Where to attach earth on this Variac - Page 1
Forum section: electronics / projects

author: windmill john
author_rank: Contributor
timestamp: April 04, 2023, 04:18:34 pm
content: Hi All,I recently bought this variable transformer:https://uk.farnell.com/multicomp-pro/mc2f-1/transformer-variable-2a/dp/1086028?CMP=i-bf9f-00001000I obviously want to earth it correctly when I build a case for it, but am unsure where; remember ‘E’ on this is live.The four mount holes are painted. Do you think I could take the two earths to the two mounting holes at the bottom of the control board? Or open to ideas.Thanks.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 04, 2023, 04:55:35 pm
content: I think your power system is similar to ours, where the neutral is grounded (earthed) back at the sub panel and never anywhere else. I'd put the transformer in a metal enclosure and the incoming ground would tie to the enclosure. The outlet on the enclosure would also have its ground tied to the enclosure on the same stud as the input. None of the connections on the transformer get grounded and I've no idea how/why they called the output "E". Only the frame of the transformer gets bolted to the enclosure and is thus grounded (earthed). Neutral should go to the bottom of the winding and line to the top, such that when the transformer is turned down, the output is at neutral. Turned up it will be near or at line. There are several taps so you can do over-voltage if you like. Power is never applied to the wiper- lots of Variacs have been fried that way! Hopefully somebody with UK experience can explain this better!

author: windmill john
author_rank: Contributor
timestamp: April 04, 2023, 05:33:22 pm
content: Thanks for that. I actually was going to make a wooden case; vented.Can you think of any reason why I couldn’t tie the incoming and outgoing earths together to a fixing inside the case that I make?

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2023, 05:40:44 pm
content: Any exposed metal part should be tied to earth, this includes the metal shaft of the variac. Scrape a bit of the paint off if you need to and attach a wire to the frame, you can get those prickly washers for this sort of purpose.

author: Gyro
author_rank: Super Contributor
timestamp: April 04, 2023, 05:58:23 pm
content: A metal enclosure (or at least, front panel) would be preferable - wooden wall thickness might pose problems with ventilation holes of a safe size too. It would also give you a good thickness mounting surface for the Variac (check the available length of the shaft, it won't be very long), provide a solid point for a proper common earthing point and ensure that the mounting screw heads are earthed.You will also want something to mount a mains indicator (trust me) and most importantly a 2Aoutputfuseholder. Variacs normally get destroyed by trying to take more than their rated current at low output settings. This situation won't blow the input fuse.P.S. As Conrad says, the 'E' marking on the wiper connection initially looks a bit confusing, but when you look the datasheet, it is clear that it is just a consequence of the ABCDE sequencing of the terminals.

author: windmill john
author_rank: Contributor
timestamp: April 04, 2023, 07:20:34 pm
content: Thanks all, you’ve given me the answer/s I need.I take on board the fuse warning and will follow.Can’t wait for the days I can provide some answers to others!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 05, 2023, 02:21:02 am
content: Though I've been known to build things on a traditional wooden breadboard, wooden chassis' make me nervous. I know the odds are low, but it seems like a fire hazard that didn't need to be one.

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2023, 05:38:50 pm
content: I have several nixie tube clocks in wood enclosures, and several antique radios that also have wood enclosures, and it was once common for TV sets to have wood cabinets. Personally I don't see it as an issue, it should be extremely unlikely for a variac to catch fire, especially if it is fused on both the input and output.

author: windmill john
author_rank: Contributor
timestamp: April 05, 2023, 06:28:24 pm
content: I’ve had a thought and I’m going for a combination.I’ll have an aluminium front, bolted through and it will have wooden sides, base and top.For ventilation and any heat, I’ll have a mesh rear; also keeps fingers out.I’m not normally adventurous! but I’ll fit a mains indicator light to the front and also a little on/off toggle switch; don’t need to, but what the heckI’m afraid I’ve just been made redundant, so this will need to wait a little.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 05, 2023, 07:58:35 pm
content: Only to the chassis/ frame. None of the terminals are to be grounded.Watch out. These things are autoransformers and give you no isolation from mains.If you want to be safe you need an additonal transformer (1:1) between the variac and your precious parts.

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2023, 09:46:08 pm
content: There are times when an isolation transformer is beneficial, but usually it's best to just not touch live connections. There are many situations where an isolation transformer won't prevent a shock anyway, better to not be careless, especially in 240V land.

author: Gyro
author_rank: Super Contributor
timestamp: April 05, 2023, 09:49:16 pm
content: It also depends whether your RCD (if you have them fitted) or isolation transformer will provide you with greater protection in any given situation (hint: it's not always the same answer).

author: Terry Bites
author_rank: Super Contributor
timestamp: April 05, 2023, 11:18:54 pm
content: True. An isolation transformer will only protect you from a current with a path to earth.The secondary terminals will fry you just the same. Safety depends on user brain power.

author: windmill john
author_rank: Contributor
timestamp: April 06, 2023, 04:31:12 pm
content: Thanks all. I’d already purchased an isolation transformer. I’ve been reading for quite a while. All my time so far is focusing on safety with no actual project. And to be honest, I’m glad I’ve read so much and seen what should happen… one hand, discharging caps etc.It’s interesting, because a few years ago, I made a lamp current limiter, got an old valve radio and at the time didn’t know where to start and did nothing.I’m glad that since that time I’ve learnt a lot more about safety!

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: April 06, 2023, 04:52:45 pm
content: Quote from: Conrad Hoffman on April 05, 2023, 02:21:02 amI have a 3A variac in a metal enclosure (32yrs old) and a 10A variac in a wood enclosure (unvented, 25 yrs old). I've used both of them hard with no issues.

author: Gyro
author_rank: Super Contributor
timestamp: April 06, 2023, 08:41:11 pm
content: Quote from: Terry Bites on April 05, 2023, 11:18:54 pmImportantly, if you accidentally ground the secondary side, for instance with grounded test equipment, itwon'tprotect you from current with a path to earth. The isolation will prevent an RCD from tripping. This is probably the most likely accident scenario! Placing yourself across both ends of a floating secondary simultaneously is actually pretty unlikely [Edit: and more dangerous than placing yourself across the primary on an RCD protected circuit].Isolation transformers were good for working on things like live chassis TVs when the consumer unit was full of fuse-wire, but time has moved on.

author: windmill john
author_rank: Contributor
timestamp: April 12, 2023, 07:10:49 am
content: Quote from: Gyro on April 06, 2023, 08:41:11 pmNot in my house it hasn’t!

author: wasedadoc
author_rank: Super Contributor
timestamp: April 12, 2023, 09:01:46 am
content: Quote from: windmill john on April 12, 2023, 07:10:49 amMe too. New build 1976 UK. A four position fuse box withe the same type rewireable fuses as windmill john. One 30A ring main, one 30A cooker circuit, one 15A immersion heater, one 5A lighting circuit.

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2023, 10:50:02 am
content: Our house, of '80s vintage also had a rewireable fusebox, in our case 6 way - 2 rings, 2 lights, immersion and cooker. It is now upgraded to an RCBO one. At 1976 I doubt there would be any VIR (Vulcanised rubber) cabling so it probably wouldn't be a difficult upgrade. Windmill john's look like PVC too, unless there are other older hidden circuits.In cases like this I wouldReallyrecommend either fitting an RCD socket in your work room or one of those in-line plug in ones. That will at least give you the same earth leakage protection as a modern consumer unit. The plug fuse takes care of the rest. Use one on the lawnmower / hedge trimmer while you're at it too.

author: wasedadoc
author_rank: Super Contributor
timestamp: April 12, 2023, 11:13:18 am
content: Quote from: Gyro on April 12, 2023, 10:50:02 amI do have an RCD socket in a garage where the submerged pond pump is plugged in. And I do use an extension lead with RCD plug when using electric hedge clippers. That has been useful once when I accidentally cut the cable!When working on SMPSs I power them from a UPS. Unplugged from mains power of course.

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2023, 11:20:27 am
content: QuoteOnly once? I ended up fitting an in-line piug near the handle so that I can easilyshortenre-terminate the cable as reguired!

author: themadhippy
author_rank: Super Contributor
timestamp: April 12, 2023, 11:59:47 am
content: QuoteOnly issue with cable from that era is the possibility of the dreaded green goo bleeding from the wires.

author: windmill john
author_rank: Contributor
timestamp: April 12, 2023, 05:21:40 pm
content: 1960s my house.Four times I cut through my mower cable! (See where I learned my wiring skills) The fifth time, I gave up and bought a cordless mower.