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Old 13th June 2005, 10:07 AM   #1
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If i wanna build a 12V 3A output power supply, should i need to use a 3A transformer also??
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Old 13th June 2005, 08:03 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
when using a capacitor input as smoothing after the rectifier the transformer will see a large charging pulse with a long period of zero current during each cycle. The power lost is based on Isquared * R and is worse in this non constant current situation.
Much literature recommends that you draw about 70% of the transformer rating to account for this.
But many designers use the full rating and because transformers can meet quite large overloads and rarely run at full power this works for most.
Answer;- use somewhere between 36VA and 60VA by 12v depending on your use.
12Vac + regulation (15%) + suppler overvoltage(6%) times sqrt2 less low current diode drop (0.7) = 20Vdc max on no output current.
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Old 15th June 2005, 05:09 AM   #3
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hmm.... i bought a 24V 3A transformer, then it should work for 12V 3A power supply. Wow, if i build a 13V 10A power supply, then i atlease have to use 130VA transformer?? It is very hard to find this kind of transformer especially PCB mounted type in Malaysia!
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Old 15th June 2005, 08:28 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi
and if you apply the 70% derating for capacitor input then you need about 180VA.
If you regulate you have to add on the losses in the regulator, at least another 30% (3 volts drop across reg.) making 240VA.
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Old 15th June 2005, 09:32 AM   #5
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Default Re: power supply

Quote:
Originally posted by jamesbooi
If i wanna build a 12V 3A output power supply, should i need to use a 3A transformer also??
This have to do if you are going to take out 12 V, 3A all the time or not. What is your application?
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Old 15th June 2005, 09:33 AM   #6
Electrons are yellow and more is better!
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Quote:
Originally posted by jamesbooi
hmm.... i bought a 24V 3A transformer, then it should work for 12V 3A power supply. Wow, if i build a 13V 10A power supply, then i atlease have to use 130VA transformer?? It is very hard to find this kind of transformer especially PCB mounted type in Malaysia!
I'd say it's hard to find in general.
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