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High Current Power Supplies
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Old 10th October 2004, 10:18 AM   #1
matt_uk is offline matt_uk  United Kingdom
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I need to build a power supply that can give out ~13V DC at 10A. To do this I calculate that a 15V and near 400VA toroidal would be needed but finding a toroidal over 300VA that is 15V seems to be a bit of a problem in the UK.

It's not actually for an audio project as such this one, it's for an amateur radio transmitter but hopefully this is the best place for a good power supply design.

Can anyone suggest any kind of alternative design, or other methods of getting a fairly high current power supply at a lowish voltage?

Any help much appreciated.

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Old 10th October 2004, 10:36 AM   #2
Upupa Epops is offline Upupa Epops  Czech Republic
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13 V / 10 A is 130 W, so you need trafo cca 200 W - PS for transmitter must not to be " hifi " oversized .
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Old 10th October 2004, 10:57 AM   #3
johnnyx is offline johnnyx  United Kingdom
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Maplin do a 225VA, 12v toroidal transformer, you can connect the secondaries in parallel for max current. It depends if you have a regulator that will drop volts requiring a 15v secondaries, cos they haven't got one of 15v as you say.

CPC have a much greater choice, order online like Maplin, they are owned by Farnell, but have a greater selection of audio - related stuff. Lookhere.

Just register with them and you can order with credit card etc. after a short while.

Hope this helps.
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Old 10th October 2004, 11:29 AM   #4
bigparsnip is offline bigparsnip  United Kingdom
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Why not just use an old (or cheep new one) computer power suply, as these will all be capable of far more than 10A on the +12V output suply, and it's only a small amount of work to get a turn on switch connected up for control.
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Old 10th October 2004, 12:46 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi bigparsnip
Which wire(s) switch on the PSU in the computer supply? Two of them become a button on the front panel that switches the power on, but which two?
Andrew T.
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Old 10th October 2004, 02:11 PM   #6
Swedish Chef is offline Swedish Chef  Sweden
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Just keep one thing in mind if you choose a computer SMPS - their load regulation (or lack of it) usually assume you are pulling out at least an amp or two on a constant basis. Otherwise their regulation can go pretty haywire and your end result may be accordingly.

"Knowing what to do but not why is no use in a changing world" - The Art of Sound Reproduction
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Old 10th October 2004, 02:20 PM   #7
till is offline till
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Reichelt in germany has up to 800VA 2*15V
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Old 10th October 2004, 03:53 PM   #8
matt_uk is offline matt_uk  United Kingdom
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Thanks for the replies, similarly to audio amplifiers noise and incosistency on the power supply is a very important factor, and as with audio batteries are the best option, so this really gets rid of the SMPS option.

Current drawn is not steady at all, changing often between about 0.6A on Receive and upto 10A on transmit (i.e. when the RF power amplifier is in use).

I have made my calculations on the following formulae found for a full wave bridge rectifier circuit with capacitor.

VAC = 0.8 x ( VDC + 2 )
IAC = 1.8 x IDC
WATTS = VDC x IDC (output)
VA = 1.4 x ( WATTS + 2 IDC* )

So VAC = 0.8 x (13 + 2) = 12 (My apologies as I said 15V in the first post)
IAC = 1.8 x 10 = 18
WATTS = 13 x 10 = 130

so VA = 1.4 x (130 + (2 x 10)) = 210??

Having gone through the calculations again it seems I was a bit off when I rushed through them first time around!! My apologies, looks like a 300VA will be more than enough, could even get away with 225VA.

Feel really stupid now! but happy that the cost of the supply has dropped dramatically! Thanks for the replies that made me go through this again and find my mistakes!


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Old 10th October 2004, 04:15 PM   #9
Brian Donaldson is offline Brian Donaldson  United States
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Buy a battery charger. At Walmart, you can get a 20 amp 12V (really 14v or so) battery charger for about $35.
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Old 10th October 2004, 05:48 PM   #10
Eva is offline Eva  Spain
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An old PC PSU would save money, space and again money on electric company bills

Old AT units are preferable over modern ATX ones since they are simpler and easier to understand and tweak

To turn on an ATX unit just short the pin 14 of the connector [usually green wire] to ground, see http://www.hardwarebook.net/connecto.../atxpower.html

Regulation may be tweaked to rely only on the 12V output and also to make it adjustable, otherwise +5V output should be loaded with >2A and some units require also load on +3.3V to properly regulate +12V output, however, negative outputs usually have its own regulation

Output rectification and filtering for +5V, +3.3V and negative outputs may be removed leaving enough space for enhancing +12V filtering [a pi filter and a common mode filter should be enough]

If EMI is a concern, just place the PSU some meters away from the RF equipment. Anyway, any PC PSU won't generate more EMI than any computer placed on the same room or even a TV set

The alternative to a 130W SMPS is a 130W linear regulator that sould be capable of dissipating up to 100W of heat in worst case conditions [providing enough input voltage margin, while PC PSUs already have excelent input voltage margin]
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