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DC to AC power inverter - wireless speakers

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lielec11

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I am trying to turn my rear surround speakers from plug in AC to a battery powered DC. I would like to create a smal circuit board that I can mounted to the back of the speakers (hopefully). The input is rated 120V, 60Hz, 11W so it's not a lot of power.

1) I have searched the web for some basic DC-AC inverter designs but am a bit confused. I keep finding videos etc. for 12VDC to 220V AC but I can't seem to find anything rated 110-120V output. Is there a reason for this?

example design: https://www.electroschematics.com/9155/12v-to-220v-voltage-inverter/

2) The parts seem relatively easy to come by but I would like to utilize a 9V or some AA or AAA batteries if possible. Can someone point me in the direction of a more detailed design guide?

3) How do you size the RC components to get 60hz? It seems every design I've found they use different equations. From my college days I seem to remember the f = 1/RC but when I test this on designs it never works out. Any insight?

Thanks in advance.
 

dknguyen

Well-Known Member
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Yes, you can RC filter a square-wave to produce a sine-wave, but that is really meant for signals, not power. The fundamental of a square wave contains only half the energy of the entire square wave so you lose half the input power by taking this route and batteries are crappy energy storage devices as it is. On the other hand 11W doesn't really warrant the complexity of a DC-AC inverter which is a very complicated project, but half of 11W is more than enough to overheat components.

Your speakers probably run internally on DC anyways. So rather than this roundabout DC-AC-DC conversion, can you just open up your speakers and disconnect the internal AC-DC supply and directly hook up your battery? That would be far more ideal. Simpler and double the battery life.

2) The parts seem relatively easy to come by but I would like to utilize a 9V or some AA or AAA batteries if possible.
You do realize that a 9V batteries will supply 11W for significantly less than 10 minutes right? AAs aren't much better. Four AA batteries in series to supply 9V will only supply 11W for less than 30 min.

3) How do you size the RC components to get 60hz? It seems every design I've found they use different equations. From my college days I seem to remember the f = 1/RC but when I test this on designs it never works out. Any insight?
I believe that you need a multi-order RC filter. Not just one RC.

1) I have searched the web for some basic DC-AC inverter designs but am a bit confused. I keep finding videos etc. for 12VDC to 220V AC but I can't seem to find anything rated 110-120V output. Is there a reason for this?
Don't know about the reason but you can just change the transformer ratio to get a different output voltage.
 
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lielec11

New Member
What do you mean by multi order? All the designs I've seen and demonstrated seem to use just one RC set for the frequency. There are other resistors and caps in the circuit for other purposes I'm sure.
 

dknguyen

Well-Known Member
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What do you mean by multi order? All the designs I've seen and demonstrated seem to use just one RC set for the frequency. There are other resistors and caps in the circuit for other purposes I'm sure.
Multi-order filters have a sharper cutoff frequency than lower-order or single-order filters since real filters can't have the ideal sharp drop off due to math/physics. It's more-or-less, stacking the same basic filter topology on itself repeatedly.

In this case, it would probably putting the same RC filter in series with itself repeatedly. Google it.

But that's moot because, in case you did not pick up on it, I'm advising you against your current approach. Such setups aren't meant to deliver power. The first of several problems it that the output voltage will vary wildly with current draw due to the resistances in the RC filter. A second, already mentioned, is ineffciency. A third is heat due to aformentioned inefficiency.
 
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Nigel Goodwin

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This formula is also wrong. You are missing a 2*pi.
You're correct his frequency was wrong - but you've mis-understood his post - the frequency part wasn't about filtering, it was about making an inverter that runs at 60Hz.

But basically his entire premise is probably no good, as running it off batteries will only last a short time, or will require huge batteries. Regardless of battery size it will be many times more expensive to run than from the mains. It's be cheaper to run mains to where he needs both short term and long term.
 

dknguyen

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You're correct his frequency was wrong - but you've mis-understood his post - the frequency part wasn't about filtering, it was about making an inverter that runs at 60Hz.

But basically his entire premise is probably no good, as running it off batteries will only last a short time, or will require huge batteries. Regardless of battery size it will be many times more expensive to run than from the mains. It's be cheaper to run mains to where he needs both short term and long term.
Oh I see. He was asking about the timing RC components for the IC to get a 60Hz square wave, not the cutoff frequency of the an RC filter to turn the square wave into a sine-wave. I guess I just kind of assumed it would be there because it's generally a bad idea to run equipment expecting a mains supply off a square wave unless you know it can take it.
 

lielec11

New Member
Thank you all for your responses. I guess I was focusing on the wrong thing first (the circuit), without thinking of the battery limitation. I guess this will never come to fruition since I'm not trying to use huge batteries, I just wanted to have more freedom with my speaker without having to add receptacles in my walls.
 

AnalogKid

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Back to the original problem - Another approach is to determine what the speakers' internal DC power rails are. It probably is a whole lot easier to convert battery power to them. This means opening up the speakers and possibly making a small modification to the internal circuit board (to bring in the external DC). If that is not a viable solution, then you're back to an inverter.

Many inverters use a Royer oscillator design. That might improve your search results.

https://images.search.yahoo.com/search/images;_ylt=AwrNHcjVfXxbmUQAL01XNyoA;_ylu=X3oDMTB0N2Noc21lBGNvbG8DYmYxBHBvcwMxBHZ0aWQDBHNlYwNwaXZz?p=12+V+120+Vac+inverter+schematic&fr2=piv-web#id=2&iurl=https://www.daenotes.com/sites/default/files/article-images/12v-DC-to-120v-AC-Inverter-circuit-diagram.png&action=close



Note - for the second schematic, the only change to produce 120 Vac is a transformer with a different primary rating.

ak
 

ronsimpson

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turn my rear surround speakers from plug in AC to a battery powered DC
Please tell us what speakers you have.

It sounds like you want to start out with a 12V battery, convert to 120VAC 60hz, then inside the speaker convert back to 12VDC.
If we knew what the DC supply is inside the speaker it might be easy to power the speakers directly form a battery.
 

audioguru

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A link to the Sony wireless but mains powered speakers was posted on another website. Their input (from the mains) power is 11W and the output to the speaker power is 50W. Huh??
 

ronsimpson

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11 watts rms = 15.5 watts pk = 31 watts peak to peak. Who would measure audio power in peak to peak watts? Anything to sale another amplifier.
 

unclejed613

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A link to the Sony wireless but mains powered speakers was posted on another website. Their input (from the mains) power is 11W and the output to the speaker power is 50W. Huh??
it's the patented Sony OverUnityPower Amplifier design (SOUPA). they have it in all their products. Sony sells some boom boxes that draw less than 1A from the wall outlet, and have an output of 800W. we know this to be true because the expert sales agent says so. what they don't warn you about is the 500W subwoofer with a 3/4" diameter voice coil that provides such earth-shattering bass, that you need to make sure your earthquake insurance is up to date before you can use this amazing Sony boom box.

EDIT: yes, Sony's spec sheets for their boom boxes are, shall we say, over-optimistic? kind of like the 1000 meter sights on an AK-47.
 
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Nigel Goodwin

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EDIT: yes, Sony's spec sheets for their boom boxes are, shall we say, over-optimistic? kind of like the 1000 meter sights on an AK-47.
Bit unfair to single out Sony, when pretty well ALL manufacturers use the same imaginary figures.

It's also originally an American idea, which started back in the 60's or 70's, all amplifiers gave proper RMS watts specifications, then there was a sudden influx of American made equipment with imaginary power ratings - perhaps America had always used imaginary ratings, I don't recall American equipment before then. Since then it's got worse and worse, with only decent quality equipment giving realistic values - in-car electronics is usually even worse.
 
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