Generic Chinese 220V TDGc2-2kVA "Variac" Current Rating? - Page 1
Forum section: electronics / general

author: TheBay
author_rank: Unknown
timestamp: July 01, 2021, 11:11:15 pm
content: Just ordered a cheap generic "Variac" from Vevor, it's a typical red TDGC2-2kVA 220V model with the optional LCD Voltmeter.It turned up extremely quick from China, literally 3 days which was surprising.I knew what I was buying and what level of quality to expect, so I opened it up when it arrived and checked it all over.The toroid transformer isn't bad at all and the brush moves smoothly, the soldering left a lot to be desired (Which I expected) so I tided all that up and re did all the wiring. I also fitted a compression gland for the mains input as it was held in with a cable tie and hot snot.As this is a 220V model I moved the live tap on the transformer around a bit near where my voltage here is (Approx 240 - 250V) And put some kapton tape over the transformer terminals as they were all bare and a bit close to the chassis for my liking.So on to my question, the configuration of this "Variac" is 0-300V so it's tapped to boost rather than 1:1.What I can't quite get my head around is the current rating of this, I'm aware these should be derated and take the KVA figures with a pinch of salt.Let's say it is actually rated at 2000kVA, then at 300V if my maths are correct I make that around 6.6A.But what I can't figure out is, does the current capability drop when you are using the "Boost" part of the transformer?What is the actual current capability before the "Boost" part of the transformer, say I have the brush exactly on the part of the winding where the Live is tapped in to the transformer would the max current capability be 8A-ish?What happens if I move the Live around to the end of the transformer so it's technically 1:1, the reason I ask is there seems to be 2 versions of these "Variacs" 0-250V or 0-300V. But the current rating on both seem to be advertised as a random number, anywhere from 6A to 10A. Typically I've only ever seen a 0-270V Variac in work which had an option on there to select 1:1 or a slight boost.I do not really need above 250V, but making a new dial would be more hassle than what it's worth. Though I think it's daft they use voltages on the cheap "Variacs" opposed to percentages which I'm familiar with on genuine Variacs.The reason I'm trying to work out the actual max current rating and whether being 1:1 or boosting makes any difference is because it doesn't come with any form of fuse or protection on the output side. Yes the primary side has a 8A fuse but that's not much use when it's run below the same voltage as the input. So I want to put a circuit breaker/fuse on the output.Also I want to remove the awful mains output socket as it seems to be an "All in one" type that can take any plug from around the world (Think of a travel "death adapter") And fit a shuttered Type G socket.I may post some pictures of the inside/outside after I've made it all "Safe"

author: bob91343
author_rank: Super Contributor
timestamp: July 01, 2021, 11:20:19 pm
content: Just reverse engineer it. Measure the wire diameter and thus learn the gauge, then decide how hot you are willing to have it get. Maybe 500 circular mils per Ampere? It also depends on whether you want the fuse for protection of major fault or are concerned about small overloads.

author: bdunham7
author_rank: Super Contributor
timestamp: July 01, 2021, 11:33:21 pm
content: Quote from: TheBay on July 01, 2021, 11:11:15 pmThe current in autotransformer windings varies a great deal with the settings. For the equations, look at Wikipedia:https://en.wikipedia.org/wiki/AutotransformerAt precisely 1:1, the current capacity of the autotransformer is virtually infinite, limited only by the brush and the input fuse, which makes sense as it is a direct connection. The winding current is just the residual current and is independent of load. As you move away from the 1:1 point, current in various sections of the autotransformer go up, but not nearly as much as they would in a transformer with separate windings. Your variac in boost mode will have more input current than output, but the maximum current at any point will be the output current, provided it has only the typical 10-20% boost.You absolutely should add output protection, preferably in the form of a slow-blow fuse. The way to burn out an autotransformer is to start it at zero into an overload or short circuit and slowly turn up the power. Since it is functioning as a step-down in this case, the input current will be multiplied many times. Since the windings are typically all the same size, the bottom of the winding can be burned out quickly while the input current remains low.

author: TheBay
author_rank: Unknown
timestamp: July 02, 2021, 11:49:29 am
content: Taken the cover off today to measure the wire diameter and took some pictures while I was at it.The wire measures approx 0.71mm which is SWG22 (UK) or AWG21 (RoW) Bearing in mind although these may seem small and look odd to those familiar with Variacs in 110V countries, this winding will be a significantly smaller gauge for 220/240V countries.I'm trying to locate a decent Type G panel mount mains socket to fit that will replace the "Multi socket" as I really do not like this.Then need to work out what size fuse/breaker to fit on to the output. Hopefully I can get some help on here regarding current rating with the information I have so far on it. I haven't measured the brush size yet but it seems to cover 2 of the windings at any given time.While I'm in there I'm going to add some insulated banana sockets on the front panel so I can easily put one of my multimeters on it to look at actual output voltage on&off load.I'm also tempted to put a pair of banana sockets on the output so I can use an external ammeter/multimeter to monitor current, I can make up a pluggable link to defeat that if I don't wish to have it going through an ammeter.I've put a bit of green tape at the top where it's exactly 1:1 so I can easily see that when adjusting it, I also scraped back some of the powder coat and bolted the earth terminal to the chassis, I'm going to add another earth terminal to the front panel to that point on the chassis and fit some star washers to all the screws. OCD made me scrub out the incorrect voltage rating on the top plate and write 240V there with a Sharpie and cover that with some clear adhesive vinyl (It had a 110V Plate fitted)A lot of things I repair are quite old and I buy lots of "Parts not working" items and repair them. And had quite a few things here that are "New Old stock" which have been sat for maybe 30+ years unused. Also when I design something I like to bring it up slowly to save blowing the arse out of expensive components. So this should be quite handy in the workshop as a companion to my "Lamp current limiter".I spotted quite a few "Halogen" lamps in my local bargain store today, so hoarded a few of those in different wattages for my lamp limiter, as the sale of halogen lamps are going to be banned soon (supposedly).

author: Ian.M
author_rank: Super Contributor
timestamp: July 02, 2021, 12:37:01 pm
content: Ignore the VA rating and *ass*u*me* the 8A input fuse is correct for 220V operation. That's only 1760VA. Apply that at full boost to get the output current rating (5.86A). All other wiper positions will stress it less.Now how much do you need to derate it for being pure Chinesium? Its *probably* good for about 3A in continuous use!Also see my comments on 'Red Peril' variacsand in Tkamiya's thread linked from that. At the very least you need to check all housing parts and the shaft are effectively grounded.

author: TheBay
author_rank: Unknown
timestamp: July 02, 2021, 01:32:02 pm
content: Quote from: Ian.M on July 02, 2021, 12:37:01 pmThank you for those links, I noticed that fuse issue before I powered it on. I hate those type fuse holders and should only be used for ELV. I plan on swapping that input fuse holder for something else. I would eliminate it but the closest value fuse I can put in the plug top would be 5A or 10A. None of those are really suitable, but saying that if I did derate it then 5A may suffice. Mine was wired from the factory with the Live input on the rear of the fuse. But that is still a hazard for a shock if you touch the end of the fuse while it is still in the holder. Or there is a swap of L/N somewhere (Very unlikely here).I just checked the shaft and it wasn't earthed. I quickly remedied that just now.The shaft comes through the bottom and has the following. Chassis > Large washer > Ball bearing race with "hat" > Nylock nut.I removed all the above and scraped off the Powder coating between the chassis and Large washer. I added a "Star" washer between the Large washer and chassis and now the shaft is earthed. I will re-pack the ball bearing race with conductive grease.I may swap out the wire that goes to the brush with a silicone / higher quality wire and sleeve it as that part moves around with the brush when the knob is turned.I noticed that this one does have a heatsink on the brush. That whole assembly is isolated from the shaft by some fibreglass reinforced plastic.

author: TheBay
author_rank: Unknown
timestamp: July 02, 2021, 02:18:06 pm
content: Pictures of the brush assembly.

author: Ian.M
author_rank: Super Contributor
timestamp: July 02, 2021, 02:33:29 pm
content: That looks quite promising, and is a pleasant surprise compared to the 500VA one Tkamiya tore down, then scrapped as not worth fixing its design/construction issues.The input fuse is only to prevent disaster if it develops a shorted turn, so I'd go with a 10A BS1362 plug fuse and initially a 5A fuse on the output, then crank it up above line voltage with something like a 1KW bar fire for the load (possibly with a fan pointed obliquely at it so you can over-run it more) and see how the section of the winding between the line tap and the wiper heats up before deciding if it needs further derating.

author: themadhippy
author_rank: Super Contributor
timestamp: July 02, 2021, 02:34:17 pm
content: QuoteOr a 7A,https://uk.rs-online.com/web/p/cartridge-fuses/0412582/

author: bdunham7
author_rank: Super Contributor
timestamp: July 02, 2021, 03:00:46 pm
content: Given that there is a 7A plug fuse available as shown above, I'd suggest using that plus repurposing the spot where the fuse is now to wire in a 5A circuit breaker on the output side, if you can live with a lower maximum output. If you need more power, those wires are wound on a single layer and their ampacity is probably higher than you might think. I'm not sure how far I'd push it, but I'd bet even money it could handle 10A on both the input and output without actually bursting into flames. Not that I'm recommending that approach--derating is safer.

author: TheBay
author_rank: Unknown
timestamp: July 02, 2021, 04:43:18 pm
content: I had no idea a 7A cartridge fuse existed! That looks perfect.I cut the Schuko plug off as soon as it arrived and binned the death adapter it came with, I fitted a heavy duty "Permaplug" to it, I'll change the fuse in that (I put a 10A) in there and I'll remove that awful fuse holder and use the hole for one of the insulated banana sockets I want to fit.I'll mount a circuit breaker on the side of the front panel.I did load it up with a 1500w fan heater yesterday with a UNI-T mains power monitor plugged in and watched the current draw, voltage stability etc and it seemed to cope fine with it. It warmed up slightly overall but I mean luke warm not hot. I then moved it to the 1:1 position and had it running at least 30 mins, then I took it all apart and checked the windings and the cabling which was fine.I don't like the wires in there as they have some really cheap insulation, when I resoldered the whole thing I noticed the insulation melted a bit too easily. But I can say they are not CCA.I have no idea where they got their solder from, it all looked like cold joints even though they were not and it had an AWFUL rubbery smell to it when heated up. I can only think they recycle solder or have some strange flux. It certainly didn't look like it had any lead content.None of the wires had been pushed through the terminals from the factory so I removed all the old solder, cut the wires and J hooked them all through the terminals for now with some fresh solder on there.That's reassuring knowing there is some hope for this one, I was fully aware that it wasn't going to be usable out of the box before I ordered it and looking at the shortfalls (that are correctable) it really should not be on sale for the general public.

author: bdunham7
author_rank: Super Contributor
timestamp: July 02, 2021, 05:20:02 pm
content: Quote from: TheBay on July 02, 2021, 04:43:18 pmSo instead of buying old junk and fixing it up, we now can buy new junk and fix it up!

author: Gregg
author_rank: Super Contributor
timestamp: July 02, 2021, 05:24:28 pm
content: It looks like a considerable amount of the winding wire has been sanded off where the brush makes contact; this may be another reason to de-rate the advertised output current.

author: TheBay
author_rank: Unknown
timestamp: July 02, 2021, 05:42:56 pm
content: Quote from: Gregg on July 02, 2021, 05:24:28 pmYes that's a very valid point, It's SWG22 and the brush seems to cover 2 parts of the winding and I don't think by looking at it that it's ground down anywhere near half. So it should still be at least the capacity of SWG22 plus a little more then minus the resistance in the windings.