Isolation transformer for variac advice.

Thread Starter

Mikuda

Joined Mar 23, 2017
10
Hello!
I'm building "isolation transformer+variac+dim bulb tester combo" according to plans in attached pictures
(from "Mr Carlson's Lab" YT channel - It's gonna be enclosed in chassis version since I wouldn't feel comfortable with such stuff exposed ;) )
device.jpg device1.jpg
I have quite nice ~230v 6.3A variac I'd like to use (see pics) and my question is what should be amperage output of isolation transformer I should look for?
Variac:
variac.jpg variac1.jpg variac2.jpg

Keep in mind I'm in Europe so 230V is normal mains here.
IF You think 230V 6.3A is too much (isn't it like ~12A for 120V in USA?) I can get similar but ~4A version of this variac.

So, what Isolation transformer power should I look for?

Thank You in advance for any comments!
 

dendad

Joined Feb 20, 2016
3,835
What power loads are you going to use?
That will determine the transformer size. If you want full power, then a 240V 6.5A (1.5KVA) transformer or bigger is needed.
Large isolation transformers can be expensive!
Also, if you can get a couple of identical mains transformers, you can connect them back to back to make your isolation transformer. In the past I used a couple of transformers from old valve TVs for that very purpose. Even rewinding an old transformer secondary can be done.
There is nothing wrong with using a lower power transformer, just as long as the load is not bigger than the power rating.
 

Dyslexicbloke

Joined Sep 4, 2010
565
Dont disagree with any of the above but you need to be talking about VA not W and you will need headroom.

The variac states 7A at 230V PRI with a fuse rating of 10A and as you have inrush currents to considder as well as the power factor I would recommend an isolation transformer of at least 110% of the primary VA of the variac, so not less than 1.6KVA

At that size site isolation transformers will be the cheapest option, two 110's back to back is an option but probably an expensive one.
230/230 units are quite common but watch the rating, most b1.5KVA units are only rated 750VA continuous.

Look up site isolation transformer.

If you want an open frame unit then Leeds Transformers, in leeds oddly enough, or Electrowind in Stoke-On-trent.

Having said all that... If your application is less than 700VA and you dont mind a bit of fiddling then two 230/110's at 1.5KVA would work back to back and will deal with the cope with the inrush but they will cope better in paralell, on the primerry and seriese on the secondary side 110-0-110 provided you move the earth point to the 0.

Tool transformers are 55-0-55 with the earth at 0 you would need to reconfigure to 110-0 & 0-110 with both earthed at 0.
Keep an eye on auctions... cant recommend any particular site, wink.

BTW... many 110V site transformers of 3KVA peek and above actually have two seperate secondrys and con therfore be reconfigured, witout rewinding to 220V. The problem is working out which ones but at £30 each second hand, would you care?

Hope that helps,
Al
 

Thread Starter

Mikuda

Joined Mar 23, 2017
10
Thank You for replies!
First of all, to be more specific I want to try my hand at building stereo tube amplifier.
So maybe we should determine what amperage I will need for my variac in the first place.
As I wrote I got two variacs: 6.3A and 4A - for FREE - so no room to be picky here ;)
Should I use 6.3A version to get plenty of headroom or 4A will be entirely sufficient for my purpose?
6.3A vs 4A will then determine necessary power of isolation transformer, and consequently it's cost/availability.
Please keep in mind we are talking 230V in Europe ;)

P.S. Am I correct that 230V / 4A is roughly equivalent of 120V / 8A? (without getting into tech language pls :) )
 

dendad

Joined Feb 20, 2016
3,835
I don't know why you want a variac for a tube amplifier. In fact you want a constant voltage so the heaters work correctly, not a variable one. Running valves (tubes) on incorrect voltages will greatly reduces their working life.
Just invest in the correct power transformer to start with.

Volts x Amps = Watts so 230V x 4A is about the same power (920W) as 120V x 8A(960W).
This only works simply with a resistive load and putting inductors and capacitors into the circuit complicates it a bit, but close enough.
 

tcmtech

Joined Nov 4, 2013
2,867
Are the tubes getting their DC directly off rectified mains line power or does it have a proper transformer in it?

As far as working with a tube amplifier all the HV DC inside is typically referenced to the chassis frame anyway so isolating the AC input from the mains does not make anything any safer.
 

Thread Starter

Mikuda

Joined Mar 23, 2017
10
Dendad - I don't want to put variac INSIDE amplifier.
I want to build general safe workbench power supply for use while working on my projects.
Such power supply (i.e as in my OP) is used and highly recommended by many technicians so obviously it must provide some kind of safety after all :)
I just mentioned tube amplifier as sample project I intend to work on, just to help determine power loads You were asking for.
 

Kermit2

Joined Feb 5, 2010
4,162
Variacs have some wonderful uses, but AC power supply for test bench is not a common one.

Use whatever AC you have and bring it to an outlet on the bench with a fuse holder in line. Anytime you think limiting the current would be called for, just pop in a low value fuse.

Have a terminal for earth ground brought out on the bench and connect a voltmeter to it and chassis of the equipment you are working on. You can then connect earth to the metal chassis while working on it should it be needed for safety.

For high voltage DC work (think tube amplifiers) you would want to leave the chassis floating after you checked it (see above) for voltage to prevent grounding yourself accidentally with hands inside, or across a tool touching bare metal.

Speaking from experience. The more complicated you make your power connection scheme (even with safety in mind) the more likely an accident becomes when used to frequently connect or disconnect equipment you are working on.
 

MrAl

Joined Jun 17, 2014
8,251
Hi,

I did a little work with tube stuff but that was a really really long time ago.

One thing i remember though is that the filament re1quires a constant voltage, and it's usually AC. Usually 6.3vac but that could vary i guess like to 12.6vac or maybe others too.

The other thing is that the DC supply is usually high, like 300vdc. They usually use a step up transformer inside the old tube amps then rectify and filter into smoother DC.

There is also a trick for getting rid of some line frequency hum in the amplifier output, but i'd have to remember how that works...it's been years since i've even seen a tube amp.
 
That schematic has a BIG problem. You MUST fuse the wiper It's OK to fuse both and make the wiper one easier to replace. i.e. front panel.

So, you have a 6.3 A 230 V Variac. I DOES NOT MEAN it can put out 12 Amps at 10 V. The current in the output or wiper MUST be fused.

I put my isolation transformer external. I cannot remember how I did the grounding because I only worked on 2 pole power at the output. I did put binding posts for the outputs as well.

Since this discussion included a lower current Variac, well my 3A @ 120 V Variac was free and I was about 10 years old and the 10 A 120V isolation transformer was about $10.00 USD, I have a 3 A Variac and still do. I added an isolated/non-isolated mode switch where I could use the Variac, voltmeter and ammeter to bring a piece of equipment up and the switch to non-isolated for full power testing.

I want to add this:
Sencore PR57 Variac manual/schematic
https://www.artisantg.com/info/PDF__53656E636F72655F505235375F4D616E75616C.pdf

link for giggles. It adds a Wattmeter and leakage tester.

The Variac was a necessity for working on the TV's that strung a bunch of tube filaments together to be 120 V and had no transformer.
I have a friend that build tube amps and he had an old McIntosh in storage and recently brought those up over hours to reform the electrolytic capacitors. The Variac was a necessity when working on Solid State amps too including the one I built.

One regret with mine would be the lack of a pilot light.
 

tcmtech

Joined Nov 4, 2013
2,867
Dendad - I don't want to put variac INSIDE amplifier.
I want to build general safe workbench power supply for use while working on my projects.
Such power supply (i.e as in my OP) is used and highly recommended by many technicians so obviously it must provide some kind of safety after all :)
I just mentioned tube amplifier as sample project I intend to work on, just to help determine power loads You were asking for.
I for one have never used dedicated isolated power for working on much of anything.

It leads to a false sense of safety and little more being pretty much every shock you are going to get is going to be from contacting two points inside the device you're working with rather than through a live line to a viable earth ground like your floor or such simply due to natural amount of resistance any footwear or wooden floors or even a dry concrete or tile floor while wearing socks will provide.

As for a variable AC source that again depends on what it is you are working with. I work with a fair amount of things where a variable AC voltage is useful. Same with powering things through unregulated DC power supplies ehre varying the AC power going into them will vary the DC output voltage.

Regarding protecting a variac I prefer a double fuse or push button circuit breaker setup where both the primary and a secondary have their own overload protection (especially when setting one up to be capable of putting out higher than line voltage on the secondary tap) to which I tend to go with a 50 - 75% overload capacity and use whatever common fuse or breaker size that fall in that range. By design they are rather robust devices and to kill decent sized one takes a lot of overloading for an extended period of time to do.
 

Thread Starter

Mikuda

Joined Mar 23, 2017
10
Thank You again for all inputs! I have a lot to read and digest now.
I'll just stick to my main question from OP for now. We can discuss design details and necessity of fuse here and there later on if You wish.
First I need to acquire correct variac and isolation transformer. Then I'll think on various wiring options.
Quick observations now:
I noticed that common "commercial" units like that Sencore PR57 or Heathkit IP-5220 have 3Amp rating at ~120V. So they produce roughly 360VA. It would mean that my ~230V 4A variac while producing 920VA would be a monster I don't need.
I have to look for 1.6~2 Amp variac max. instead, and for corresponding isolation transformer.
Are my thoughts correct?
 
Probably.

VA is related to size and there's a big difference between 3 and 10 A. Your isolation transformer may be a higher VA.

I used something like http://www.ebay.com/itm/Acme-T-3-53...40V-Sec-Transformer-1-0-KVA-1-Ph/272427739065 which didn't take up bench space. 1KW Isolation and 120V 3A Variac.

The only amp I worked on that would have needed it was a 250 W/Channel commercial amplifier. My amp is about 100 W/channel.

You can look here: https://toroid.com/Products/Tube-Amplifier-Transformers and there is a 363 W tube transformer. Here http://www.hammondmfg.com/guitarLinePWR.htm is some idea of tube amplifier transformers too. Torroid.com made a custom transformer for my amp back in the 1980's.
 

tcmtech

Joined Nov 4, 2013
2,867
I noticed that common "commercial" units like that Sencore PR57 or Heathkit IP-5220 have 3Amp rating at ~120V. So they produce roughly 360VA. It would mean that my ~230V 4A variac while producing 920VA would be a monster I don't need.
I have to look for 1.6~2 Amp variac max. instead, and for corresponding isolation transformer.
Are my thoughts correct?
Theres nothing wrong with having a larger unit doing the work. The energy loss differences between the two at idle are only a few watts.
 

Thread Starter

Mikuda

Joined Mar 23, 2017
10
Ok! Thanks for all advice! I decided to look for something a bit smaller anyway. Just from practical point of view - size and weight.
 
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