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Need help making an 18v 650mA DC Power Supply

wakkun

Oct 3, 2014
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I need help making a power adapter that can supply 18VDC at 650mA , I need this for a 2.1 audio system that I have at home. I was thinking of just buying one but what's the fun in that right?! Plus I'm an engineering student and this would be a good experience to level up.

Can anyone please help me design a circuit that I can make? I can get most common parts locally.

P.S. - I'm not very good at theory but I'm adept with practicals provided I have guidance.
 

Gryd3

Jun 25, 2014
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If you're an engineering student, you should have some background knowledge already correct?

Will you be building a regulated power supply? Will it be a linear or switching supply?
 

wakkun

Oct 3, 2014
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Yes I have a bit of knowledge on Electronic circuits but not enough to design a circuit myself.

I require a Linear Regulated Power Supply.
 

Gryd3

Jun 25, 2014
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Well... because you're a student, I am going to be vague ;)

I'm assuming your going from mains down to 18V DC?

Two ways to do so:
-Linear supply - Typically requires a small transformer and a rectifier to drop the 110/220 down to the 22V+ Range. This will be rectified to DC and filtered with a capacitor.
This will result in a very rough DC voltage a little higher than what you need. You feed this to something like : http://www.cdil.com/datasheets/lm7818.pdf which will regulate it at 18V
Add another capacitor on the output side to help reduce any noise on the output to keep a smoother 18V and your golden.

-Switching supply - Typically requires an inductor and a couple other components, but has the advantage of running cooler, being smaller, and more efficient.
You can also do away with the transformer... but using a transformer can isolate you from the mains voltage making it safer...
 

wakkun

Oct 3, 2014
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Oh wow this is real life stuff, I can't translate it! Can you make it a bit simpler?
 

Gryd3

Jun 25, 2014
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Sure thing ;)

One way or another you need to reduce the voltage coming from the wall from 110/220 down to the 18V you want.
A linear supply, like the LM7818 I linked above will do this by turning itself partially on to only allow some of the voltage to pass through... There is a big issue with this, in that voltage does not just 'stop' at a component. The LM7818 will get warm... HOT very very HOT if it needs to dissipate too much energy, so care must be given when designing the circuit to prevent the LM7818 from having to deal with too much of a difference in voltage. This is why you can't just feed 110V into it and get 18V out... the amount of power that the LM7818 must dissipate will be very very large... thats 92V that get's 'eatin' up by the LM7818. Now a transformer will be chosen to take the 110V input, and drop it down to something a little close to what we need. Unlike the LM7818, the transformer does not 'dissipate' energy, but transforms it. If you are unfamiliar, if 110V 1A goes into a transformer, and you want 11V out... then you can take up to 10A from it! (You need to account for loss... but the product of voltage and amperage will remain the same on both the input and output of a transformer in an ideal product)
So the finished project would be a transformer to get you something closer to 18V... but the transformer will provide AC, so you need to rectify it with Diodes or a bridge to turn it into DC. You then need to smooth out the voltage with a capacitor before you work it down further to your desired 18V.

Take a quick read here: http://www.learningaboutelectronics.com/Articles/How-to-build-a-DC-power-supply.php
Then ask some specific questions you may have and I can further clear things up for you.
 

davenn

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Sep 5, 2009
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Oh wow this is real life stuff, I can't translate it! Can you make it a bit simpler?

I have to suggest for your safety and long life, if you find that too difficult, then you should really be considering of you should really be playing with mains voltages. buy a plugpack and learn to experiment with low voltage circuits before getting into mains powered stuff


cheers
Dave
 

Gryd3

Jun 25, 2014
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I have to suggest for your safety and long life, if you find that too difficult, then you should really be considering of you should really be playing with mains voltages. buy a plugpack and learn to experiment with low voltage circuits before getting into mains powered stuff


cheers
Dave
I should have caught on to that sooner... I jumped on the 'I'll help you learn it train' without thinking about the OPs experience... I'm a little tired :s

Take Davenn's advice with some seriousness. If your learning, this is not a good starter project as the voltages can be lethal if you accidentally touch something or hook something up wrong.
As an example... a transformer that can be used to give you 20V to work from, could potentially give you a 700V surprize... or over 2000V if your working with 220V from the power-grid.
 

wakkun

Oct 3, 2014
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Yelp that does sound...scary. I guess I'll take davenn's advice for now. Any recommendations as to where I can get a decent plug pack?
 

Gryd3

Jun 25, 2014
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Well... I'm not sure what area you live in, but a store like RadioShack, TheSource, Futureshop etc. will have universal adaptors.
They will either be rated for a specific voltage, or have a switch allowing you to pick what voltage you want.
Remember that the voltage needs to be matched exactly, but the adaptor can be (and often is) over-rated for mA... Even if the adaptor is rated for 2100mA, your audio system will only take what it needs.
 

davenn

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all cool wakkun :)

you just need to learn to walk before you learn to run ... so to speak ;)
stick your country into the spot in your profile so its there for future reference
you could look around for some old laptop computer power packs there are for some laptops an 18V and ~ 4 Amp one see if you can pick up a second hand one
As Gyrd3 said your equip will only draw the current it needs.

If you are really keen to learn about electronics, and that's to be encouraged, have a look through some of the stuff in the resource section tab near the top of the page.

Don't be afraid to ask us all questions, we are here to help :)

cheers
Dave
 
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