12VDC battery to 12VAC inverter?

[Troy Jollimore]

It's been so long since I've dabbled in electronics...

A friend of mine is looking to power some fly zappers that are scattered throughout his LARGE property. They take 12VAC with a 2A draw. Right now he's powering each one with a car battery (recharged by a good-sized solar panel), inverting to 110VAC, then stepping down to 12VAC. This seems to be a bit inefficient, and he wants something that would do it more directly. Of all the searching I've done, I just can't seem to find a 12VDC-12VAC inverter, or even a transformer that would do (equal windings) in the schematics of normal 110VAC inverters that are readily available.

I've no idea about wave forms or isolation, but I doubt that it's an issue for this application.

Is what he doing now (inverting and stepping down) as inefficient as it seems? Would it be more efficient and cost-effective to build the new inverters? Also, my father (an ex-electronics guy) thinks that the 2A draw would be a bit much for the batteries over the run of a day, even with the solar panels. Besides running around swapping out batteries, it may be more efficient to just run AC power lines out to his property and power them directly... Think so?

And finally, how would you go about building one of these beasts, and about how much would it cost?

 

[stevez]

Regarding efficiency. Transformers are usually relatively efficient in terms of energy use but there are losses. It does seem like a waste of material to have to generate 110 vac then step down to 12 vac however low cost 12 vdc/110 vac inverters are extremely common. I'd argue that even low cost, store bought invertors are likely to be more reliable that something you construct yourself - unless you are really experienced. Making a special 12/12 invertor may cost more in the end.

It does seem that a well designed 12vdc/12vac "convertor" would be more efficient but how much more requires some data. It might be worth a look inside the powered devices to see what they really need. Wouldn't it be nice to find a rectifier/filter that powers some kind of internal switching supply with 9 vdc. I've been there and done that. All you'd need to do then is come up with a 9 vdc regulator.

Remember that if solar is the way you are recharging the batteries that you'll have to generate and store at least as many watts as you'll be consuming. If the load is on 24/7 then the solar panel will need to deliver much more than 24 watts to account for night time, position of the sun, clouds, etc.

Clearly you have a rough but good plan for evaluating the alternatives. You just have to start getting data (how big is your solar panel, how much does it cost, etc, etc, cost of AC lines, etc etc) and completing the analysis. Without the data it's hard to offer an opinion.

I do see solar panels powering remote loads (1,000 ft from power) and it seems that in those cases someone determined that solar was the best choice for that application. What that tells me is that it's an economical alternative. The fact that I don't see more of it tells me that it's a good choice but only sometimes.

Good luck, as for more assistance if you need it.

 

[kurtjo]

I would have to agree with stevez, you have very little to gain by converting directly to 12VAC. It's the 24VA per unit that posses the real problem.
Also, keep in mind that if you run these things all night, it will cause "deep cycling" of the batteries. Automotive lead acid cells don't hold up well in that situation even if you can recharge them during the day, they would have to be replaced fairly often.
You might consider replacing the 12VAC units with 120VAV ones and running regular AC line power, as trying to supply 12VAC would require some very heavy cable to prevent voltage drops at 2A per unit.

Good luck, I hope it works out.

 

[chemelec]

Inverter Project

There is a Good Mos-Fet Inverter Project on my website. By using an appropriate transformer you can Easily get thet 12 volts AC Output.
Go to: **broken link removed**

Gary

 

[Oznog]

You would indeed save a lot. Inverters take a lot of idle power, 300 mA (4 watts) is pretty typical but many go much higher. Added to inverter inefficiency (10%-15% or so), plus transformer losses, it's substantial.

A 12VDC to 12VAC inverter isn't too difficult, particularly if you don't need output isolation. That is, neither power wire connects to a metal part of the case which would touch something metal connected to the battery ground, like the body of the solar panel.

However, I wonder if your device actually requires AC. It's very common for small transformers to put out AC but it gets rectified inside the device anyways. If it's going into a rectifier, it doesn't matter if it's AC or DC.