Sizing Decoupling Capacitors for 240AC to 5VDC Conversion

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but it should be handling 4amps from the simulation.

 

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we designing backwards, thus now the transfomer and rectifier is giving some headaches.

 

[Phrak]

Yea, I have heard of LM317. Is it better than 7805.But I think will stick to the 7805 for now.

I have attached my final(hopefully) designed circuit. The filtered output to 7805 is 9-11V at around 1 A.

The inrush current from rectifier is around 6A and repetitive 4A peak cycles after that.

Is really 3A rectifier enough to do this? or a 5A is better?

It's your design, but it won't handle 1A without dropout unless you are willing to put up with a 25VA+ overkill transformer. You've used an ideal transformer in your spice model. However, there's nothing magical about 1A, and you probably won't use all of it anyway.

 

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So if I use an underated transfomer or bridge, at full load, the voltage will drop?

It won't effect heating or maybe damage??

 

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I think there is a mistake of using Vrect/ 2 pifC.

Impedance of capacitor is : 1/ 2pifC

I=V/R = Vrect / (1/2pifC) = Vrect x 2 pi x f x C

 

[Studiot]

I do apologise you are quite correct.



Well done for spotting that.

 

[Pagedown]

So if I use an underated transfomer or bridge, at full load, the voltage will drop?

Will it cause heating or maybe damage??

Anyone can answer me pls?

 

[vk6kro]

This may be an English problem.

If a transformer is used at full load then it is being used properly, but at the limit of its ratings.

So, a 10 amp transformer can be used to give currents up to 10 amps although it is considered bad practice to operate components at the current they are rated at.
Normally a safety margin is allowed. So, you might design this power supply so that the load never exceeds 7 amps if the transformer was rated at 10 amps.

If the load were 20 amps and you used a 10 amp transformer, then the transformer is said to be under-rated for that load.

What actually happens depends on the transformer. It may get very hot due to heat dissipated in the transformer not being able to escape fast enough. This can cause the transformer to fail eventually.

It may fail instantly, although this is less likely.

The voltage out of the transformer would probably be lower than if the load was normal and this is due to the extra voltage drop across the resistance of the wire in the windings as well as saturation effects in the iron of the transformer.

Semiconductors which are used beyond their ratings are very likely to fail instantly or in a very short time.

I like to use 30 amp bridges for power supplies, even if they are only going to deliver a few amps. These are very cheap and good insurance.

 

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I am suspecting the simulation. How correct is it? cause even with lower loads the current in the secondary before the bridge is at least 1.5A.
for full load, it goes until 3-4 A.
even with lower capactitance.
Is it so in reality?

 

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Due to my earlier designing of 1A, the transformer requirement at least 60VA(tested in multisim) is too bulky and expensive.

I have been trying now to design towards 0.2A since my microncontroller and accessories use a lot of less current. But from simulation, the secondary side still takes in around 1A. Is this true??the 7805 will at least take 1A??

 

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Attached is the simulation, full load is 0.2A. and the current drawan from rectifier is 900mA.

 

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Latest update : Without the PI-filter, the inrush current is less. Simulated using multisim, using just a RC capacitor input filter, I can use a lower VA transformer to supply the same amount of current.