How to get 9V AC 7VA from 18V 5Ah Li-ion Powertool battery

Thread Starter

GrahamRobinson

Joined Jul 8, 2018
6
I would like to power a MIDI Synthesizer module (the Alesis NanoSynth) from a battery to allow for fully portable use.
The snag is that the synth requires 9V AC. This is currently provided by the original ‘wall wart’ power supply [marked input: 240V ~ 50Hz output: 9V ~ 0.78A 7VA]. Under no load, my multimeter reads its output as 10.1V AC.
It was suggested, and it seemed plausible, that I should just give it 12 or 13 V DC and it would be fine. However feeding from a freshly charged leisure battery, as well as a couple of different DC-DC converters, just made it howl.
I have got it to work by putting the battery DC through a regulator to provide 12.9V DC that feeds a 75W car cigarette lighter inverter into which the synth’s mains power supply is plugged. Not only is this ugly but is seems quite wasteful of battery. With the regulator, inverter and power supply under no load the battery at approx 20v is delivering 0.25 amps. Plugging in the synth increases the drain to about 0.51 amps. It would also be safer not to have 240V AC involved at all.
It seems that a pure sine wave is not required as it does seem to be quite happy running of the cheap non-sinusoidal inverter. Though the power supply might be smoothing things out a bit??
One thing I do not know is whether the 50Hz is required by the synth as some sort of ‘clock’ signal.
I was wondering if I could adapt a simple 12VDC to 220V AC inverter circuit. Such as this one: https://www.electronicshub.org/12v-dc-220v-ac-converter-circuit/ by replacing the step-up transformer with an isolating 1:1 transformer and adjusting the input voltage. I am not knowledgeable enough to be able to find centre-tapped 1:1 transformers. I only need 7VA and that circuit is claimed to be able to deliver 35W. Is that just comfortable headroom or are the MOSFETS overkill?
Another one of the many things I do not know is whether these simple circuits provide a stable frequency and voltage under varying load.

Any guidance on finding a suitable circuit would be much appreciated. I would be glad to answer any clarifying questions.
Many Thanks Graham
 

dendad

Joined Feb 20, 2016
4,452
If it is similar this model (Alesis Micron), it does look like you need an AC input.

MicronPS.png
I have not found a copy of the NanoSynth circuit.

There are many circuits like this around..
inverter_100w_12v-220v_by_ic-4047_irf540.gif
so you could base a design on it, just wind a transformer for 18V to 9V instead.

But having a delve into the sysnth's power supply circuitry may show if you do need AC or not. An external supply generating the +/- DC volts could be made instead.
 

Dodgydave

Joined Jun 22, 2012
11,285
Chances are it will have a bridge rectifier and voltage regulators inside , so you could feed 12V DC into it and it will be ok. Ideally i would open it up to see what it has on the input side or get the service manual download.
 

Thread Starter

GrahamRobinson

Joined Jul 8, 2018
6
Many thanks Dendad for your reply. I think I lack the skills to wind my own transformer. But if I can find a 1:1 transformer with a centre tapping I assume that I will be able to adjust the input voltage to get the required AC output.
 

Thread Starter

GrahamRobinson

Joined Jul 8, 2018
6
Dodgydave. Many thanks for your reply. I have tried a variety of batteries and DC-DC converters. Unfortunately they all cause the synth to emit a medium to high pitched howl. Cheers Graham
 

Janis59

Joined Aug 21, 2017
1,834
My choice would be in that situation the MC34063 and downcoverter according this tablet`s datasheet reccomended one.
 

dendad

Joined Feb 20, 2016
4,452
Here is another idea.
synthPS.jpg
Use a switch mode supply to go from your 18V to around 5V or 6V, then feed it into the converter above.
This time, just use a center-tapped transformer from a wall wart, feeding the secondary winding, and take the output from across it. DO NOT connect the original primary. Insulate it as there will be high volts on it.
Across the low voltage new primary, there will appear a square wave of about twice the DC input volts, hence the input supply is around half what you want.
I've not build this so the values may need to be adjusted but it should work ok.

Also, do not have the 5V or battery supply connected to the synth ground. The only connections are the 2 ends of the transformer winding.

It would probably be a good idea to add a bidirectional 12V tranzorb across the 9V output, along with a bit of filtering too. But try it and see.
 
Last edited:

Dodgydave

Joined Jun 22, 2012
11,285
Dodgydave. Many thanks for your reply. I have tried a variety of batteries and DC-DC converters. Unfortunately they all cause the synth to emit a medium to high pitched howl. Cheers Graham
Can you open up the synth and look to see how the AC is used, like draw the input side out, or take pictures of it?
 
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