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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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29th Jan 2019, 7:14 pm | #1 |
Tetrode
Join Date: Jun 2011
Location: Liverpool, Merseyside, UK.
Posts: 54
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Isolated Variable AC Power Supply
As I'm now working on valve equipment, I thought a Variac would be a good idea but I'm told I'll need to isolate it too. Is the all-in-one solution like this the best approach though?
https://www.aldetronics.co.uk/produc...n-transformer/ I can't find any cheaper alternatives but I'd hope this one will cover all bases as it's up to 6A / 1,500VA. |
29th Jan 2019, 7:17 pm | #2 |
Guest
Posts: n/a
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Re: Isolated Variable AC Power Supply
Nice bit of kit, but for a fiver you could make a lamp limiter. With common sense it will be as safe, if not safer as you know there is a bit of danger there. Too many people assume an isolating transformer cures all ills.
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29th Jan 2019, 7:21 pm | #3 |
Octode
Join Date: Jan 2014
Location: Oban, Scotland, UK.
Posts: 1,129
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Re: Isolated Variable AC Power Supply
If your budget allows for such (mine doesn't!) then I'd get one....
But not all valve equipment needs such isolation as the ht is transformer-derived in the first instance and an isolated supply still won't save you from an HT belt..... If your workshop doesn't have an RCD I'd be fitting one first. I also use an isloated variable AC power supply (self-made) but only up to 250W - there's not a lot of stuff out there that uses 1kW+ Each to their own of course. |
29th Jan 2019, 7:36 pm | #4 | |
Tetrode
Join Date: Jun 2011
Location: Liverpool, Merseyside, UK.
Posts: 54
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Re: Isolated Variable AC Power Supply
Quote:
I guess I'll need some higher wattage bulbs as I intend to work with amplifiers but I've just read through this and it seems ideal: https://www.vintage-radio.com/projec...p-limiter.html I'll have to do some research before I go for something so expensive as the unit referenced but I take it that I'll still need a Variac to raise the voltage over time to the full working voltage of th device under test? |
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29th Jan 2019, 7:54 pm | #5 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,998
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Re: Isolated Variable AC Power Supply
I've never bothered faffing around with Variacs or lamp-limiters (preferring to pre-emptively replace any suspect degradable components before showing the thing power) though I *do* have a rather 'industrial' 1.5KVA isolating-transformer, which I consider an essential piece of PPE when working on any AC/DC radios/tellies.
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29th Jan 2019, 8:24 pm | #6 |
Nonode
Join Date: Jun 2008
Location: Coningsby, Lincolnshire, UK.
Posts: 2,819
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Re: Isolated Variable AC Power Supply
I find my variac very useful, at the moment I’ve got an American Motorola TV on the bench, so it’s powering that on 117v while I work on it, I’ve also had tv’s and radios with duff dropper sections that I didn’t have replacements for, and have been able to power them on lower voltage until I got round to sorting replacements. I also have a separate isolation transformer which said variac is connected to.
Regards Lloyd |
29th Jan 2019, 8:51 pm | #7 | |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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Re: Isolated Variable AC Power Supply
Quote:
John |
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29th Jan 2019, 9:04 pm | #8 |
Heptode
Join Date: Jun 2010
Location: Zurich, Switzerland
Posts: 538
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Re: Isolated Variable AC Power Supply
Phykell,
a lamp-limiter is ok in the workshop, but not as insulating as the most variacs, but the cheapest solution for isolating transformer of the home workshop are two equal/similar -against turned; ie 24V toroidal transformers... rgds, Karl |
29th Jan 2019, 9:30 pm | #9 |
Octode
Join Date: Jan 2014
Location: Oban, Scotland, UK.
Posts: 1,129
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Re: Isolated Variable AC Power Supply
This is the basis of my own DIY isolated and variable PSU which allows both a low voltage AC and a high voltage (variable to 300V) output.
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29th Jan 2019, 9:50 pm | #10 |
Tetrode
Join Date: Jun 2011
Location: Liverpool, Merseyside, UK.
Posts: 54
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Re: Isolated Variable AC Power Supply
It's a shame that there aren't any other, similar units, as it would be really convenient to have everything in the one box. It's just a shame it's not a fair bit cheaper but there's probably a lot that goes into one considering it's acting as a variac and isolation transformer.
I'm convinced about the lamp limiter already with the only downside being that I'll probably need a 200-250W bulb or look at running 100W bulbs in parallel. |
29th Jan 2019, 11:02 pm | #11 |
Tetrode
Join Date: Jun 2011
Location: Liverpool, Merseyside, UK.
Posts: 54
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Re: Isolated Variable AC Power Supply
This one might be more like it:
https://www.aldetronics.co.uk/produc...olated-variac/ It's half the power of the other one (750VA) and with fewer features but it's £160 which isn't too far away from a decent Variac. I'm a bit concerned that 3A is the same fuse rating as a 100W valve amp though. Or a 1500VA version for £210: https://www.aldetronics.co.uk/produc...olated-variac/ |
30th Jan 2019, 12:08 am | #12 |
Nonode
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,587
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Re: Isolated Variable AC Power Supply
Both 'out of stock' I notice.
Alan |
30th Jan 2019, 12:46 am | #13 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,553
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Re: Isolated Variable AC Power Supply
I built a unit with moving iron meters and a 6 amp Variac and run it from a 1000VA transformer. They would be hugely heavy if they were a single unit.
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30th Jan 2019, 7:34 am | #14 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,662
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Re: Isolated Variable AC Power Supply
I use my variac/iso tfmr ( 8A variac/500VA tfmr) every day, wouldn't be without it. Mine is an all in one homemade jobbie with a "breakout box", LL. It's useful for amp repair work, slowing drills down when drilling big holes, forms part of my cap tester/reformer, great for ad hoc variable PSU's when prototyping circuits, etc, etc, it cost about £40; variac's come up on the forum cheap occasionally as do iso tfmr's.
If it were me I'd get a variac first, the iso tfmr isn't as important, as mentioned amps have tfmr's anyroad. The ability to bring up the power slowly whilst monitoring current draw is the important thing with gear that has old caps in, but it depends how you work. You should check any gear for shorts etc prior to powering up, so you could get away with just a LL at first, whilst keeping a look out for a variac. A home made unit is going to be cheaper, ready made is going to cost, there are medical units out there for not a lot though. If you do buy something like that, check the earthing and wiring first before you use it. Andy.
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30th Jan 2019, 9:42 am | #15 |
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Re: Isolated Variable AC Power Supply
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30th Jan 2019, 11:13 am | #16 | |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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Re: Isolated Variable AC Power Supply
Quote:
We could talk about the variac described here too if you're interested: https://www.vintage-radio.net/forum/...d.php?t=153415 You could turn it all into a nice useful piece of steampunk, complete with a light on the top I might even have a suitably sized empty Mahogany box (ex large old wattmeter).
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30th Jan 2019, 11:22 am | #17 | |
Octode
Join Date: Jan 2014
Location: Oban, Scotland, UK.
Posts: 1,129
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Re: Isolated Variable AC Power Supply
Quote:
Based on 50% efficiency (or even 30% efficiency) your 100W amp will really consume just 150W - certainly less than 200W i.e. under 1A. |
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30th Jan 2019, 9:23 pm | #18 | |
Tetrode
Join Date: Jun 2011
Location: Liverpool, Merseyside, UK.
Posts: 54
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Re: Isolated Variable AC Power Supply
Quote:
I probably need a larger Variac though as I have three 100W amplifiers to work on at the moment plus the odd Linear L50 and Conchord... |
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31st Jan 2019, 5:09 pm | #19 |
Hexode
Join Date: Nov 2003
Location: Ossett, West Yorkshire, UK.
Posts: 482
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Re: Isolated Variable AC Power Supply
I have an isolation transformer and a variac but rarely need the variac.
Unless you are repairing American or other amplifiers that need 110 to 120 volts then you do not really need an isolated supply or a variac. The amplifiers own transformer will provide isolation though it is wise to make sure the primary is not leaky to earth before you start. Valve amplifiers are very forgiving and easy to repair, unlike some transistor amplifiers that suddenly smoke or destroy speakers for no apparent reason. For repairing 110 to 120 volt equipment a small tool transformer will suffice and they are usually rated at 500 watts or more and they are relatively inexpensive. The lamp limiter is a good idea when repairing old radio equipment, especially AC-DC sets. It needs to be a filament lamp, LED and CFL are no use for this application. It needs to be the same voltage as the supply to the equipment being repaired and a 100 watt will suffice. A short circuit or other major fault will be immediately apparent as the lamp will be quite bright. Once the fault has been repaired the lamp is taken out of circuit. For many years I used an old outdoor floodlight that had a 300 watt halogen lamp and nothing else. I just checked the auction site and they are still available Last edited by See_Mos; 31st Jan 2019 at 5:19 pm. |
13th Feb 2019, 5:14 pm | #20 |
Tetrode
Join Date: Jun 2011
Location: Liverpool, Merseyside, UK.
Posts: 54
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Re: Isolated Variable AC Power Supply
For the 100W+ valve amps 200W bulbs are still easily found so I built myself a dim bulb unit out of spares and it works fine - it's a bit ugly though
One other idea did occur to me - whilst I'd like the combined variac and isolation transformer for convenience, I thought an alternative would be to buy a basic variac, replacing the isolation transformer feature by using a UPS instead. A quick check shows that I could buy a new APC 1400VA UPS for £125 or less. I'll make sure I use the dim bulb unit as well to avoid a sharp inrush of current. The UPS can then double up as a backup PSU for my PC, etc. Newer UPS devices tend not to include a transformer but others actually have an isolation transformer. For the former case, with the mains-in disconnected and running from battery, the DUT is effectively isolated. Does the above sound like a sensible plan? |