Electronics > Beginners
power input for small transformer, good practice
FlyingHacker:
--- Quote from: Yansi on July 01, 2018, 11:41:02 am ---
*Some freaked out countries even put another fuse in their crazy looking plugs, to compensate for the simple and dangerous design of their plugs. ^-^
--- End quote ---
I actually think the UK plugs are the safest design out there. They are huge, but they have a ground that makes first and breaks last, and the have insulation on the love and neutral which prevents any thin metal object (e.g. metal ruler) that may have slid down between the plug and the wall from shorting out. The fuse in the pug is a brilliant idea, though I would still put a fuse inside the piece of equipment as well.
I have seen tons of blown glass fuses in equipment at 120V here in the US, and none of them have ever ruptured the glass. No argument the HRC are better, but I am just not sure how important they are at 120V, especially when in a proper, covered fuse holder.
mariush:
If you want low noise, wouldn't it make more sense to go with a toroidal transformer?
At the very least, it would allow you to easily wrap it in some conductive material like copper foil and then ground the foil, and you got yourself a nice shielding. Even without extra shielding, the magnetic field would be inside the toroid and all that..
At 10VA the transformer would also be smaller and if my memory is correct, at such low VA rating, the output voltage should not vary so much with the load (compared to standard transformer)
You may have higher inrush current
C:
--- Quote from: Cerebus on July 01, 2018, 12:02:59 pm ---Oh, and breaker panel fusing usually 15-30A in 220V countries, typical IEC cable rating 6-10A. So the breaker will open when the cable from the wall is overstretched will it?
--- End quote ---
The main purpose of a breaker or fuse is to prevent fire.
So how hot will your typical IEC cable rating 6-10A get with a breaker panel fusing usually 15-30A.
Heat of cable = time + power.
What is the temperature rating of the insulation?
To have possibility to start a fire you have up to when insulation fails.
And NO the fuse or breaker is not there to protect the source of power in a properly designed power system. The power source has a fuse or breaker that serves this function.
Simple electricity rules apply.
Cerebus:
--- Quote from: C on July 01, 2018, 10:20:17 pm ---
--- Quote from: Cerebus on July 01, 2018, 12:02:59 pm ---Oh, and breaker panel fusing usually 15-30A in 220V countries, typical IEC cable rating 6-10A. So the breaker will open when the cable from the wall is overstretched will it?
--- End quote ---
The main purpose of a breaker or fuse is to prevent fire.
So how hot will your typical IEC cable rating 6-10A get with a breaker panel fusing usually 15-30A.
Heat of cable = time + power.
What is the temperature rating of the insulation?
To have possibility to start a fire you have up to when insulation fails.
And NO the fuse or breaker is not there to protect the source of power in a properly designed power system. The power source has a fuse or breaker that serves this function.
Simple electricity rules apply.
--- End quote ---
From what you've said I suspect that you don't understand that we've been talking about the breaking capacity of a fuse, not its load capacity. The load capacity of the fuse is the current it opens at (very approximately - the fuse's rating is more properly specified as some value of I2t). The breaking capacity is the flowing fault current that it can effectively interrupt or, put differently, the largest fault current that it guarantees to interrupt. That's why I say you have to consider the supply in choosing the breaking capacity of the fuse.
The earlier example was of a 1A load capacity fuse with a 1.6kA breaking capacity versus a 1A fuse with a 35A breaking capacity. If the current flowing in a fault exceeds the breaking capacity of a fuse, that fuse will fail to open the circuit and can even induce a higher current fault itself, tripping the next bit of protection upstream. So, an example for the latter case would be that 1A glass fuse with a breaking capacity of only 35A. It's very easy to generate a fault current of >35A with a simple accidental dead short - loose wire, loose screw, that kind of thing. The 1A HRC fuse would break that fault current, the 1A glass fuse wouldn't and might even become a lower impedance than the fault it was trying to isolate.
A fuse that fails to isolate a fault in a point downstream of itself becomes a liability to points upstream of it and part of ensuring that does not happen is to choose a correct breaking capacity based on the upstream's potential for supplying a fault current to a downstream fault. Yes, part of the criteria for a fuse is "Does this have the breaking capacity to protect the upstream supply?". So in a "properly designed power system" properly chosen fuses/breakers protect both the load and the supply.
Cerebus:
--- Quote from: FlyingHacker on July 01, 2018, 08:41:52 pm ---I actually think the UK plugs are the safest design out there.
--- End quote ---
Until you stand on one while barefoot. Then they become the most painful plug design out there.
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