Determine Fuse in power supply 1

[Zoegabh]

Hi all,,

I have question about determine fuse,
I would like to build power supply which the output is 12V@25A and 5V@1A
I will try to build linear power supply,
My transformer is 220V -> 24V (500VA).
Could you tell me what is the fuse value should be?

Thank you

regards

 

[MacGyverS2000]

Why build when you can purchase?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

Just buy two switchers!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zappedagain]

For starters, make the fuse blow at a current less than the current limit for your wires on the mains. Otherwise one of the wires will become the fuse! This sets your maximum fuse value.

Calculate your in-rush current when the supply first starts up. Make sure the I^2*T value support this current for a long enough time (typically a half cycle of the mains frequency for a switcher, probably longer for a linear). This sets your minimum fuse value.

At that point you can select any value in between these limits to meet other requirements.

John D

 

[LionelHutz]

Normally, you size to protect the transformer. There may be certain requirements depending on your location and the type of approval you want to achieve.

The transformer rated current = 500VA/220V = 2.27A

A value such as 3A or 4A time delay should be OK and allow for transformer inrush.

You might also need other protection inside the supply. If one of the outputs is shorted or components fail shorted you do not want parts to burst into flames.

On another note, your transformer is too small for the linear supply you want to build. 24VAC rectifies to about 28VDC. 12V @ 25A with a linear rectifier will require you to pull 25A off the raw 28VDC. 28V x 25A = 700VA.

 

[Noway2]

Remember too that fuses have a voltage rating that specifies their ability to clear the circuit and extinguish the arc, so you need a fuse with a voltage rating higher than your circuit operating voltage.

 

[jimkirk]

I agree with Keith, get a couple switchers. With a 24 (or more) full wave rectified raw supply, a linear regulator putting out 12 volts at 25 amps will be dissipating 300 watts (or more). Putting aside the fuse issues, you probably don't want to do that.