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12VDC to 9VAC - How to do it? Anybody? Answered

Well, I've posted this in numerous places and no one seems to have a clue. Myself being electronically illiterate, still, with all the technology out there, you can't tell me this can't be done.
I live off-grid. We all know how inefficient the inverter is. It's a huge drain on my battery bank, won't function when they get below a certain voltage, yet all my peripherals that operate directly from the battery bank, operate fine.
That being said, I have been trying to come as close as possible to eliminating the inverters and I have two peripherals left: 1) cordless telephone and 2) modem
The problem is that they run from that pesky wall charger/converter from 120VAC to 6.5 and 9VAC respectively. Now, notice these units run on LOW VOLTAGE AC, not DC. That is, they are powered by the AC. My guess is that inside each unit, there sits a rectifier converting it right back into DC. In fact, for the phone, I see no other way because it charges a NiCad battery. 
So.... does anyone have an inkling as to an efficient way to convert 12VDCD to low voltage AC. Preferably variable, but not absolutely necessary.
There is nothing on the market that I have found. So if you're looking for something to have to break into the electronics market with, here ya go. I'll bet there are hundreds of thousands 'off-gridders' who would want this technology.


Many of them use cheap converters from 120 volts to 9 or 6 volts and there may be no need to convert from 12 volts DC to 9 volts AC if the phone or modem only converts it to DC.

Inside of the Phone or the modem as long as it doesn’t go through a transformer it is a simple adjustable DC voltage regulator something like a LM317 you need.

A circuit something like this set to what you need.

In fact the 9 volt may run fine on 12 volts DC and the 6 volt will need a voltage regulator, look inside the phone cradle there should be only components diodes capacitors and resistors no transformer if this is right.

If you don’t know what to look for and can post a picks of the Inside of the phone cradle or the modem, and if you can get picks both sides of the circuit boards I may be able to tell you.

Adjustable LM317 1.gif

The phone requires 6.5VAC. I have taken it apart and I see nothing on the circuit board that looks like any type of transformer I'm familiar with, though if there are more than the two I generally see, than I could be mistaken.
Tried to upload a pic, but after 15 mins it still didn't load, so gave up.
The general consensus seems to be that it likely does not have one, and that I can probably put DC in at approx. the same voltage as the AC. Ergo, I will once again try it.
I have a choice from my battery bank to bring in 6,12,18 or 24VDC. I also have a variable DC step down transformer with which I can run 3,6,9 and 12VDC. So that being said, I would guess the 9VDC would be the optimum voltage for the phone (6.5VDC) and then the 12VDC directly from the batteries to the modem (9VDC)
After doing so, I'll report back on the progress or lack there-of.
Thanks everyone for the input, and I wish I would have come here first, but you live and sometimes learn.

Well, partial success with the phone. I ran the wiring directly off the batt. bank, via an automobile variable step-down transformer. I set it for 9V. The phone worked great, that is, until I started the generator to charge the batteries. Then it went haywire, started ringing, when I answered was nothing but static yada-yada. I'm thinking that the increase in 'Amps' is the culprit. I even turned down the voltage to 7.5 then 6V, to no avail. I have various ceramic resistors, up to about 6 ohms. Would that do the trick? To tie them/it in to the power feed in series? Or better to use both sides in parallel?
I guess I need to give thought to the changes that take place as soon as I start the charging procedure. It also kicks out the inverter that I use for the modem.

Sounds like you may need a filter, is your generator a little gas engine running a 12 volt alternator?

Well, it's actually a large gasoline eng. (Onan 2 cyl) running a large generator. (4000 W)
What is a filter and where do I install them?
I'm building a waterwheel which will then serve all the power needs here, but that is a slow process right now and am probably looking at late fall if I'm lucky, or sometime next spring. Trying to get away from the fossil fuels as much as I can. But it should power two fairly large generators about 5000W each.
The one I'm using now is not a modified sine wave as are most of them and will hate giving that up.

OK I know the beast, I have a Coalman 3750 that is 3500 watts.

We call anything under 4 cyl a small engine.

The cause of the static and stuff while on the generator is most likely being caused by the sparkplug ignition feeding through your system. Start by checking your engine ground the engine its self needs a good ground.

Then you can get the filters from an automotive stereo store put them on the input of your low voltage sensitive electronic devices. Ask for an engine noise filter they should know what it is.

This use to happen a lot in old cars and these old Onan 2 cyl generators are just like the old cars.

Yeah, technically I guess you're right. I was comparing to the coleman 5000 watt I have which has a briggs one cyl. which one man can move. This onan is a monster compared to what I'm used to, and it was all two of us could do to drag it on a sled to my shed.
Yeah, I remember the problems in the old cars. And we had to buy suppression everything and nothing seemed to help until those filters came out, at least to my knowledge.
So guess that's the answer, at least I hope. However, though I didn't notice at first, I even get a hum on the line just coming off the batt. bank. Not anything I couldn't live with if I had to.
OK, the filter may solve the noise problem, but I think I mentioned that the phone starts acting whacky as soon as I start charging, in-so-much as it starts ringing when no one is calling, and the display lights begin blinking. Kinda the way the electronics did on the movie "Close Encounters" when everything went haywire.
The guess I had, being that little else is left, (seemingly) was that the biggest change during charging is an increase in amps. And since I'm running my 12V through a DC step down transformer to get 9V, that the voltage is relatively unaffected by the increase in the charging level voltage but not so with the amps. Would that be correct?
BTW, I wanted to thank you JM for the help and your time & patience. I know it may be frustrating trying to help someone who has just enough knowledge to create problems but not quite enough to solve them. lol Apparently there are a lot of knowledgeable people here, and I hope in some manner I will be able to contribute at some point.
The 'powers-that-be' try as hard as they can to thwart ways around modifying their equipment so we'll have to buy what they offer, even to the point of legislation. But I see this kind of exchange as being the saving grace for that.
Thanks again

Yea I know the beast easier to move with a front end loader.

Although I am not off the grid I prepare to be since we are blackout as many as twenty times a year for as much as a week at a time. Many of the farmers around here have off grid systems for blackouts see my instructables.

You are about the same age as me and judging from your avatar from the US, I am a Canadian preper and a member of safe America. We help ourselves so we can help others. I do not believe in dooms day but I believe in disasters like New Orleans.

Not to be technical I think it is phasing voltage out of phase from current although the voltage may be steady the current may be bouncing or visa versa.

Are you inverters off or in standby mode when not in use?

I keep my inverters off when not in use. And in fact, they draw voltage for something even when off at the switch on the unit. So I have them switched behind the unit on the 12V side so there is no ambient draw.
My goal is to only use the inverters as needed when 120VAC is required with no way around it, but shut them down there-after. Right now, the modem is the only thing preventing me from doing that, simply because I use the internet a great deal. I'm somewhat isolated and it's easier and cheaper to order much of what I need from there, do research, stay in touch with my kids etc. and so on.
I have now put the phone on it's own battery. That has solved the humming problem. Next I'll attach that battery to the main bank and see if I get the hum. If not, then I'll charge the bank with that battery still attached and see if I get it then. If I do, I'll just have to have two batteries that I can use, with one on charge, the other dedicated to just the telephone. They are small motorcycle batteries and work well so far for this.
I may take the same approach with the modem except I don't need to step down the DC. And hope it will be ok to apply the DC where it requires or at least has low voltage AC going in.

I can't predict the future, and have no idea what will happen for certain, and don't necessarily trust all I hear from a political standpoint, but what I do see, deeply concerns me, so I try to be prepared for all eventualities. My motto is: "Be prepared for the worst, but hope for the best."
And yes, I guess the avatar is a dead giveaway as to my nationality. :)

I can't speak directly to it - but I bet you'd get away with powering the low voltage ac devices with slightly higher dc voltages.

AC voltage is pulsating, and if you average that waveform down to a dc input, you get the equivalent dc voltage; such that 9VAC is equivalent to 9VDC but they have different overall amplitude.

If you power it directly from 9VDC the rectifier will cause a 2-diode-drop in voltage that you would need to compensate for. The only time you would need to worry is if the device internally uses the first step of AC input to transform to a higher voltage, which dc wouldn't do.

Long story short, some devices will work (particularly electronics) on DC input, many will not work or may be damaged by DC input. (anything with a passive internal transformer)

Thanks Frollard,
Amazingly enough, I have posed this question in excess of a dozen places and you are the first to have even replied. So if for nothing else, thank you for that! But notwithstanding the advice, as I mentioned, I am near illiterate electronically. So I don't really know what a passive internal transformer is or looks like. However, in a personal discussion about 6 mos. back, I was also told that possibly I could power these units directly with DC. And, I did so. And, it worked, at least temporarily. As luck would have it, the telephone burned out (I even smelled the burning wires), so I immediately unhooked the modem, for fear of the same. Yet, I have no clear evidence that running DC to it was the culprit for certain. But I replaced the phone, with the exact same model and just haven't had the nerve to try it again... yet.
As I said, in regards to the phone, it would have to convert the AC back into DC, or otherwise somehow get a DC pulse to charge the batteries, and a rectifier is the only means I am aware of. At this point I'm probably willing to sacrifice one more telephone for the sake of science, lol. The modem is a different story, as it has no batteries, and converting the DC seemed the safest way to go. Yet, if you can tell me how to recognize that internal transformer, I may even have the courage to try it as well. Of course I know what most transformers look like, is this any different?
Thank you again for the response, and if it interests you, I'll be happy to inform you as to the status of the phone after I use it for awhile. The last time it worked for about a month.
Also, I'm currently building a waterwheel to power my cabin 24/7. mechanically, I'm sound, but when I get to the wiring, hooking the generator(s) etc. likely I will need some guidance there. Would you be game?

What to look for inside on the circuit board:

Power will come in from the jack and then do one of two things: (excluding any fuses...there should be a fuse on quality electronics somewhere right at the start)
a) go into 2 or 4 diodes to rectify it into pulsed DC, then across a capacitor to smooth out the dc, then through a regulator
b) go into a transformer immediately (possibly after a fuse) - looks like wire wrapped around a stacked metal section - and the output of the transformer will go through the diodes, then cap, then regulator.

If it's A (most likely), then you can supply it DC directly and it won't be hurt in most cases. If it only has 2 diodes, you will need to get the polarity correct because it's only half-wave-rectifying. Each diode will cause a voltage drop, usually 0.7 volts each - so if you input 9vdc it will have to go through 2 diodes in the bridge to make the circuit, which means there is about 7.6 volts before it hits the regulator. If that regulator relies on more than that, it will 'drop out' or stop regulating and just fail to supply sufficient voltage.

Here comes the trick: like I said AC can convert to an equivalent DC but the AC waveform AVERAGED (using root-mean-squared fancyness) will result in smooth equivalent voltage.
i.e. 9VAC is actually +- 12.75 volts at peak, which can for short periods overcome the diode limitations because it has that higher peak.

What does that mean? You might need to apply slightly higher DC voltage to make it work.

I must be going mad! OK, working only with the modem, which definitely requires 9VAC. (the phone, IS actually DC input 6.5V, not sure how I originally saw AC) Upon opening, after the power jack and directly in front is a bank of what could be four diodes. The breadboard designates them as "D12, D13, D14" etc. To the left of them, is a donut type transformer, i.e. a round ceramic 'donut' wrapped in copper windings, and several caps in the vicinity. So it sounds like it is as you describe.
So bottom line is, I should be able to put 12VDC directly to it, and it 'should' operate.
I'll try and let you know.
I owe you guys my eternal thanks, and if I can help you in any way, do not hesitate.
My forte is: Auto Mechanics ('retired'), Survivalism, how to make women hate you, and...someday, off-grid living. Need info or resources on any of these, I'll do everything I can.
(I am also a writer, songwriter and self-styled musician of sorts) And I dabble in just about anything.
I hope to post my waterwheel here in the future, but I'm a one man show here and progress is painfully slow.

If you are correct (and you probably are), that both devices do some internal rectification, then it does not matter if you supply AC or DC to the device! A rectifier will just pass DC straight through without doing anything to it.

You could (if you don't care about manufacturer warranties) is open up the two devices and look at the circuit, to see if you can find a bridge rectifier near the power jack.

One way to test this would also be to simply provide the appropriate DC voltage to each device (4.6VDC and 6.4VDC, respectively), and see whether it operates normally.

If you really try converting DC to AC, and then (in the devices) back to DC, you're going to have efficiency losses, heating, blah blah blah.

There do exist inverters which produce low voltage AC, although the only example I have actually seen is one that outputs an AC waveform with an amplitude of 24 V(rms), and it was on this page:
where I saw evidence of this particular artifact.

Also if you feel like you are crying out for some particular power converter, that you think the world really, really needs, but nobody understands, and nobody cares, etc, perhaps you could tell your sob story to someone at powerstream.com.   The reason I am suggesting this is because I suspect they have engineers capable of understanding your problem.

Although I have never contemplated what it would actually cost to get them to build you a custom power converter.  I am guessing such a custom solution would be outside the price range of mere mortals.

I kind of like Frollard's idea of finding a way to make the make the modem and cordless telephone run on DC, instead of AC.

However to do this you must become like the Ancient Egyptian god Anubis, for you will need to peer into the heart of these devices, and see what is inside.  I am guessing you will find the usual bridge rectifier, plus filter capacitors, plus linear regulator IC.  (Very often, but not always, this IC will have the name 78nn, where is nn is a two digit number indicating the fixed output voltage, e.g. 7805 is a 5 volt regulator.)

Anyway, once you look into the heart of your cordless phone, and see how it converts power internally, then you will be prepared to help guide this cordless phone to its way to its "new life" running on low voltage DC power.

This task of looking into the heart of an electronic thing, if only at the power supply circuits, may be a challenging task for a person who is, as you say,  "electronically illiterate".

But that does not mean it cannot be done.

Alternatively, you could just try to live with the power losses associated with using the inverter you have.  BTW, you did not mention how much power was actually being lost that way.

Technically this isn't too hard:

You need to use the DC to drive an oscillator to produce AC - this can produced by a simple astable oscillator (millions of options available on line) or even a 555 timer driving a suitable power transistor to provide the needed current.

After that a suitable transformer to make the required voltage and you should be up and running.

You just need to make sure you have the necessary current available for the devices your trying to charge.