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Old 11th May 2008, 03:10 AM   #1
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Default dc power supply help needed

hello all been awhile but got a ? i just bot these 2 kits for a project im doing, could someone tell me what i can change to make the 12v go from outputs a maximum current of 125mA@12V and 200mA@10V or less to 1Amp? and the 9v from maximum of 100 ma output current at 9VDC and can produce up to 275 ma at 3VDC to about 500ma or higher? im not sure what part will change the amp's or what part to use. thank you

0-12V Variable Power Supply

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0-9VDC Variable Power Supply

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Old 11th May 2008, 03:15 AM   #2
TheMG is offline TheMG  Canada
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That kit is not really going to be able to supply greater currents I'm afraid. First of all the transformer may be too small, secondly at greater currents you'll have a significant amount of heat to deal with, so a larger heatsink will be a necessity.

Other than that, decreasing the value of R2 may be able to get you higher currents (and also higher voltage at low currents).

That being said, this is a fairly primitive circuit, it doesn't even count as a regulated supply, and seems to rely only on the base current for current limiting, which might have been planned or might just be a side effect of the simple circuit.
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Old 11th May 2008, 03:19 AM   #3
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thers the specs now image wasnt working
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Old 11th May 2008, 03:21 AM   #4
TheMG is offline TheMG  Canada
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I edited my post while you posted, read above for more insight.
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Old 11th May 2008, 03:25 AM   #5
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is there anything i can chane or do to make it a variable 0-12v 1amp i have a bag of stuff i can dig through for parts?
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Old 11th May 2008, 03:31 AM   #6
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im going for this but with parts i have. just need a varib 0-12v 1amp
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Old 11th May 2008, 03:34 AM   #7
TheMG is offline TheMG  Canada
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Well first things first throw in a larger heat sink. At 12V the transistor has to drop 4V (16V-12V) so 4V*1A= 4 watts. Doesn't seem like much but trust me that little sink is gonna heat up like a mutha.

As for resistor values, you're going to have to calculate them, or experiment. To calculate you'll have to lookup the transistor specs (most importantly the hFE value) and work it out from there.

And that's all assuming that little power transformer can handle that much current, which it likely can't.
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Old 11th May 2008, 04:56 AM   #8
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the transester is the to220 7812 ic i think. im new to this so i dont understad much. is it posable to take a 12v 2amp dc adapter and make it Variable?
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Old 11th May 2008, 06:15 AM   #9
TheMG is offline TheMG  Canada
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Yes, easily. Have a look at the LM317 http://www.national.com/mpf/LM/LM317.html
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Old 12th May 2008, 05:02 AM   #10
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ok thanks but that doesnt help me much. i just need 2 dif. veriable adapters 1 that will do 5-9v for a peltier and 1 0-12v to drive 2 12v pc fans. and iv got a few dif. adapters laying round but not sure how to make them variable, or just boost the 2 i posted as i need more then the M.amp's they output.

my project is creating a mini fridge like this but 5v isnt enuff iv pushed it to 7.5v so far and cooling the hot side isnt to bad with my design. but i need the veriable so i can use it like a fridge thermastat (colder = more voltage) wich means peltier hot side needs more cooling so i could turn up the 2 fans if its variable.
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