Vari-what? Something I hadn't seen before - Page 1
Forum section: electronics / general

author: TerraHertz
author_rank: Super Contributor
timestamp: November 20, 2014, 01:19:26 pm
content: I've been gradually scrounging bits to bring my Zeiss microscope to life. One thing it lacked was an illuminator. The process of getting a working 100W mercury arc lamphouse and power supply is taking a while, however I did recently get a Zeiss tungsten bulb lamphouse. So needed a power supply for it, which is simple since all it needs is an AC or DC 0 to 12V supply.Generally I just use a bench power supply for the lamp on another microscope, however I also had an old Olympus microscope lamp supply with a few minor problems that I'd never used. So opened it up to see if it could be made to work, as a dedicated supply for the Zeiss.Something about it turned out to be quite surprising.Here's the unit, with a knob that's obviously meant to adjust the lamp voltage.Here it is with the back removed.Hmm.... so how does the knob vary the output voltage? There's a simple transformer, a meter, a mains switch, and nothing else visible. No electronics at all. Mysterious...Taking the front panel off reveals the answer. That 'transformer' is actually a variac, of a form I'd never seen before.Twin secondary windings, each with their own wiper on an arm that forms a series connection between the windings.With the knob at one end stop, no turns of the windings are in the output loop. At the other end of travel, all the turns are in series in the loop. Simple but clever.The only thing wrong with the unit turned out to be a broken zero-adjustment coupling on the moving coil meter, which was easily fudged to 'good enough'.The output pair of banana sockets has the standard 3/4" pin spacing, but wouldn't you know it, the Zeiss plug does not.Cutting odd plugs off semi-antique gear is something I'm loathe to do, so I made a quickie adapter from a standard banana pair terminator, a banana receptacle, and hot-melt glue. Horrible, pics below.But it works.

author: Zero999
author_rank: Super Contributor
timestamp: November 22, 2014, 01:24:42 pm
content: That's an interesting variac.I assume the secondary is isolated right? Is the primary winding inside?

author: TerraHertz
author_rank: Super Contributor
timestamp: November 22, 2014, 09:58:44 pm
content: Quote from: Hero999 on November 22, 2014, 01:24:42 pmYes, it's fully isolated. The two identical primary windings are inside the secondaries.Electrically it's trivial. But still, a fully isolated variac is pretty neat. I don't know why this topology isn't more common, since there are many applications where it would be useful. Plus the coils can be wound before assembly onto the laminated core. I'd expect the patent, if any, is expired by now too.The only downside is that the control rotation range is only about 100 degrees, instead of the nearly 360 degrees of a standard toroidal variac.The twin primaries mean it can be wired for 120 or 240V supply - another thing standard variacs can't do.I wish they could - a few years ago I acquired a large three phase variac (and I have three phase power here) but curses... it is for 120VAC. And no, it won't run on 240VAC, as it saturates. So I have this lovely big 3-ph variac that is useless to me, but I can't bear to dispose of it. Just in case I ever do need a 120VAC 3-ph variac. Weighs a ton, I can barely lift it.

author: SeanB
author_rank: Super Contributor
timestamp: November 23, 2014, 08:31:48 am
content: Probably only usable when you can live with the limited turn range, and can handle the non linear voltage with rotation. Making it for a toroid will mean that you need to have an inner winding and then a set of clamshell housings to provide the fixed height single layer winding needed for the control winding. Doable but increases the amount of wire used by quite a lot.For the 3 phase variac get a transformer ( 3 phase) that has a secondary of 60-100VAC ( or use a 400VAC to 240VAC delta primary star secondary one with the primary reconnected in star and the secondary reconnected in delta), and use the variac on this. I did this with a 115VAC 400Hz variac, supplying it with 12VAC 50Hz, and it makes a very nice AC voltage source. Does not saturate as the VHz product is about the same. Current is what the variac is capable of, 5A, and the supply transformer is a toroidal one as well.

author: TerraHertz
author_rank: Super Contributor
timestamp: November 23, 2014, 08:51:57 am
content: Quote from: SeanB on November 23, 2014, 08:31:48 amNot sure what you mean there. The shaft angular range, or the limited space for secondary turns?QuoteHuh? Because the tap point is not constant across the turns? It's really not noticeable, the output voltage relationship to the shaft position seems linear to human senses. I guess if you graphed it precisely, it might show up. Who cares?QuoteYeah, that's the point. An isolated toroidal variac is not practical.QuoteOf course, and I do have a big 3-ph 240V transformer with low voltage independent windings I could connect to the 120V 3-ph ganged variac. Was just wishing that when i found a free giant 3-ph variac, why couldn't it be 240V not 120V? Typical.