Isolated Variable AC Power Supply - UK Vintage Radio Repair and Restoration Discussion Forum

phykell · 29th Jan 2019, 7:14 pm

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

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.

kellys_eye · 29th Jan 2019, 7:21 pm

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.

phykell · 29th Jan 2019, 7:36 pm

Quote:

Originally Posted by merlinmaxwell

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.

If there's a cheaper way then great

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?

G6Tanuki · 29th Jan 2019, 7:54 pm

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.

Lloyd 1985 · 29th Jan 2019, 8:24 pm

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

John M0GLN · 29th Jan 2019, 8:51 pm

Quote:

I find my variac very useful

I find mine very useful as well, small variac with isolating transformer and I've added a rectifier, capacitor and moving iron voltmeter with switched output which gives me AC / DC variable, also a 6V transformer so I can power most of the things I need to.

John

karesz* · 29th Jan 2019, 9:04 pm

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

kellys_eye · 29th Jan 2019, 9:30 pm

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.

phykell · 29th Jan 2019, 9:50 pm

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.

phykell · 29th Jan 2019, 11:02 pm

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/

ajgriff · 30th Jan 2019, 12:08 am

Both 'out of stock' I notice.

Alan

Refugee · 30th Jan 2019, 12:46 am

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.

Diabolical Artificer · 30th Jan 2019, 7:34 am

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.

30th Jan 2019, 9:42 am

Quote:

Originally Posted by Refugee

They would be hugely heavy if they were a single unit.

Perhaps they use an isolating Variac, two winding's on the core.

Herald1360 · 30th Jan 2019, 11:13 am

Quote:

Originally Posted by phykell

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.

I've got a spare NOS 200W ES incandescent GLS bulb if you fancy chancing the shipping......

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).

kellys_eye · 30th Jan 2019, 11:22 am

Quote:

Originally Posted by phykell

I'm a bit concerned that 3A is the same fuse rating as a 100W valve amp though.

If the 3A fuse is in the mains lead then it's rated as such to protect the LEAD, not the equipment and, as a measure for power consumption, is not useful.

Based on 50% efficiency (or even 30% efficiency) your 100W amp will really consume just 150W - certainly less than 200W i.e. under 1A.

phykell · 30th Jan 2019, 9:23 pm

Quote:

Originally Posted by Herald1360

I've got a spare NOS 200W ES incandescent GLS bulb if you fancy chancing the shipping......

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).

Thanks I'll give that some thought and PM you if I don't go-ahead with the combined unit

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...

See_Mos · 31st Jan 2019, 5:09 pm

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

phykell · 13th Feb 2019, 5:14 pm

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?

29th Jan 2019, 7:14 pm	#1
phykell Tetrode Join Date: Jun 2011 Location: Liverpool, Merseyside, UK. Posts: 54	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 Guest Posts: n/a	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.

29th Jan 2019, 7:21 pm	#3
kellys_eye Octode Join Date: Jan 2014 Location: Oban, Scotland, UK. Posts: 1,129	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, 7:54 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	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.

29th Jan 2019, 8:24 pm	#6
Lloyd 1985 Nonode Join Date: Jun 2008 Location: Coningsby, Lincolnshire, UK. Posts: 2,819	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, 9:04 pm	#8
karesz* Heptode Join Date: Jun 2010 Location: Zurich, Switzerland Posts: 538	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
kellys_eye Octode Join Date: Jan 2014 Location: Oban, Scotland, UK. Posts: 1,129	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.

29th Jan 2019, 9:50 pm	#10
phykell Tetrode Join Date: Jun 2011 Location: Liverpool, Merseyside, UK. Posts: 54	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
phykell Tetrode Join Date: Jun 2011 Location: Liverpool, Merseyside, UK. Posts: 54	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
ajgriff Nonode Join Date: May 2004 Location: Halifax, West Yorkshire, UK. Posts: 2,587	Re: Isolated Variable AC Power Supply Both 'out of stock' I notice. Alan

30th Jan 2019, 12:46 am	#13
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	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.

30th Jan 2019, 7:34 am	#14
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,662	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. __________________ Curiosity hasn't killed this cat...so far.

31st Jan 2019, 5:09 pm	#19
See_Mos Hexode Join Date: Nov 2003 Location: Ossett, West Yorkshire, UK. Posts: 482	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
phykell Tetrode Join Date: Jun 2011 Location: Liverpool, Merseyside, UK. Posts: 54	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?