So I wish to build a variable output mains power supply as a learning exercise.

I have determined the transformer that I need and now I am looking at the rectifier bridge.

I have looked at these diodes http://uk.farnell.com/on-semiconductor/mur410rlg/diode-ultra-fast-4a-100v-axial/dp/2441588

Which should easily allow my supply to deliver 24v @ 2.5A

Am I right?

Do I need to look into protecting the rectifier from a short?

I don't see many online guides dealing with this.

Also when it comes to earthing. Should I ensure the body of the case and transformer are connected to the death pin of the 3 prong plug. Earthing 24v on the secondary coil seems overkill.

  • \$\begingroup\$ "Earthing 24v on the secondary coil seems overkill." In that case you are building a Class I product with an Class II output. To protect it from a short, you have the option of resettable fuse, normal fuse or thermal PTC type fuse. \$\endgroup\$
    – winny
    Commented Jul 30, 2016 at 9:03
  • \$\begingroup\$ @winny is that really bad? If I ensure the secondary circuit is well insulated and the 240v is earthed would that be sufficient? \$\endgroup\$
    – James
    Commented Jul 30, 2016 at 9:14
  • \$\begingroup\$ Not really, most power supplies are built this way but it requires double isolation. To meet the requirements, you must be able to withstand 500 Vac between earth and the secondary. \$\endgroup\$
    – winny
    Commented Jul 30, 2016 at 9:20
  • \$\begingroup\$ This type of supply (mains->50/60Hz transformer->rectifier->cap->regulator) generally turns into a design excercise in managing heat dissipation. When you allow for ripple and input voltage variation you will find that you have to dissipate a surprising amount of heat. I really recommend that you use off-the-shelf or scavenged (e.g. laptop) AC-DC converters. \$\endgroup\$
    – user57037
    Commented Jul 30, 2016 at 16:08
  • \$\begingroup\$ @mkeith thanks for the tip. Where is most if the heat generated? I guess in the regulators because I worked out the diodes to only reach about 5°C above ambient. \$\endgroup\$
    – James
    Commented Jul 30, 2016 at 17:23

1 Answer 1


The MUR410 diodes should work OK for you in this application, although you don't really need an ultra-fast diode for a 50/60Hz rectifier application.

It is not common practice to provide short circuit protection to rectifiers in power supply circuits like you are building. However, it would be good practice to place a fuse in the primary circuit to the transformer.

Grounding the case of the transformer is not a bad idea. But you don't want to ground the output of your power supply either at the secondary coil or the output of the rectifier(s) since most power supplies are used to provided ground-isolated voltages. Still, you should provide some connection means on the power supply to make the ground connection convenient on the occasions when it is needed. On commercial bench-top lab supplies this is usually done by including a green or black 5-way binding post which is electrically connected to the mains ground wire.

  • \$\begingroup\$ Ok thanks. I will look at other diodes that could perform the same function. Obviously I need a low voltage drop, but how fast do they need to be \$\endgroup\$
    – James
    Commented Jul 30, 2016 at 9:17
  • \$\begingroup\$ Do you think a 1N5008 would be sufficient? \$\endgroup\$
    – James
    Commented Jul 30, 2016 at 10:19
  • \$\begingroup\$ It's not that the MUR410's won't work in this application, it's just a little bit of overkill. But if you already have them, use them. I can't find the 1N5008 you refer to. If you are rectifying 50/60 Hz just about any rectifier diode with sufficient power, voltage and current specs will work. Speed is not important. Look at the speed specs for the 1N4001 as a guide. These are the jelly-bean rectifier diodes, though not a high enough current rating for your application. \$\endgroup\$
    – FiddyOhm
    Commented Jul 30, 2016 at 11:32
  • \$\begingroup\$ sorry I meant the 1N5408. It seems to be the higher current version of the 1N4008. I don't have any diodes to hand but one guide seemed to recommend BYV29-500 which seem very specialised. \$\endgroup\$
    – James
    Commented Jul 30, 2016 at 11:43
  • \$\begingroup\$ The 1N540X family is appropriate for your application. However, I would not use the 1N5408 specifically because it is the 1000 volt member of this family. The forward voltage drop may be too high for your application. Use the 1N5401 or 1N5402 instead. \$\endgroup\$
    – FiddyOhm
    Commented Jul 30, 2016 at 13:02

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