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Old 12th Sep 2019, 11:14 pm   #15
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Earthing a part metal enclosure for mains transformer ?

Quote:
Originally Posted by Herald1360 View Post


I disagree.... the peak inrush is determined purely by how saturated the TX core gets if switch on occurs at the worst point in the mains cycle relative to any remanent flux, the resistance of the primary winding and the primary inductance assuming a saturated core. Whilst a small TX will have much lower absolute inrush than a large one, relative to FLC it will still be "large".
On a couple of points, I mentioned in post #3 about the need for two nuts and lock washers on the chassis earth stud, this also prevents anyone unscrewing the screw from the outside if the head is exposed.

For power supplies of equivalent power rating, in the case of switching on a transformer primary across the mains power the peak inrush current will always be lower than switching on a switchmode PSU (without inrush current mitigation circuitry) because in the latter case you are applying the voltage directly (via the rectifiers with have a very low internal Z), from the mains source (a very low internal Z) to discharged capacitors which by their nature oppose a change in applied voltage, and it could be longer for them to get to the peak voltage than for the transformer's flux field to get established and the current to drop (if the timing of the power switch on was at zero crossing, encouraging a high peak initial primary current) and there is the primary resistance present in the transformer case to help limit the peak current and its not insignificant in small power transformers.

In most cases with transformer power supplies, you can work out the running power and current, and use a slow blow fuse 1.5 times that value or a fast blow fuse 2 x that value and not get nuisance (non fault related) fuse blows.

This is not the case for the switchmode psu (without inrush mitigation).

The method I use to select the fuse, (as outlined in the article I mentioned that has a 65W psu with no inrush mitigation) is to have the supply normally loaded, start with a fast fuse, with a low fuse value that blows at turn on. Increase the fuse value in steps until it survives at least 3 to 5 turn on attempts. Then use a fuse twice that current rating if a fast blow or 1.5 times that rating if a slow blow. In the long run, if it is sailing too close to the wind, the fuse you have selected will fatigue & fail after a few hundred turn on cycles, and you might need to increase one step up in current rating. Its all a much easier arrangement if the switchmode psu has inrush mitigation. Most if not all desktop computer psu's have it.

So what I'm getting at here is that if you compare two mains operated power supplies of equivalent power output, one a transformer, the other a swichmode psu , without inrush mitigation circuitry, mostly you will find the switchmode unit has a higher fuse rating to withstand the inrush currents.

Last edited by Argus25; 12th Sep 2019 at 11:20 pm.
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