240 to 18v tranny any cheaper?

[EofN]

Thank you so much for your amazing information, I appreciate it very much.

This looks like an interesting path to explore, I like the idea of using mosfets, perhaps with some tweaks here and there, like you said the waveform is not critical. Perhaps it can be tweaked for higher efficiency so that I can use smaller heat sinks.

The device in question does indeed convert to 5V, 12V and -12V rails, using voltage doublers, which is why it has to be AC (the device cannot be modified internally)

The waveform does not have to be a perfect sinewave, but I would want to low pass filter any square components to make it round - ish. It is for powering vintage audio gear that requires AC.

I very much doubt the wave shape matters.

Have you looked at the schematic? It probably just converts it to DC anyway. If it needs AC then it'll be to drive a voltage doubler.

A 12VAC square wave has similar peak voltage as a 9V sine wave, just use an H-bridge and an oscillator.

Here's a self-oscillating MOSFET H-bridge, complete with inrush limiting and inductive clamping, to protect the MOSFETs.

^{Astable inverter MOSFET bridge.asc}

I've searched all over the internet, but have only been able to find standard transformers that go from 120V AC to 9V 2A. They cost about $20 if I buy them in bulk. I noticed that there are a glut of high amperage 12V DC power supplies, and thought it could be possible to use them with an add-on adapter.

That's not hugely expensive.
The voltage probably isn't critical. It's fine to go with 10V and more amps is no problem. I found this in Digikey.
https://www.digikey.com/en/products/detail/hammond-manufacturing/187D10/2360624

[bdunham7]

- power supplies that supply 9V AC at 2A are extremely rare and cannot be obtained cheaply. Please post a link if you know of one, I would be very happy to find one.

I'd suggest looking for older used stuff as they probably will be rare these days.  Think Goodwill, not Walmart or eBay, not Amazon.  I dion't have time to check my private stash at the moment but I might even have one somewhere.  1 to 1.5A will be more common, but this one is 25VA which means about 2.8A.

https://www.ebay.com/itm/335178150196

[CaptDon]

I forget if the keyboard is a Korg or a Yamaha but I bought exactly a 9VAC 2amp power brick for our keyboard player in our band. I got it from Ebay and it may have been $18 with tax and shipping. He is still using it and it is exactly what the keyboard called for. If I wanted to go cheap I would maybe still look for a 12.6VAC 2amp raw transformer. They are very common known as '25VA' size (12.6 X 2 = 25.2VA) and I would unwind a few turns off the secondary. You're just wasting time home brewing an inverter circuit. Are you a serious musician playing out for money? If so, don't bring anything unreliable to a gig. Equipment failures leave a lasting impression on the venue and your fans!! Is $20 or $30 really a problem to buy something off the shelf and rated for the job?

[CaptDon]

Ebay GE 22-155 alarm transformer 9VAC 25VA $6.35 plus $9 shipping. I bet he would ship several for the same $9 total. Do you need the coaxial power plugs to plug into your audio gear? You will still be doing some soldering etc to finish the project.

[EofN]

Yes, I do a lot of soldering! I assemble a lot of custom power connectors. Actually, I restored old audio gear of specific types, and have trouble finding these power supplies. I like the idea of the cheap alarm system supplies, except I don't like how they look, with the exposed terminals. Thanks again for the input!

[mikeselectricstuff]

Do you actually need 2A ?
Digikey do a 1.3A one :
https://www.digikey.co.uk/en/products/detail/ideal-power-ltd/77DA-12-09/15667498

[EofN]

Yes, it must be 2A, I tried smaller ones like 1.5 and it gets super hot. And not in a good way.

Do you actually need 2A ?
Digikey do a 1.3A one :
https://www.digikey.co.uk/en/products/detail/ideal-power-ltd/77DA-12-09/15667498

[mikeselectricstuff]

UK source/voltage , but they do exist
https://poweradaptorsuk.co.uk/products/2000ma-2a-9v-18va-ac-ac-ac-output-mains-power-adaptor

[BrianHG]

Thanks for the suggestions, however,

- Anything other than 9V AC is completely out of the question.

- power supplies that supply 9V AC at 2A are extremely rare and cannot be obtained cheaply. Please post a link if you know of one, I would be very happy to find one.

Your stuck if your output needs to be 9vac isolated.

For 120vac to 9vac, a number of solutions are possible.

For 12dc to 9vac, are you allowed to modulate the GND, IE: full differential output.  There may be a disastrous looping ground conflict in your wiring is the 12v source isn't floating.  Otherwise, you can only make around 4vac from a 12vdc power source.

To make real 9vac from a DC source, you will need a power supply with +13v and -13v output, with a 2 amp capable class D amp, plus a reference 60hz sine wave.

This is not for the beginner.

[coromonadalix]

https://www.ebay.com/itm/372230910429?    ac to ac 9v 2 amp adapter
https://www.ebay.com/itm/122413179451?


just to be sure, ask the seller if they are really 9VAC  not 9VDC

[BrianHG]

Ohhh, now we are doing power supplies, ok, here are 3 amp 9vac supplies.
And even better, for audio, they are split bobbin supplies:
Atari 800 power supply

[BrianHG]

Even cheaper:
Cheaper Atari 800 power supply

Just make sure you are not buying the Atari XL series power supplies which have a DIN connector for their output.  They are DC regulated supplies.