DIY Linear PSU - Safety Considerations

Thread Starter

RobinGriffiths

Joined Aug 16, 2011
14
Hi,

I'm designing a simple dual rail PSU using linear regulators. I have a couple of safety/best practice questions.

1. Fuses. I presume the best place to put fuses is between secondary and rectifier rather than after the regulated outputs?
2. Is it safer to mount in an earthed metal enclosure, or unearthed ABS or similar?
3. Is it safer to allow ground to float, or connect to mains earth ?

Thanks,
 

AlbertHall

Joined Jun 4, 2014
8,486
I would put the fuse on the mains supply to the transformer.
I would prefer a metal box, connected to mains earth, and also earth the outputs.
 

Hymie

Joined Mar 30, 2018
710
I would recommend fuses in each of the secondary transformer output windings before the bridge rectifier. This allows any overload condition to be better limited.

e.g. consider a mains transformer with two secondary windings, say each 12Vac at 35VA protected by a 3.15A fuse. Given that a typical fuse will operate at twice its nominal current rating, the worse case continuous load within the circuit would be around 70VA.

Now consider that the transformer primary winding (at 240Vac) protected by a 0.5A fuse – it is possible that a continuous fault load in the secondary circuit could reach 240VA, and the 0.5A fuse is likely to be prone to nuisance tripping due to the switch on inrush current.

If mounting within a plastic enclosure, you may need to ensure that the transformer core is earthed if there is only Basic insulation between the primary winding and the core. A metal enclosure will provide better protection against a transformer overheat condition which could melt/deform plastic material. If using a metal enclosure, it is good practice to earth the accessible metal parts even if they are adequately separated from the mains.
 
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