Transformer issue

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HI,

Sorry if this sounds stupid but what exactlly am i doing wrong, I am trying to test the voltage ouput of some transformers i recent found. They are rated as 43v secondaries and have 2x 120v primaries. Since the output volatge would be too high instead of running the primaries after each other to make a normal 240 input i have been trying to run them in parraell,.

The issue is i cannot test even one of the units, I plug one in with nothing attached and it blows the 13a mains fuse, i then though maybe it needs some load the other end so i rigged up a bridge rectifer and with 1 ohm 5 watt resistors on the output ( read somewhere this stops in rush current blowing fuses) going into a big capacitor.

Flick switch again, transformer humms for a couple of seconds and then the bloody 13 a fuse goes again.

They are bg transfomers have no VA rating but they weight 3 kilos each, any ideas on how i can get the thing up long enough to do some tests.

phil
 
filholder said:

HI,

I am trying to test the voltage ouput of some transformers i recent found. They are rated as 43v secondaries and have 2x 120v primaries. Since the output volatge would be too high instead of running the primaries after each other to make a normal 240 input i have been trying to run them in parraell,.
phil
You must use the primaries in series. You cannot wire them in parallel without saturating the core and burning up the transformer. Don't even try.
filholder said:

The issue is i cannot test even one of the units, I plug one in with nothing attached and it blows the 13a mains fuse, i then though maybe it needs some load the other end so i rigged up a bridge rectifer and with 1 ohm 5 watt resistors on the output ( read somewhere this stops in rush current blowing fuses) going into a big capacitor.

Flick switch again, transformer humms for a couple of seconds and then the bloody 13 a fuse goes again.

They are bg transfomers have no VA rating but they weight 3 kilos each, any ideas on how i can get the thing up long enough to do some tests.

phil
It is fortunate the fuses blew, rather than burning out the wires. The transformer cannot be used with the primaries in parallel on 240 volt, 50Hz. Test it with primaries in series. If the secondary voltage is too high, sorry, get another transformer with lower voltages.
 
Thanks guys,

So to recap is there anyway to run them at a lower voltage?

the
[/quote\]
I think wiring primaries of both transformers in series(with each trafo's primaries wired in series too) could be a possibility to reduce output voltage twice
[/quote\]

anyone ever tried something like this?
 
Hi Filholder,

you must run the 120 +120Vac primaries in series when in the UK using 240Vac mains supply.

If you choose to run two transformers in series then the transformers must be almost identical. A very small mismatch is acceptable. BUT the loading must also match and the load on each must also be in phase.

If you cannot meet these requirements then the transformers will not share the input voltage evenly.

Light Bulb Protection Circuit


A good safety rule for first connecting a new PSU to the mains is to wire in a light bulb in series with the primary of the mains transformer.

If the load is light and correctly wired up, the light bulb briefly flashes and cools and the filament reverts to a small resistance and passes most of the mains voltage to the transformer.

If the load is high, either due to a miss-wired connection or a heavy current demand, then the bulb flashes on and runs at a glow or brighter. The heavier the load the brighter it glows. The higher resistance of the hot filament reduces the voltage fed to the transformer and possibly saves you from a catastrophic failure.

Go wire up a plug top feeding a light bulb then into a three pin socket outlet.
The neutral and earth passes straight through from plug top to the socket outlet.
The live from plug top goes to light bulb terminal and from the other terminal goes to the live terminal of the socket outlet. Insulate it and fix the holder and socket back box to a mounting board.

So you have an extension lead consisting of plug top, light bulb in holder, socket outlet. Plug into the mains and the light bulb should be off.

Use this every time you experiment with a modification and/or build a new circuit. You can select the wattage rating of the light bulb to suit the size of load you are testing.

It will stop you blowing fuses and potentially save you a wad of money on destroyed components. It should even stop reversed caps exploding.
 
Hi Davidlz,
start your reply.
go to the "quote" box. It's two lines above the reply box.

Copy and paste or just type the previous posting into the dialogue box. Then OK.

You can copy before you start your reply or copy, after you start your reply, from the postings that appear at the bottom of the reply page.

It always puts the quote at the end of your current reply. So you can either paste the quote in place first and then compose your message around it or highlight and drag the quote to it's intended position.
 
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