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My own power supply is just about finished. Since i live in Europe the voltage here is 230v, from that it goes into my step down transformer that steps down the voltage to around 20 V AC and then into the power supply, etc.

If I check in a normal PC switching power supply the AC ground/earth is connected to the chassis of the power supply. Since i have a wooden chassis for my power supply how do i connect the earth wire to be "safe"? I understand all the risks of working with AC voltages and i try to be as careful as I can. Can I just put some "metal piece" inside of my wooden chassi and connect earth to that?

And just to clarify the AC wire I have contains of 3 wires, One live wire, one ground wire. and one green earth wire. Looks like this:

enter image description here

I have the exact same colors of my wires as the pic.

Pictures of the transformer I'm using: enter image description here enter image description here

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  • \$\begingroup\$ DO you expose any of the internal electronics to the 'outside world' in any way? Any wires/connectors coming out? Any user controls made of anything metallic? \$\endgroup\$ – brhans Mar 29 '16 at 15:17
  • \$\begingroup\$ No, from the 230v ac soldered to the transformer (with shrinking tubes). From transformer soldered to the PCB.s bridge rectifier. So the only thing that is "exposed" is the soldering joints of the bridge rectifier if that counts. \$\endgroup\$ – Xane Mar 29 '16 at 15:19
  • \$\begingroup\$ What sort of power supply have you built - linear or switching? \$\endgroup\$ – Andy aka Mar 29 '16 at 15:22
  • \$\begingroup\$ Linear one, based of the LM317. So nothing advanced. \$\endgroup\$ – Xane Mar 29 '16 at 15:23
  • \$\begingroup\$ So - thought experiment - what happens if the insulation in your transformer fails between the primary & secondary windings ... ? \$\endgroup\$ – brhans Mar 29 '16 at 15:24
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Suggested earthing point.

As this isn't a commercial project your primary concern is to keep the system safe for you and other occupants of the house. Dry wood is normally a good insulator but if it gets wet the situation changes. You also need to consider any other possible electrical paths out of the enclosure: for example, is the transformer bolted to the chassis and can the bolts be touched from the outside?

You could consider earthing the transformer frame as a safety precaution. This will

  • blow the fuse or trip your RDC (ground-fault detector) should the transformer frame ever go live.
  • earth any transformer bolts protruding though the case.
  • trip your RCD should the case get wet.
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  • \$\begingroup\$ Thanks for your answer, This seems like a smart way to do it. Since my transformer is rated for 4A max, Should i use a 4A fast blow fuse? \$\endgroup\$ – Xane Mar 29 '16 at 15:32
  • \$\begingroup\$ @Xane Keep n mind that transformers will have higher in-rush current the moment you turn it on. The current will also depend on the state of magnetization of the transformer when you turn it off, so a too small fuse will sometimes blow and sometimes not. Here's an answer I got to similar question few years back. \$\endgroup\$ – AndrejaKo Mar 29 '16 at 15:35
  • \$\begingroup\$ Thanks for the tip, since i have many fuses laying around, I can try using a 4A slow blow one, If that blows i can try with a 5A fast blow or something similar. \$\endgroup\$ – Xane Mar 29 '16 at 15:36
  • \$\begingroup\$ Well this answered my question, Thank you folks! :) \$\endgroup\$ – Xane Mar 29 '16 at 15:54
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If you have an insulated case then you don't need to earth the case but you may need to earth other things.

I made it as a lab power supply for powering various circuits.

In other words the output of this power supply will likely be connected to open bench circuits which can be touched. That means that it needs to provide adequate safety isolation from the mains.

Your next step is to look up the specifications of your transformer to see whether it is suitable for doing that and if-so whether the transformer is class 1 (in which case an earth connection to the transformer frame is needed) or class 2 (in which case an earth connection to the transformer frame is not needed).

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  • \$\begingroup\$ Thanks for your answer, i really doubt that i will find the specs of the transformer since i found it in an old amplifier. but i can try and look. \$\endgroup\$ – Xane Mar 30 '16 at 14:39
  • \$\begingroup\$ Does the transformer have a metal body? if it does then in the absense of better information I would advise earthing said transformer body. \$\endgroup\$ – Peter Green Mar 30 '16 at 15:06
  • \$\begingroup\$ Yes it has a metal body. i can take a picture of it a while. \$\endgroup\$ – Xane Mar 30 '16 at 15:09
  • \$\begingroup\$ Updated question with pictures \$\endgroup\$ – Xane Mar 30 '16 at 15:39
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the wire is there to detect earth faults and safely remove any charge on the chassis. since you have a isolating chassis, there is no need for an earth connection at all. You have a Class II device: https://en.wikipedia.org/wiki/Appliance_classes#Class_II Of course you need a double insulation and whatnot to meet regulations, something, that's probably not going to work out with wood

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  • \$\begingroup\$ I see, So i can skip the earth wire completly since I'm using a wooden chassi? \$\endgroup\$ – Xane Mar 29 '16 at 15:14
  • \$\begingroup\$ Its not just the chassis which determines the appliance class. \$\endgroup\$ – brhans Mar 29 '16 at 15:18
  • \$\begingroup\$ @Xane, you have to make sure that you design the circuit such that it does not require a safety ground. Per the link: "The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing." \$\endgroup\$ – user65586 Mar 29 '16 at 15:19
  • \$\begingroup\$ I just read that on the wiki hehe, Well then i should be safe, i have went through everything many times. And there is no way that i can get a shock with my current setup. I was just a little unsure about the green earth wire if i needed it or not. Thank you \$\endgroup\$ – Xane Mar 29 '16 at 15:22
  • \$\begingroup\$ It's not needed for operation anyway, so yes you CAN skip it. If you SHOULD is another matter. As the links says: No single fault should allow a high voltage to be accessible from the outside, you're also skipping a very easy way to detect such faults. Maybe you should build an inner chassis to connect to earth. \$\endgroup\$ – Christian Mar 29 '16 at 15:23

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