Variac Enclosure / Projects / Forums

Forums

4hv.org :: Forums :: Projects

« Previous topic | Next topic »

Variac Enclosure

Move Thread

LAN_403

Ruben

Tue Nov 23 2010, 04:30AM

Registered Member #3263 Joined: Sat Oct 02 2010, 04:43AM
Location: Sydney, Australia
Posts: 34

Hi All,

I'm planning a DRSSTC, but first I need something to power it with:

I picked up an open frame 240/280V 10A variac on eBay. It appears ancient, and very solidly built. In addition I scrounged an unmarked transformer from a TV on the side of the road.

Testing showed a 2.27:1 ratio (I assume it was used in a step down capacity, although I could be wrong) it weighs 2.5 kg, so hopefully it wil be good for ~250VA. This will provide me with an isolated 0-123v supply, in addition to the main variac output.

While I was cleaning up the variac, I accidentally broke one of the carbon brushes (and then cursed a lot) I managed to fashion another one using the brush from an old vacuum cleaner. It is visually identical, and testing to 2.5A showed no ill effects.

I lack quality tools and cash, so construction is mostly from MDF i have lying around. I'm aiming for something as robust as possible, with a tokenistic effort at keeping the weight down:

Gluing timber posts to screw into, the case taking shape:

No table saw, and old MDF means that the edges of the box are not particularly square or pretty. MDF itself will not wear well and is likely to split and fray over time. Thus I plan to use aluminium L-Section around the edges of the box for durability and aesthetic value.

I have illegitimate access to a band saw and bench sander (with set square) at uni, allowing for nice mitre cuts (I made a rookie mistake and didn't account for the 1.5 mm thickness of the Al, and thus cut everything 3mm short the first time -- RAGE):

*******

Panels sprayed matte black (2 coats exterior, one coat interior) and mounting panel meters, switch and E-Stop. I'm amazed at how much nicer painting things black makes them look.

The 15A toggle switch is DPDT, meaning I can switch between the 240 v and 280 v inputs to the variac. Everything up to the isolation transformer was wired with 12AWG, probably overkill for 10A, but it was what I had on hand.

Wiring everything up:

I threw away the tiny screws supplied with the carrying handles, drilled out the holes, and sunk M4 bolts right through the top MDF panel. Reason being, this sucker is heavy! I don't have any scales in the house, but I'm reckoning 20 kg.

***********
Finished!!

I like the look the aluminium gives, but note to self -- any future attempts at fully mitred corners must be damn accurate! I wasnt happy with the way the corners turned out, hence the gaffer tape

Mads Barnkob

Tue Nov 23 2010, 07:56AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Good idea with a isolated output, it will be needed when you want to measure directly on the bridge or GDT while the coil is running. (to avoid grounding the entire bridge through your scope)

But the power capability of such a small transformer will make it smoke fast if you start pushing that DRSSTC, so better make two outputs so you can run the DRSSTC just through the variac.

For my own variac I have made a "war time button" that basicly just switches over to a 20A fuse instead of 10A for 5 minutes, then a timer switches it back, future upgrades might be a temperature controller taking care of that instead.

Ruben

Sun Nov 28 2010, 01:40PM

Registered Member #3263 Joined: Sat Oct 02 2010, 04:43AM
Location: Sydney, Australia
Posts: 34

*Updated* -- More pictures of box construction

I have a few parts in hand but most are on order, or still to be purchased:

â€¢ 10A breaker on variac output
â€¢ 3A breaker on 123V isolation transformer.
â€¢ Fuse/breaker on input side GET
â€¢ Carrying Handles GET
â€¢ Bezel mounted neon â€˜liveâ€™ indicator GET
â€¢ LCD panel meter (requires 5V input) WAITING
â€¢ Analogue current meter on main variac output WAITING
â€¢ 15A DPST toggle switch WAITING
â€¢ Emergency stop button WAITING
â€¢ 80 mm fan and grille WAITING

To Do:

â€¢ Finish sanding box
â€¢ Cut holes for panel meters, neon, toggle switch E-Stop and fan/grille
â€¢ Drill holes in AL flashing, and pilot holes in MDF
â€¢ Dismantle box and spray inside & out matt black
â€¢ Reassemble, mount all components, wire things up.
â€¢ Test for heating at high load, especially DIY brush repair (2400W kettle as load)
â€¢ Close box, attach AL flashing, and handles.

Thanks Mads, I am definitely planning two outputs, one fused at 10A (variac output) and one at 3A (isolation transformer secondary). A power override button may come later, but for now I'm a little cautious, mostly due to my DIY brush replacement.

Ruben

Sat Dec 18 2010, 10:32AM

Registered Member #3263 Joined: Sat Oct 02 2010, 04:43AM
Location: Sydney, Australia
Posts: 34

*Updated* -- More Construction pictures.

I've now received everything I need except for a fan grill, and construction is almost complete; I expect to have it finished tomorrow.

Everything is wired up, and I load tested the set-up this afternoon (2000 W toaster) -- No smoke! Also no sparking or heating on my repaired brush which I'm quite happy about.

Mains voltage here is 250 V (measured), and I noticed that when loaded with the toaster, the output dropped by ~10v at full scale to 240 V-- is this normal behaviour for a variac, or is something amiss?

*edit* -- Just remembered that power was coming through a 10 m extension cord. I'll be testing tomorrow without it.

Inducktion

Sun Jan 23 2011, 05:08AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Ooh....That's really cool. I wish I had a variac. The case for it all looks fantastic though, almost like a professional briefcase or something. A bomb. Yes. :D

Adam Munich

Sun Jan 23 2011, 04:43PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I have to agree, that looks fantastic.

Proud Mary

Sun Jan 23 2011, 08:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Looks like good work to me, Ruben.

As for the corners, there are different sorts of corner protectors, like those used on flight cases, for example, which you could add later if you wanted to.

I see them advertised inexpensively on ebay as "steel ball corners" in both bright zinc plate and black.

James

Sat Jan 29 2011, 10:33PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

That's really slick! When I was a kid I used an old coffee can :)

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob