Isolating Transformer.

[ben]

On the lookout for an isolation transformer for the workshop, I saw this listed locally. Seems to be rated for 1.6A, which seems a little low, especially if dealing with televisions that tend to take a big gulp at switch on what with degauss and rectifiers. Or am I being pessimistic?
Would welcome your thoughts!

[joebog1]

It should handle it OK. I do agree though, that something larger is more appropriate.
It is rated at 400 watts so as long as you dont exceed this for any longer than a few seconds it should be fine. Make sure it doesnt have the dreaded internal thermal fuse!!.

Joe

[cmjones01]

400VA should be absolutely fine for servicing radios and TVs. Exceeding its ratings for a fraction of a second with a degaussing coil won't hurt it at all: the limit on the transformer's rating is thermal, so there'll be no time for it to overheat with such a short overload.

Chris

[ben]

Thanks. Might be tempted but the seller wants 80eur for it, which I think is a bit high. Wanted to get some second opinions before haggling!

[Ed_Dinning]

Hi Ben, 80 Euros seems a bit heavy for what is a standard control panel transformer. That would not be that new.
Try some local scrap yards that do electrical equipment and strip it out yourself

Ed

[buggies]

400 volt primary tap - is that not a bit unusual?

[G.Castle]

Originally Posted by buggies

400 volt primary tap - is that not a bit unusual?

For use across two phases of a three phase supply.

Greg

[broadgage]

Originally Posted by buggies

400 volt primary tap - is that not a bit unusual?

Very common for control panel transformers such as this. A common application is to derive a 230 volt control circuit from a 3 phase 3 wire circuit with no neutral and 400 volts between phases.

Large industrial 3 phase motors are often supplied via a 3 core cable, this being significantly cheaper than 4 core in larger sizes. A small transformer in the control panel is a LOT cheaper than say 100 M of 4 core 240mm cable, when 3 core could otherwise be used.

[Tim]

Whatever rating of isolating transformer you have, there will come a time when you need a bigger one.

[G6Tanuki]

The 400V thing can be useful.. Some of us use such transformers backwards and feed the 400V into a bridge rectifier to get a decent HT for valve power / linear amps.

[dougietamson]

I picked up a couple of 800W microwave ovens (freebies on Gumtree) for the transformers for upcycling.
I was thinking it might be an idea to remove the HV secondary coil and replace it with custom wind for 240v with the option of some taps at 115/120 and maybe some lower ones eg 90v for a substitute for a variac. Would just need to wind a few turns to get the turns/v ratio for the existing primary.
The laminations are usually butt stacked and welded but can be opened up easily with an angle grinder, the cut is well outside the magnetic path.

[Radio Wrangler]

No no no.

Microwave cooker transformers are rather specialised. Like electric welding transformers, they are designed to control their output current. They have loose coupling from primary to secondary. The magnetrons in microwave cookers will just run away to damaging power levels if run from a straight forwards voltage supply.

Consequently, anything you do on the core of a microwave cooker transformer will have lousy voltage regulation. It is designed to be this way.

David

[tritone]

Hello,

I got one here that might be OK for you,

It gives out a good 252Vac (unloaded) on the Sec side with a nice healthy 240Vac on the Pri line side,

250VA

Let me now, as it's been sitting aroung gathering dust,

( B.T.W. the Isaac Albéniz piano score, Op 165, No. 2 is not up for grabs. )

F.O.C. you pay postage.

[G6fylneil]

It's a current model available from RS for £160. Control transformers have good regulation to supply high inrush currents, and are expected to run 24/7 in hot crowded control panels without adding too much heat themselves. They usually long outlast the machines they are fitted to and designers don't skimp on them. That one could have easily come out of a £100K machine.

[dougietamson]

Originally Posted by Radio Wrangler

No no no.

Microwave cooker transformers are rather specialised. Like electric welding transformers, they are designed to control their output current. They have loose coupling from primary to secondary. The magnetrons in microwave cookers will just run away to damaging power levels if run from a straight forwards voltage supply.

Consequently, anything you do on the core of a microwave cooker transformer will have lousy voltage regulation. It is designed to be this way.

David

Thanks David, is the the coupling done via the shunts?

I've only ever just reused the laminations, splitting them up into E andI's and interleaved them.
Only issue I thought about is you have no idea whet the lams are made of so default to 'garbage' and hope their typically larger size/core area makes up for it.

Doug.

[G6Tanuki]

As noted, microwave ovens transformers are rather specially designed to deliver the current requirements of a Magnetron.

There was a thing a couple of decades back where radio amateurs experimented with using these transformers to power valve linear amplifiers. The widely varying current demand in such applications showed up the poor voltage regulation, meaning that the amplifiers were often very much non-linear!!

For isolation transformer purposes use a proper commercially made one, that has a statement of isolation and has been individually Hi-Pot tested

[Techman]

Originally Posted by ben

the seller wants 80eur for it,

That's ridiculous! They're dreaming and I would let them keep it at that price.

An example of what you can get is one of those BT units that ebay has been awash with for years. I've just checked and there's a few on there now. I got given one a few years back and I'll also post an example link to one on ebay for reference only. They contain a filter network and an isolating transformer - keep in mind that if used 'as is' that the earth is continuous through the unit, but obviously can be removed depending on your usage. There must be thousands of these out there on the scrap market and there's higher powered versions available if you search hard enough.
https://www.ebay.co.uk/itm/125530813...Bk9SR5zU88XwYA

Edit to say - they used to go for a fiver until folk started to realise what they were!

[Techman]

Admittedly the one shown (and the one that's the same that I was given) is only rated at half an amp, but I've used mine to power up various radios etc. with no problems.

I seem to think that forum member 'Woodchips' had one or two great big ones for sale a few years ago for something like £25. I could be mis-remembering this and also I think it would probably have been collection only, but it may still be worth asking him if he has anything available - it seems that he's been getting rid of a lot of 'stuff' in recent times.

[Ed_Dinning]

Note that microwave transformer often use cheaper and thicker lams than other types.
This increases the losses and they may well run hotter. In a microwave the run on a duty cycle somewhat less than unity

Ed

[dougietamson]

Originally Posted by Ed_Dinning

Note that microwave transformer often use cheaper and thicker lams than other types.
This increases the losses and they may well run hotter. In a microwave the run on a duty cycle somewhat less than unity

Ed

The ones I have are 0.5mm thick, same as most of my re-used power transformers. I've noticed output transformers have thiner lams.

I've been working on experimental builds of a PT and OT from a single MOT.
The MOT lams are 90mm wide so giving a 30mm tongue width.
The experimental build designs are for a push pull guitar amp OT, min freq was 70Hz so not needing so much iron. Output valves are 2 x 6L6GC, core area of 30x32mm. The PT core is bigger at 30x45mm and I "went to town" on the windings, 6.3v, 12.6v, 40v and an HT with a few taps 320-300-280-260-CT-260-280-300-320v. I had a couple of lams left over.
I tested the OT in an existing 6L6GC push pull amp and sounded good.
Still got some metalwork to do for a frame to hold the lams tight and mount them...