Do I need to wire up a ground for my diy bench power supply? - Page 1
Forum section: electronics / beginners

author: PotatoBox
author_rank: Regular Contributor
timestamp: May 05, 2020, 12:01:43 am
content: So I'm making a basic linear power supply with a transformer and an lm317. I wasn't planning on wiring up the mains ground since I'm using a plastic enclosure but I need to drill holes for the mounting screws in the enclosure, thus exposing the metal screws on the outside of the enclosure. Should I wire up the mains ground to the mounting screws of the transformer? Should I also wire up the mains ground to the ground of secondary side of the transformer with all the lm317 circuitry?

author: Ian.M
author_rank: Super Contributor
timestamp: May 05, 2020, 12:18:31 am
content: All exposed metalwork, and the transformer core if its got an exposed core, should be grounded. Also, put a green binding post for ground next to the ones for the output, but leave the secondary side non-grounded, so you can choose whether or not to ground the output (and even what side of the output to ground) depending on your application.

author: PotatoBox
author_rank: Regular Contributor
timestamp: May 05, 2020, 01:56:08 am
content: Quote from: Ian.M on May 05, 2020, 12:18:31 amWhy would I need a mains ground connected to a separate output? What kind of projects would require this?

author: Brumby
author_rank: Unknown
timestamp: May 05, 2020, 02:31:13 am
content: The reason to keep the ground separate on the secondary side is so that your output will be floating. This isessentialwhen you want to connect two or more power supplies in series.(1)You canchooseto have it earth referenced by connecting either the positive or negative output terminal to the earth terminal.(2)By hard wiring the earth to the secondary side, you remove that ability(1)and that choice(2)

author: PotatoBox
author_rank: Regular Contributor
timestamp: May 05, 2020, 02:51:23 am
content: Quote from: Brumby on May 05, 2020, 02:31:13 amI don't plan on having two power supplies in series (I'm only building this single one) but how would you go about this?

author: cdev
author_rank: Super Contributor
timestamp: May 05, 2020, 03:40:26 am
content: Its safer. Also, in the future you'll likely have times come along where you'll find you need the extra voltage or current.

author: PotatoBox
author_rank: Regular Contributor
timestamp: May 05, 2020, 05:53:59 am
content: Quote from: cdev on May 05, 2020, 03:40:26 amHow is it safer? Are you referring to the fact that the AC mains wiring (if the insulation is fraying) may come in contact to the secondary side of the transformer as well as the lm317 and essentially making its way to the output of the DC + and - binding posts?

author: Ian.M
author_rank: Super Contributor
timestamp: May 05, 2020, 11:05:12 am
content: Grounding exposed metalwork and the transformer core is for safety. If its a split bobbin transformer, (or otherwise has reinforced insulation that meets the safety requirements for a double-insulated device) there is no need to ground the secondary circuit, provided its impossible for any of the mains wiring to come in contact with the secondary side, (and visa-versa) even if any of the wires break at their terminations.Bringing out the ground on a separate terminal that can be tied to either output terminal if required is for convenience and versatility, not safety.

author: PotatoBox
author_rank: Regular Contributor
timestamp: May 05, 2020, 08:38:58 pm
content: Quote from: Ian.M on May 05, 2020, 11:05:12 amI'm using a transformer that looks like this with an enclosure around it:Is the transformer enclosure connected to the core? Will I need to ground the transformer enclosure?

author: Ian.M
author_rank: Super Contributor
timestamp: May 05, 2020, 08:55:39 pm
content: Quote from: PotatoBox on May 05, 2020, 08:38:58 pmMost probably yes, though it may not be a very good connection due to the oxide and/or varnish coating on the laminations.QuoteYes, but simply grounding one mounting lug should be good enough if you use a ring terminal on the wire and a shakeproof washer and a Nylock nut on the bolt so the ground connection cant easily loosen due to vibration.

author: djacobow
author_rank: Super Contributor
timestamp: May 05, 2020, 09:03:44 pm
content: I'd definitely leave the secondary floating -- as others said, that's a feature, not a bug. As for grounding the transformer, I guess if you're using metal screws to hold it in place that are accessible outside the box, then you have little choice but to do it. But since you are using a plastic enclosure, consider using nylon screws, and then it doesn't matter.

author: PotatoBox
author_rank: Regular Contributor
timestamp: May 05, 2020, 09:27:41 pm
content: Also this is a bit off topic but what would be a good way to restrain the power cord? I remember seeing these round rubber cots where you drill a hole into the enclosure and fit it and provides strain relief for the power cord. What are these called?

author: Gyro
author_rank: Super Contributor
timestamp: May 05, 2020, 09:37:15 pm
content: Your description sounds like a 'grommet'. These will protect the cable from the sharp edges of the hole but are not sufficient to secure the cable. The restraint needs to be able to prevent the cable from pulling out, being pushed in, or rotated (any of these can damage your internal wiring). A snug fitting P-clip screwed to the inside of the case works well.Alternatively you could fit an IEC connector to the back of the case and use a standard plug-in mains lead. This is useful for storage, and for not tripping over the mains lead when carrying it around.P.S. You can get IEC panel connectors with built-in fuse holders (and switches) which can solve two problems in one.

author: fourfathom
author_rank: Super Contributor
timestamp: May 05, 2020, 10:01:35 pm
content: Quote from: PotatoBox on May 05, 2020, 09:27:41 pmA "Heyco" strain-relief is pretty common:But I do like the IEC power connectors with the built-in fuse.

author: johnkenyon
author_rank: Regular Contributor
timestamp: June 03, 2020, 09:04:07 am
content: Quote from: fourfathom on May 05, 2020, 10:01:35 pmFor years (going back to 1988) the box that held a 24V 50VA transformer "base station" for my temperature controlled soldering iron had one of those strain reliefs and a 3 metre cable.I drilled a hole and installed an IEC inlet connector when I got fed up with having to loop and unloop the cable. (no built in fuse - already had that installed on the front panel).If you have a permanent work bench - use a cable+strain relief, if the unit gets returned to a storage location (=shelf) after use, go for an IEC inlet.