hi everybody ...

for many electronic applications, we need a DC supply ... and we often need an internal power supply rather than powering the project by batteries ...

for small size applications, where the area is an important factor, the transformer of the power supply is really a big problem ...

the transformer i used in my current project has the dimensions of 7*3.5*4 cm which are very very big dimensions with respect to my circuit dimensions ...

I am asking if there is a special type of transformers that is smaller in size or if there is another technology that can discard the Big transformer ...??

please help and thanks in advance

afesheir

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Build an isolated SMPS.For low power at high frequencies these can be small. Rohm has non-isolated pre-built low power SMPS.

I would recommend an isolated supply,but another alternative is use capacitive reactance and a zener see here.

http://uk.geocities.com/ronj_1217/tless.html

 

You can use a switching power supply (Like the one in a computer) which is smaller than the traditional power supply of equivalent power. They work by rectifying the AC and then chopping it at 200Khz or so to drive a much smaller transformer (The higher the frequency, the smaller the transformer can be).
http://computer.howstuffworks.com/power-supply1.htm

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You can get little AC/DC modules in small packages. Here is a link to such a device provided by DK in a previous post.

http://www.v-infinity.com/dklist_ac_dc%20conv.asp

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For small DC projects that I need to run off the mains, I always use a wall plug adapter(plugpack supply), this is many times safer as the mains voltage is confined to the wall transformer & the circuit usually needs only a 78xx regulator & a few caps, very small PCB area needed.
This also gives flexability to your project as it can be run off battey as well, be it car battery or whatever.

 

A wall plug is normally best for most applications, if you need a bipolar supply then go for a wall plug with an AC output and use a voltage doubler.

For more power laptop powers supplies are better, even if you need to put another switching regulator on the output.

Large transformers are normally only needed for things like audio amplifiers.

I still like to use a small transformer to build internal power supply but that's only because I've got lots of them in my junkbox.

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This Circuit can be Modified to operate Small Projects.
http://www3.telus.net/chemelec/Proje...er/110-chg.htm

CAUTION, SHOCK HAZZARD, it is NOT Isolated from the AC Line, So it is Not Suitable for some projects.

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Corrected.

A 2.2µF mains capacitor is nearly the size of a small transformer. The only use this circuit is for is something with an MCU or CMOS logic like a timer. 100mA is a bit optimistic, it'll be more like 86mA once you take into accound for the 14V drop of the load and rectifier, then there's the issue of the capacitor bein on the lower end of its tollerance range.

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first of all, sorry for my late reply but i had my internet connection disconnected the last days ..

thanks to all .. but i couldn't understand the idea of the transformerless power supply ..!!!!!!!!

thanks agaon to all of u ..

afesheir

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I have heard this many times, but i'm stil curious why SMPS not appropriate for audio amplifiers ?

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It is, many PA amplifiers (and a few HiFi amplifiers) use them - but SMPSU's aren't cheap, and can make the unit more expensive. You also have the problems of the RFI they produce, so you need good screening and filtering.

It's far, far, easier to throw a conventional PSU together, a transformer, a bridge rectifier, and a couple of electrolytics - job done, and it doesn't kill itself at the slightest provocation.

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Is that ture these days?

I thought large SMPes are cheaper than huge transformers and filter capacitors.

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It can be, depending how good the PSU needs to be - it's a LOT more complicated, and bear in mind you're not talking about a regulated conventional supply. A large toroid made in China won't be very much money.

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What max voltage & max current and max power? Does the V & I need to be adjustable?
Does it need to be isolated from the AC line? Are you working at 50 Hz?
Continuous or intermittent use?