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Question about transformers

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iceblue

Member
Sorry if this is covered somewhere already. I did a search but didn't have time to go through all the results I found.

I have a transformer which needs to be replaced. The primary is 230V and secondary needs to be 12V. There is a 5A fuse on the primary side and a 4A fuse on the pcb to which the secondary side goes. I'm not sure if this fuse is before or after rectification. All the transformers I have found are rated something like 12-0-12 or 6-0-6 and I am not certain which I should use. If I took a 6-0-6 with 4A output current and then used the two 6V outputs would I get the required 12V? And would the current rating be ok or too low?

I'm a bit confused by the whole 6-0-6 and 12-0-12 labelling.

Thanks
 

MikeMl

Well-Known Member
Most Helpful Member
Use the 6-0-6 and leave the center-tap unconnected.
 

Pommie

Well-Known Member
Most Helpful Member
Choose a transformer with a 12V secondary that is the same physical size as the one it is replacing. I think you will find that one rated at 12V 4A will be a lot bigger than the original. A 6-0-6 can be used, you just leave the 0V disconnected.

Mike.
 

iceblue

Member
Thanks guys. That really helps. The original one was mounted in resin inside a plastic housing so physical size isn't an issue as I will have to place it in a new housing anyway.
 

RODALCO

Well-Known Member
Best practise is to fuse before the rectifier on the secondary side of the transformer.

It protects the transformer in case a rectifier diode fails.

And of course have the primary fused as well, as already done, although 5 Amps looks a bit on the high side. better to fit a 1 amp slow blow fuse there.
 

iceblue

Member
Thanks. The transformer is powering a house alarm system with backup battery. So most of the time I'd say it would not be drawing too much current. The fuse on the secondary side is located on the PCB and I suspect it may be before the rectifier. I will look into changing the fuse on the primary side. The power supply was originally used on a very old alarm system and when the alarm got replaced they left the power supply in and just replaced the rest of the system so I am quite sure that the new system will draw much less current than what the supply was designed for.
 
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