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Old 21st February 2005, 12:03 PM   #1
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Default can i use this kind of power supply?

Dear friends,
I want to build a power supply where instead of using a very heavy toroidal transformer I am thinking to use a small transformer with same voltage ratings but reduced current ratings but by adding more capacitence like 20,000uf per rail..?

can I use like that...?

Here is an example so that you will understand>>>>
My peak current output will be about 12 amps so for that the following are the two ways for me which one to choose

CASE 1: Using a 500VA toroidal with 22-0-22 with 10,000uf per rail

CASE 2: Using a 120VA toroidal with 22-0-22 with 40,000uf per rail


I am expecting the second case to work better what do u say?
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Old 21st February 2005, 12:58 PM   #2
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A transformer has an efficiency of less than one. It depends on the core configuration and core material etc.
Typically a toroidal transformer has better efficiency than an EI type.
The typical EI might have an efficiency of say 85% while the toroid might be 95% .

So your 120 watt transformer can only provide about 100 watts into the circuit. At 22 volts that means about 5 amps rms! At 44 volts that is 2.5 amps !! At that load the voltage drops and the core starts to heat up. So the 500VA would be better than the 120VA unit.

Capacitance only cuts down on the ripple and helps in some energy storage for short periods. As you get deeper into the process of rectification and use of capacitors it gets more complicated than you want - I think !
I am simplifying things to make you grasp some of the basic facts.
If you really want to understand this well , you should search the Net for a tutorial. A post on the DIY forum could not do justice to the subject.
Be careful with electricity . It can hurt even at low voltages and can be lethal at high voltages. We never thought about how powerful water can be . We have now seen it after the Tsunami. Electricity is equally dangerous .
Be careful and have fun!
Cheers.
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Old 21st February 2005, 08:17 PM   #3
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No.

Not only is the transformer smaller, large capacitors will labor it even more by forcing huge charging current pulses.

Your best hope would be choke input, but even then don't expect more than 80W DC output (i.e., approx. 18VDC at 4.4A or 36VDC at 2.2A).

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