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> How to make power supply plus and minus 12V DC, From 240V AC to +&- 12V DC
kevy
Posted: November 27, 2006 09:35 pm
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I am a college student need to build a power supply for my project.
The project is require the + and - 12V dc.
I need to convert the housing voltage (240V AC) to that voltage (+ and - 12V DC).
I am looking for guidance.
Can someone teach or provide me the schematic diagram on how to contruct a circuit for this power supply?
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kellys_eye
Posted: November 27, 2006 10:29 pm
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Are you sure you want + and - (relative to zero) volts?



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tekwiz
Posted: November 28, 2006 01:40 am
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Nick Mulder
Posted: November 28, 2006 06:52 am
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QUOTE (kevy @ Nov 27 2006, 09:35 PM)
I am a college student need to build a power supply for my project.
The project is require the + and - 12V dc.
I need to convert the housing voltage (240V AC) to that voltage (+ and - 12V DC).
I am looking for guidance.
Can someone teach or provide me the schematic diagram on how to contruct a circuit for this power supply?

You'll need a transformer with a centre tap - eg. 30V across and 15V either side of the tap - the centre tap becomes a floating ground - send the other two outputs to a bridge rectifier and you'll end up with a DC voltage a bit higher than the original range, using capacitors to GND you'll smooth out this supply (essential) - maybe this bit is cheating in terms of your project, but then these two signals are sent to 7812 and 7912 voltage regulators, depending on the transformer you get and the current draw of your project you'll probably need some good heatsinks here ...

This is pretty standard stuff and many cct examples can be found on the net where you will find your capacitor values and placement info and also info as to what is happening ... If you were keen I imagine you could study power supplies to PHD level, so dont expect it to be super elegant/efficient etc...

sounds like your using op-amps ?
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kevy
Posted: November 30, 2006 07:39 am
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Thanks for telling me. I got the idea now. biggrin.gif
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Skeith
Posted: December 02, 2006 09:05 pm
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aslo assuming you cant get a center tapped tranny you can use a single tap around 14vac, rectify it, filter it and then pass it through 12v regulators. lm7812 for +12v and lm7912 the negative variant for -12v.

They both bias to the rectified ground thus becoming common and seperately output 12v and -12v. This setup will only handle about 1A - 1.5A depending on the regulator.


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Nick Mulder
Posted: December 02, 2006 10:31 pm
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QUOTE (Skeith @ Dec 2 2006, 09:05 PM)
aslo assuming you cant get a center tapped tranny you can use a single tap around 14vac, rectify it, filter it and then pass it through 12v regulators. lm7812 for +12v and lm7912 the negative variant for -12v.

They both bias to the rectified ground thus becoming common and seperately output 12v and -12v. This setup will only handle about 1A - 1.5A depending on the regulator.

I'm certainly no expert so I'm interested in your post here as it would mean I dont have to buy centre tapped transformers...

What becomes the ground reference ? Youd have say 20V or so from your rectifier and you can send these alternately to the regulators to get two 12V pot differences - but how are they referenced relative to each other ?

The 'grounds' on the regulators would be in one case the -ve from the rectifier and +ve on the other.

Can you draw a schematic ?
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Skeith
Posted: December 03, 2006 06:56 am
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You should be able to get the regulators for free as samples from the internet. There is a free samples section in general discussion that is stickied that lists companies that give free samples. Try fairchild thats where I got my regulators from.

If you plan on drawing more than about 100ma from the regulators I suggest putting a heatsink on them as they can get pretty hot.

Get the 1Amp versions if you can.


A circuit like this should do. Will be nice and clean too, specialy if its for opamps or audio
user posted image


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Nick Mulder
Posted: December 03, 2006 08:48 am
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ok

this is way easier
(I need to read up more!)

I thought the 7912 reg needed a 'negative' voltage with respect to its gnd connection and that the extra reletive point of the centre tap gave it (and was required)

Still a bit confused as my cct gives the 7912 this -ve voltage and it works, so the 7912 can regulate from 30V +ve and 30V -ve across its inputs ??

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kellys_eye
Posted: December 03, 2006 04:22 pm
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^^ Won't work....



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Skeith
Posted: December 03, 2006 06:37 pm
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QUOTE (kellys_eye @ Dec 3 2006, 10:22 AM)
^^ Won't work....

oops right wrong rectifier, needs a voltage doubler. This should do it.

user posted image

Its basicaly 2 half wave rectifiers stacked to give you double voltage. The you make your floating ground in between both of the capacitors. And from here as you can see there is (-) voltage on negative side of the bottom cap and (+) voltage on the positive side of the top cap. Then this is passed into the regulators.


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kellys_eye
Posted: December 03, 2006 08:38 pm
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Now THAT'll work laugh.gif


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Nick Mulder
Posted: December 03, 2006 09:14 pm
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QUOTE (Skeith @ Dec 3 2006, 06:37 PM)
QUOTE (kellys_eye @ Dec 3 2006, 10:22 AM)
^^ Won't work....

oops right wrong rectifier, needs a voltage doubler. This should do it.

Its basicaly 2 half wave rectifiers stacked to give you double voltage. The you make your floating ground in between both of the capacitors. And from here as you can see there is (-) voltage on negative side of the bottom cap and (+) voltage on the positive side of the top cap. Then this is passed into the regulators.

righto,

understand it this time - out of interest: what voltage will the 14V tansformer give at each rectified output wrt GND ?

thanks for this info... I wish I found it earlier when I was making my supply
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Skeith
Posted: December 04, 2006 10:23 am
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unloaded around 22vdc. ut as soon as theirs a load on it the voltage will drop.
Also keep in mind that unless you are using a low dropout regulator you need at leats 2-3v above its output for it to function properly.

I used a 13.7vac transformer to build a battery charger for my gell cells and its rectified output was along the lines of 22vdc.

The regulators can handle i think its 35v input. Just put a heatsink on em.


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