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El-Cheapo Mains Isolating Transformer--Better than nothing!

 
Alastair
(@alastair)
Reputable V-Ratter Registered

Reading a few threads recently indicated the risks of our hobby--especially with some of the more oddball late 40's Philips sets currently under renovation, that have an odd heater and focus arrangement, whereby the chassis --isnt quite 'neutral'

Worth making up a Mains-Isolating transformer--el-cheapo style like I did recently to enable me to connect my 625 line Bush TV62 via its HDMI conversion to a freeview box and DVD player...... :thumb

I had lying round, two toroidal transformers that had two secondaries of 55V each. One was 600VA, the other 500VA.

These I connected back-to-back as it were, Mains to the 600VA's primary and the secondaries of this first transformer connected to the secondaries of the 500VA second transformer.

'Isolated' supply is taken from the second transformer primary. Even though the transformers are rated at 600VA and 500VA, I would de-rate this to 400VA for safety reasons--just in case-- so for all the usual sets we would deal with this would be fine..... :qq1

Ive run the TV62 for several hours,--Its 140W consumption hardly making either transformer warm, and the supply the set 'sees' is 242V constant.

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Topic starter Posted : 21/04/2014 11:01 pm
Red_to_Black
(@red_to_black)
Noble V-Ratter Registered

That sounds fine :thumb

Just a word of caution for anyone considering wiring two "110V site" transformers back to back, these are centre tapped 55V-0v-55V, 0V to Earth.

You need to be careful trying to wire these back to back and I would recommend missing both CT earth connections off the secondary's of both transformers by using a two wire connection.

This is for two reasons, 1) to isolate the earth completely, and 2) if both transformers are not exactly phased the same (secondary windings) it is possible to burn out or at least damage the transformers.

"This is my multimeter. There are many others like it, but this one is mine. My multimeter is my best friend. It is my life. I must master it as I must master my life. Without me, my multimeter is useless. Without my multimeter, I am useless."

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Posted : 21/04/2014 11:10 pm
Alastair
(@alastair)
Reputable V-Ratter Registered

Ah--Now that's interesting, I hadn't considered the use of 'site-transformers' and I had no idea they were centre-tapped and more importantly earthed either!
--I wondered why the PAT Tester in work never liked 'em!

With my toroids, I bought them some years ago at Maplin, and both are the same make (Antrim) with the same secondary colour-codes, I just colour-matched each secondary connection, seems to work out OK, but did wonder whether to earth one side of each secondary or not--I didn't bother in the end....

I'm guessing that if for some reason you used a couple of transformers--and got the secondary phasing wrong, using the correct rated mains-fuse for the mains-side transformer,--this should blow....
--Just a thought.....

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Topic starter Posted : 21/04/2014 11:25 pm
Red_to_Black
(@red_to_black)
Noble V-Ratter Registered

I'm guessing that if for some reason you used a couple of transformers--and got the secondary phasing wrong, using the correct rated mains-fuse for the mains-side transformer,--this should blow....
--Just a thought.....

It would more than likely trip the thermal cut-out first, although it would likely also overheat the windings and the inter-connection, lower voltage= higher current on that type of transformer.

Besides you want a fully floating supply for isolation purposes.

"This is my multimeter. There are many others like it, but this one is mine. My multimeter is my best friend. It is my life. I must master it as I must master my life. Without me, my multimeter is useless. Without my multimeter, I am useless."

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Posted : 21/04/2014 11:29 pm
Red_to_Black
(@red_to_black)
Noble V-Ratter Registered

Here is a diagram of a typical site transformer.
The secondary winding's centre tap are connected to earth via the 3-pin plug at the primary side.

"This is my multimeter. There are many others like it, but this one is mine. My multimeter is my best friend. It is my life. I must master it as I must master my life. Without me, my multimeter is useless. Without my multimeter, I am useless."

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Posted : 22/04/2014 12:16 am
Doz
 Doz
(@doz)
Noble V-Ratter Registered

I pull a similar trick for small valve projects, only I use 6v transformers , so I can tap the heaters off the "middle" voltage.

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Posted : 22/04/2014 10:11 am