What is the primary device you plan to power?

How much total power do you want to have available?

Without more info, too many options to list. Have you tried here?

http://www.maplin.co.uk/searchtemplate.asp?criteria=240V%20TO%2012V%20TRANSFORMER

TRUE for ideal transformers and for most non-critical applications, like this one.

However, many real transformers do have a preferred primary/secondary arrangement where the windings are specifically designed to minimize loss, maximize flux coupling, and improve heat dissipation.

I don't know about world-wide suppliers, but in the U.S. you can get 120-VAC output inverters that plug into a car's cigarette lighter. They range up to 100 watts or so and cost around US$ 40. Unless this is a design project, you can simply buy one.

Tornado,

Op wants 12v to 240v step up.

Jon

Oops on my part; thanks for the correction!

Tornado,

Welcome.

Jon

Same in the UK. A 150VA inverter 12VDC → 240VAC, complete with overload protection and manufacturer's warranty, is less than £40GBP. It isn't worth building one if one values one's time.

Northern Tools' catalogue no.182, p112, a 150W inverter off-the-shelf for £21.84GBP (usual disclaimer). Sold to the CR4 subscriber in the funny hat?

Here you go follow this link: http://www.m-99.co.uk

or if you need more power follow this link: http://www.sailgb.com/p/Waeco_8100_012VS_UK_1000w_modified_12v_inverter/ (you will need to copy and paste into your browser bar)

If you google inverters and you will find they are available in the voltage you're looking for. I just picked the first link.

http://www.voltageconverters.com/220_volt_power_inverters.html

You're probably already aware that you cannot use a transformer directly from DC so unless it's a project is probably cheaper to buy one, I do not know the wattage your looking for however on the page I gave you the link to a 400 W continuous duty inverter is $50.00 U.S. And they ship to most countries.

Europium's right .. Transformers are bi-directional. A 240V to 12V step-down transformer is also a 12V to 240V step-UP transformer.

That being said ... A transformer will be of no use to you. The voltage in car is all DC, and transformers can be used only for AC. As a matter of fact, transformers that have DC applied to them act as short circuits (more precisely, their resistance is that of the winding wires to which DC voltage is applied).

Transformers work on electromagnetic inductance ... a variable voltage/current in a coil induces a magnetic flux inside, along the length, of the (primary) coil. The magnetic flux follows the transformer's magnetic core (like electrons follow an electric conductor) and passes through another coil (the secondary). Magnetic flux that passes inside a coil along its length generates an electric voltage/current in the secondary.

DC voltages across a coil don't induce magnetic flux in a coil (derived from one of Maxwell's Laws), so no variable magnetic flux makes it to the secondary coil, and no voltage/current gets induce there.

All that to say that you'll have to use another method to generate 240Vdc from 12Vdc. You'd need a voltage-regulator chip of some sort to accomplish that in teh easiest way possible. Or better yet, get one of those devices mentioned above at an automotive store.

Cheers!
DZ

DreadZontar,

Op Said: "What ime trying to build is an inverter for use in a car"

The key word there is "inverter".

transformers that have DC applied to them act as short circuits (more precisely, their resistance is that of the winding wires to which DC voltage is applied

I have an idea that Op knows all about this:

The primary of a transformer would be useless if it did not look like a short to dc or ac that has a dc component.

Part of the ac sinewave that produces the greatest strength of the magnetic field in the 50/60 Hz transformer and subsequent juice in the output winding is essentially dc. (that would be the part of the ac around the peaks.)

For 50 or 60 Hz the dc from the battery is switched through one half of the center-tapped input winding and then through the other half producing reversal of polarity. Thus the single output winding outputs both halves at the desired frequency.

Of course the desireable sine waveform costs more.

Jon

"That being said ... A transformer will be of no use to you."

Give me a break. They have been uses for a least 50 years for car radios (with vacuum tubes). Back then they used a vibrator to alternately switch the DC to the primary to get AC. Nowadays we use electronics. With a rectifier on the output, you have DC again. Don't tell an engineer it can't be done.

-S

Dread,

"DC voltages across a coil don't induce magnetic flux in a coil"

Does this mean I should throw away all of my relays solenoids and other electromagnetic devices because they don't work?

Jon

Steve,

Perhaps you should be asking for a center-tapped 24 Volt to 240 Volt Transformer.

In this poor example of an Inverter you can see how the ac is derived from alternately switching the 12 Volts dc between the two halves of the 24 Volt input windings to get the 240 Vac.

The 12Volt source, transistors and transformer windings determine the output power.

There is no feedback control in this circuit so just forget about it and focus on the transformer.

Jon

Another thing that no one mentioned. You really cant just use a 240V to 12V transformer backwards. If you did you only get about 170VACrms(240V peak). Remember the 240VAC mains is an RMS rating. You need to be able to step 12V up to the 340V to recreate the full 240VAC waveform.You would need a 240V to 8.5V transformer. More precisely 240V to 17VCT. Unless you are building a strictly square wave inverter, but why would you?

Just a thought.

IanR,

We have to ask op about the desired waveform of his output.

But:

So if I process the 12 Volts dc to produce the 240V square wave output using a certain transformer I couldn't apply 240Vrms to the HV winding to get something like 12Volts rms at the low voltage winding?

The inverter is not doing the RMS thing so I guess it doesn't really apply to the squarewave. I think the amplitude and duration of the dc pulses are the operatives here.

Back in 1956 I used a 120Vrms to center-tapped 12Vrms filament transformer in reverse to make an inverter that used 6Volts dc to produce 120 volts to power a household radio. Worked great. Has something changed?

Jon

"Back in 1956 I used a 120Vrms to center-tapped 12Vrms filament transformer in reverse to make an inverter that used 6Volts dc to produce 120 volts to power a household radio. Worked great. Has something changed?"

I never said it wouldn't work. Just that in order to produce a true 240Vrms sine wave, you needed to be able to produce the peak voltage of the waveform. I was assuming (yeah 4 that's a letter word, I know) is that the OP is trying to produce a sine wave or some close facsimilie. If you were trying to produce a square or a modified sine wave, you wouldnt need to generate quite as high of a voltage. Considering the date (1956) can I assume the radio was a valve unit? I know that vacuum tubes are not that picky about plate voltage, so I can see a tube radio running quite happily off of 84Vrms. ;) or was your inverter a square wave unit? Still, outside of some possible extra noise, I can't see how the radio would mind.

IanR,

I didn't catch the part about op trying to do a sine wave.

"I cannot find 12v to 240v step up transformers from any supplliers in the uk. What ime trying to build is an inverter for use in a car, i can find 240v step down transformers to any other lower voltage but what would i need or do i have to wind my own. please excuse my inexperience."

So basically we can assume he is looking for a tranformer to jam a 12 to 14 Volt squarewave into and get some semblance of 240Volts that trashy waveform doesn't bother.

He is trying to DIY it or he would go for a nice professional unit with frequency stability and feedback for overcurrent protection and Voltage regulation.

Makes sense?

Speaking of feedback, let's see what he has to say.

Jon

http://www.gadgets.co.uk/item/CARINV/12V-240V-Car-Power-Inverter.html

Try Newark Electronics and Digi-Key.

change the 12v dc to ac and use the step down backwards to get the 240

Mold doc,

change the 12v dc to ac and use the step down backwards to get the 240

That is what he wants to do but he is having trouble finding the right transformer.

Jon

http://www.naaptol.com/buy-online/WO-shopping-best-deals-W2391O/automobiles/auto_accessories/car_inverters.html http://www.wisegeek.com/what-is-a-car-power-inverter.htm http://www.amazon.co.uk/Car-Inverter-120-Watt-240V/dp/B000LZ1JF4 http://www.nextag.co.uk/car-inverter/zzukzB1z0--search-html

Bior,

It seems he is looking for a transformer to MAKE a do-it-yourself inverter.

Jon

There are transformers and there are transformers. The best suited for this application are saturated core types, regarding efficiency and stability. Honestly, there are a host of mfr's and models/capacities available at reasonable prices; a route that I would recommend.

Bill,

We are probably in agreement that

There are transformers and there are transformers. The best suited for this application are saturated core types, regarding efficiency and stability. Honestly, there are a host of mfr's and models/capacities available at reasonable prices a route that I would recommend.

The OP probably knows that but is asking for info on where to find one that fits his application. No doubt he would like to hear about a company to contact and a part number.

Jon

I spent some time looking for a mfr., but to no avail. Would guess that the mfr's of inverters probably make their own transformers today. I did a design paper in college on the subject and subsequently built several inverters in different power ranges with good success and wound my own toroid transformers. Frankly, I would not be so inclined to do that again should I want such an inverter called for here.

That brings me back to now just buying something off the shelf or a kit. Nevertheless, I still have not seen a load spec from stevemcgowan. So that leaves me at an impasse.

Bill,

I thought it was strange that people were suggesting he buy ready-made inverters when the he was looking for a transformer. But then he could buy one and use the transformer from it for the project.

Hello.... Stevemcgowan........ are you there?

hmmmmm...

Jon

Bill,

He may have hurt himself.

Jon

kudu, yeah maybe he's taking the day off with lots of nicks on the ol' mug.

An adder to my earlier: I certainly am tolerant for those who are from outside the US, AUS, GB etc. that leave posts with grammar that's sometimes difficult to understand. What I don't get are those from the inclusive that seem to be OMG, including those whom reply, that are slap full of poor grammer and misspellings. Guess they don't even take time for the provided spell checker, huh? Some posts come from what apparently are students in the US. I have a problem with those that begin with "Tell me" or "Give me" and the like. Too, many of those are badly educated (grammar). They want us to tell them how to make their frabitz, when they don't even have a mediocre command of the english language.

Sorry. Know this is not a blog area for other.

Bill,

For sure!

I notice that a lot of Engineer minded folks don't always have the best language skills but they are sharp enough to overcome it with good tools.

I sometimes worked for a brilliant Ukrainian Engineer who studied Electrical engineering in Harbin, Manchuria when it was Soviet dominated and his family was part of the Soviet representation there. His accent and language skills were adequate but you had to pay attention. His schematics were made using 3 different conventions so his draftsman was kept quite busy sorting that out. I built prototypes for him and he explained his designs in detail and he often made improvements after his draftsman had his drawings. (I could see the steam rolling from his neck while changes were being made on drawings he had completed. All by hand in those days.)

There was a little movie I saw long ago about a brilliant but humble engineer who had an Apalachian sound and look so others never considered him for having anything of value to offer. After saving their "bacon" they realized there was a "gem" under that outer coating of "dirt".

Jon

The co. I had been last working for hired a couple of Ukranian jokers, said to be electrical engineers (one was simply a tech). Sort-of designed a new PCI interface system that would only be plagued with lots of bugs. Did NO documentation. Both bolted, striping all those files from their hard drives. They also made off with dev. tools the co. paid for. One of those fubar situations.

Bill,

In the Soviet System an "engineer" can be little more than a simple technician or electrician. I have known a few of those.

They also have the idea that property belongs to the one who has posession. A Soviet hotel manager would have a home filled with the best the hotel can provide. Hefty ladies went to orphanages to get "their share" of the donated food to take home.

Laws were for those who were caught and had no friend in a position to override.

etc etc etc.

Jon

I have 2 which I bought in Aldi, 1 is 150 watt, the other is 500watt I think.

For a SIMPLE home-built square-wave inverter UNDER 150W, use a transformer like this one

http://www.maplin.co.uk/Module.aspx?ModuleNo=46757

*** item N30CF 240V/9V 150W (dual secondary)***

along with a couple good MOSFETS like

http://www.maplin.co.uk/Module.aspx?ModuleNo=33843

*** item N10AH IRF540A (30A 100V 2 pieces) ***

Finally, add some drive and control circuitry, the input/output connections and wiring, a heatsink, a fan, and a nice housing.

I've made many similar devices. They worked and some were worth the effort as a LEARNING EXPERIENCE. If you just need the inverter and NOT the learning experience, I suggest you just BUY one. This one is 300W and is cheaper than the PARTS!

http://www.maplin.co.uk/module.aspx?TabID=1&criteria=usb&ModuleNo=224796&C=SO&U=Strat15

Good Luck!

Can't you just buy a core and wind it with ration 1:20 using proper magnetic wire?

Stan

Guest,

He already mentioned doing his own transformer.

Jon

Can you go 12V to 120V then 120V to 240V?

What I do here is find a UPS system with the wattage I want. Remove the gel battery and run the wires out to your car battery. Use big wires. Put a computer fan on the side of the UPS to cool it, in your case you will need to put a switch on the fan, run it from the car's power. A good UPS will put out a sine wave power. Folks will throw out a perfectly good UPS because the battery died.