Having a fuse on the line input and on the DC output of a PSU is standard practice (well at least the mains input).
However, I have seen designs that include an additional fuse(s) between the transformer secondary and the bridge rectifiers or ones with fuses between the bridge and the caps.
Is one of the latter topologies actually necessary/of benefit?
If they are desirable, how would one calculate their values?
As a side issue, when mounting an unshielded toroid with 'flying leads' is it possible to mount the toroid upside down so the leads are at the bottom - i.e. in contact with the case? I'd guess that this would dangerous but I haven't found a website that mentions this possibility or that warns against it.
Cheers.
However, I have seen designs that include an additional fuse(s) between the transformer secondary and the bridge rectifiers or ones with fuses between the bridge and the caps.
Is one of the latter topologies actually necessary/of benefit?
If they are desirable, how would one calculate their values?
As a side issue, when mounting an unshielded toroid with 'flying leads' is it possible to mount the toroid upside down so the leads are at the bottom - i.e. in contact with the case? I'd guess that this would dangerous but I haven't found a website that mentions this possibility or that warns against it.
Cheers.
A fuse between the secondary and the rectifiers can protect the transformer from a shorted rectifier. A fuse before the reservoir cap can protect the rectifier from a shorted cap. Generally, a fuse protects the things near it in the circuit and may provide some weaker protection for things a bit further away.
The fuse value is calculated from the expected RMS current, as for any circuit. This can be difficult when rectifiers are concerned. You could use a guess of 2-3 times the DC current.
The fuse value is calculated from the expected RMS current, as for any circuit. This can be difficult when rectifiers are concerned. You could use a guess of 2-3 times the DC current.
Fuse the mains close rated. (300VA on 220Vac can use T1.25A or T1.6A, you will need a soft start)
Then a fault at the output of the transformer blows the mains fuse.
There is a big advantage in fusing the output of the PSU at the PSU, not at the amplifier.
Once these three fuses are fitted I cannot see any advantage in fusing before the smoothing capacitance. They would need to be so big to survive start up, that they never blow quickly enough to protect anything that survives the first few microseconds of fault condition.
Then a fault at the output of the transformer blows the mains fuse.
There is a big advantage in fusing the output of the PSU at the PSU, not at the amplifier.
Once these three fuses are fitted I cannot see any advantage in fusing before the smoothing capacitance. They would need to be so big to survive start up, that they never blow quickly enough to protect anything that survives the first few microseconds of fault condition.
Pol,
divide VA by Vac and you get the normal maximum current that the transformer will draw.
Choose a fuse around that value and you have selected a close rated fuse.
Alternatively, for motors and transformers and similar inductive loads
the fuse value can be VA / Vac * 3
The T rated three times fuse value will generally allow a toroid transformer, charging up a capacitor input filter, to start and run without nuisance blowing of the mains fuse.
I don't like this alternative. I prefer close rated fusing, except when I get down to lower than 60VA. Then I'll not bother going any lower T250mA (240*0.25 = 60VA)
divide VA by Vac and you get the normal maximum current that the transformer will draw.
Choose a fuse around that value and you have selected a close rated fuse.
Alternatively, for motors and transformers and similar inductive loads
the fuse value can be VA / Vac * 3
The T rated three times fuse value will generally allow a toroid transformer, charging up a capacitor input filter, to start and run without nuisance blowing of the mains fuse.
I don't like this alternative. I prefer close rated fusing, except when I get down to lower than 60VA. Then I'll not bother going any lower T250mA (240*0.25 = 60VA)
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