Hi all,
So I purchased this 12.5V / 240V 1A step down transformer with a VA rating of 12.5.
_**http://www.altronics.com.au/index.asp?area=item&id=M2152**_
And it says that a 250mA fuse is required.
But if I try to calculate the amps drawn on the primary side, the VA rating / primary voltage (12.5/240) I get around 0.05 amps (or 50mA) which is a lot smaller than 250mA.
Doesn't a 250mA fuse seem awfully large? Are my calculations wrong or am I missing something?
Thanks
When you turn it on, you will get a phenomenon called "inrush current", which will be more than the apparent rating of the transformer would suggest. You will also probably get currents higher than the rated 1 A secondary current, when you switch on or off whatever it is that you have connected to the secondary.
The fuse is not really intended to protect against a transient overload of the device. It is protect against against a short-circuit fault such as the insulation failure of the transformer winding. If you get a shortcircuit fault of the transformer winding, you will get a current of several amps, or more, which the fuse is intended to promptly interrupt.
Also, your calculation of the primary current, based on the turns ratio of the transformer coils, overlooks the fact that these transformers have relatively poor efficiency compared to a theoretical "ideal" transformer, so the primary current under normal "rated load" operating is going to be higher than the 50 mA you calculated.
michinyon:
When you turn it on, you will get a phenomenon called "inrush current", which will be more than the apparent rating of the transformer would suggest. You will also probably get currents higher than the rated 1 A secondary current, when you switch on or off whatever it is that you have connected to the secondary.The fuse is not really intended to protect against a transient overload of the device. It is protect against against a short-circuit fault such as the insulation failure of the transformer winding. If you get a shortcircuit fault of the transformer winding, you will get a current of several amps, or more, which the fuse is intended to promptly interrupt.
Also, your calculation of the primary current, based on the turns ratio of the transformer coils, overlooks the fact that these transformers have relatively poor efficiency compared to a theoretical "ideal" transformer, so the primary current under normal "rated load" operating is going to be higher than the 50 mA you calculated.
Thanks for the explanation.
So the 250mA fuse is mostly just for inrush current and not just for preventing me using more than the transformers rating?
Along with the 250mA fuse on the primary side, can I also put a fuse of about 1.5 amps on the secondary side as well (not a fuse of exactly 1 amp, as you said it's not a perfect transformer)? To prevent "transient overloads" from too much current draw during use in-case of a mistake during my project (since drawing 1.5 amps on the secondary side would be drawing 75mA on the primary side which is getting a little higher than 50mA but still not enough to blow the fuse on the primary side which is intended for inrush current)
If this isn't really applicable or practical, what other measures can I take to prevent myself drawing more than 1 amp~ on the secondary side by accident?
What are you trying to protect , the transformer or the load.
A 1.5A fuse wil easily deliver 3A before blowing if the transformer can supply it.
Do you know short circuit current ?
Boardburner2:
What are you trying to protect , the transformer or the load.
A 1.5A fuse wil easily deliver 3A before blowing if the transformer can supply it.
Do you know short circuit current ?
I would like to protect both if it's at all possible.
You can put another fuse on the output side, if you like.
You want to try to design your system so that the transformer does not become overloaded. If you are in doubt about that, perhaps you need a bigger transformer. Particularly if you cannot easily ascertain in advance what the peak output current is likely to be, then have a fuse.
The rating of the transformer is fairly conservative. You need to think about voltage overload ( which might cause insulation failure directly ), and current overload ( which will cause overheating, and insulation failure indirectly ). In any likely application of this transformer, current overload is more likely to be an issue. It then depends to some extent on ambient temperature, ventilation, and duration of the overload.
michinyon:
You can put another fuse on the output side, if you like.You want to try to design your system so that the transformer does not become overloaded. If you are in doubt about that, perhaps you need a bigger transformer. Particularly if you cannot easily ascertain in advance what the peak output current is likely to be, then have a fuse.
The rating of the transformer is fairly conservative. You need to think about voltage overload ( which might cause insulation failure directly ), and current overload ( which will cause overheating, and insulation failure indirectly ). In any likely application of this transformer, current overload is more likely to be an issue. It then depends to some extent on ambient temperature, ventilation, and duration of the overload.
Alright, all makes sense. Pretty much covers any questions I had in this thread.
Thanks again.
The purpose of the primary-side fuse, is to prevent causing a short-circuit on the mains supply. And to prevent the mains supply getting into the secondary side of your circuit, and causing a potentially fatal shock risk. Preventing you from overloading the secondary circuit, is not the main consideration.
If you put a 50 mA fuse on the primary side, you'd get the fuse blowing up often, which would be annoying. If they recommend a 250 mA fuse, that is what I would use.
michinyon:
The purpose of the primary-side fuse, is to prevent causing a short-circuit on the mains supply. And to prevent the mains supply getting into the secondary side of your circuit, and causing a potentially fatal shock risk. Preventing you from overloading the secondary circuit, is not the main consideration.If you put a 50 mA fuse on the primary side, you'd get the fuse blowing up often, which would be annoying. If they recommend a 250 mA fuse, that is what I would use.
Alright, makes sense. Thanks for your help.
One more question - With AC do I have to use a different kind of fuse? And should it be slow-blow?
With AC do I have to use a different kind of fuse?
No.