Fuse positioning in PSUs - Page 2

DF96 · 10th November 2012, 05:20 PM

Quote:

Originally Posted by Robert Kesh

My last post was intended to be humorous, by the way.

Other people say very similar things and are completely serious. As you are relatively new here we can only guess what was in your mind. If SY said it I would know he was joking, or had enjoyed a particularly good lunch!

polsol · 10th November 2012, 07:18 PM

Hi Andrew,
Thanks for your response. I guess the problem is chosing a mains fuse 'just right' to prevent spurious blowing is a matter of experience.

AndrewT · 11th November 2012, 10:09 AM

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)

polsol · 11th November 2012, 01:20 PM

Thanks once more Andrew for all your help.

10th November 2012, 07:18 PM	#12
polsol diyAudio Member Join Date: Aug 2004 Location: Cape Town and Kuwait	Hi Andrew, Thanks for your response. I guess the problem is chosing a mains fuse 'just right' to prevent spurious blowing is a matter of experience. __________________ Tony M

11th November 2012, 01:20 PM	#14
polsol diyAudio Member Join Date: Aug 2004 Location: Cape Town and Kuwait	Thanks once more Andrew for all your help. __________________ Tony M

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11th November 2012, 10:09 AM	#13
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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)