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Selection of Fuse and Bridge Rectifier

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alok1982

Member
Hi,

I want to select a fuse (slow blow) and Bridge Rectifier for an AC to DC converter. It's and SMPS (Flyback topology)

Input voltage = 90Vac to 265Vac / 50Hz-60Hz (Universal Voltage)
Output Voltage = 12Vdc / 1A

Can you please guide me with the selection process, formulas, etc.

Thanks!
 

Colin

Active Member
Fuses under 500mA are very unreliable. The fuse you suggest will never blow.
It is more likely to fail by sheer strain. It is much better to add a fusible resistor to the primary.
 

Colin

Active Member
When power supplies and transformers are turned ON the inrush of current is very large and you will see tiny fuses such as 315mA, stretching and dipping. This has the effect of weakening them and quite often they fail after a period of time for no reason.
Secondly, a small fuse will take a lot of current to actually burn out and merely shorting a low-current supply will not cause this current to flow and nothing will be protected. The supply will just over-heat.
 

MikeMl

Well-Known Member
Most Helpful Member
When power supplies and transformers are turned ON the inrush of current is very large and you will see tiny fuses such as 315mA, stretching and dipping. This has the effect of weakening them and quite often they fail after a period of time for no reason.
Secondly, a small fuse will take a lot of current to actually burn out and merely shorting a low-current supply will not cause this current to flow and nothing will be protected. The supply will just over-heat.
Sounds like you are picking the wrong fuses for the application. If you expect a short-duration in-rush and a small steady-state load current, there is a fuse for that. If you have a supply that is current-limited, why fuse it?
 

ChrisP58

Well-Known Member
Most Helpful Member
The input fuse on a switch mode power supply or, most any power supply, is not so much to protect against an output overload, but an internal failure.

A common failure, in addition to capacitor wearout, in switch mode supply's is mosfets failing short. The fuse should blow before such a failure takes out system wiring or PCB traces.
 
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