AC-DC Inverters

[zking786]

How can I build an AC-DC inverter. It should convert 12V DC to 240V AC. I'm trying to build a backup power supply which can handle 2-5A of current drawn. Anyone know of a simple circuit which can handle 2-5A?

[5614]

You said an AC-DC inverter... but then said it should convert 12V DC to 240V AC which is an AC-DC converter.

 

Which do you want?

[zking786]

Sorry!

 

For clarification: DC-AC inverter.

 

 

But don't inverters typically work both ways?

[YT2095]

the Transformer part of the inverter circuit is somewhat "reversible" yes, but only at the AC stage.

[5614]

The hard part is converting the DC to AC, after that you can just use a normal transformer. You can use a 555 ICC to get the "AC" effect. I say "AC" because it isn't technically AC although it is pulsed and will work through a transformer. See here:

http://www.uoguelph.ca/~antoon/circ/555dcac.html

 

I'm not so sure about the 2A - 5A current though. What is your 12V power supply from?

 

You can always buy these things, e.g:

http://www.tlc-direct.co.uk/Main_Index/Transformer_Index/Inverters/index.html

[YT2095]

you`de need a Good car battery for 2A @ 240vac (a good 40 amp draw in an Ideal circuit at DC levels).

 

the pulsed DC sorts itself out in the transformer anyway due to back EMF outputing an integrator wave (easily converted to a Sine wave with a twin T RC filter

[zking786]

I'd be using a 12V 24AH battery (maybe 26AH), definitely lead-acid. I guess I'd need to convert the DC to AC using a rectification system -- this would be the hard part. Then I'd use a transformer to convert the AC to 240V. The transformer's easy, but how should I rectify the DC voltage efficiently? YT2095, can you better explain why there would be back EMF? Also. what's a twin T RC filter?

 

Lastly, I'm trying to build the rectifying circuit from old electronic components. What do I need and where can I find them? I have quite a few transformers, so the AC part shouldn't be a problem.

[YT2095]

twin T filter: http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/tt.htm

it`s the Band Pass filter you want.

also a rectifier isn`t used in a DC to AC convertor, only in AC to DC convertors.

what you need is a DC Chopper with a High current output, then to the step-up transformer.

[zking786]

Where would I find a DC Chopper?

 

Does the chopper also alter the frequency, or does it just chop the voltage curve at certain points (based on the frequency).

 

How does a DC Chopper work?

[YT2095]

in effect, a Chopper takes the DC current and presents it to a load at regular intervals (I`m assuming you`ll want 50Hz with your 240vac?).

so effectively you`re presenting your 12vdc in "slices", 50 "slices" per second to the Input side of the transformer.

that then Induces a magnetic Field in the Core, when that field collapses (the dc pulse "Slice" is turned off again") that magnetic pulse gets transfered to the Output winding of the coil.

then it all starts off again

edit: I`ll point out also that Choppers are used in Switch Mode power supplies too, but they`re AC to DC (not what you`re after).

[5614]

And a 555 ICC is quite capable of giving out 50Hz. Maybe feeding the output into a relay would be a good idea for this size voltage and current (as opposed to transistors or nothing).

 

You could use say a 9V battery for the 555 circuit and then just connect your car battery to the relay and coil of the transformer.

[Externet]

Hi.

Go to a computer repair shop, and politely ask for a defunct UPS -uninterruptible power supply- that is destined to the garbage can, or get a rain check for whenever they have one.

That's it.

Usually discarded when its internal batteries die, you can connect yours instead and done. 12V in, 240V out. I have AC in my car with one of those.

Miguel

[YT2095]

And a 555 ICC is quite capable of giving out 50Hz. Maybe feeding the output into a relay would be a good idea for this size voltage and current (as opposed to transistors or nothing).

 

You could use say a 9V battery for the 555 circuit and then just connect your car battery to the relay and coil of the transformer.

 

a trip 5`s ok if your not fussy about freq, a relay is WAY out the question! basicly you`ll need a semi con driver (fets are good), as for the 9 volt batt, use the 12v anyway but a 7809 regulator for the trip 5, although I`de tend to go for a PLL to set my freq instead.

[zking786]

Oh... so the chopper is at the core, where the coils are wrapped.

 

Also, I'm trying to create a UPS (connecting the car battery to the inverter to produce back up power). I have a couple of computer power supplies, though. But don't these take 240VAC and generate 12VDC? I need to do the reverse. Or is the power supply I'm discussing not a 'UPS'?

[YT2095]

no, the Core is the laminated soft iron plates in the transformer, the "Chopper" justs pulses the DC into the transformer, as a transformer cannot work on DC levels, and requires Change at the input to affect the magnetic field.

else it just gets one pulse when you apply the batt and that`s the end of it.

[5614]

Why won't a relay work? I have one capable of working at 50Hz.

[YT2095]

you`de burn the contacts out in no time flat under load (seriously, I`ve tried this before).

basicly as a mechanical device, moving the weight of the contact arm and lever assembly 50 times a second doesn`t allow for much seperation, and so arcing occurs at the contacts, they just burn out

I`ve even tried 400A rated contacts, same thing.

not to mention the Noise it makes!

[5614]

That sounds about right, it's not something I've ever tried before though. And yeah I can imagine the noise, that would really get to you!