Hi,

I'm currently working on a model railway controller for my dad. I'm using 50VA transformer with a single 230V primary winding and two 15V secondary windings. I'm going to use one winding for the controller circuitry and use the second winding for a seperate AC outlet.

By my calculations, each winding will produce around 1.6amps @ 15v. So i'm going to use a 1.6amp circuit breaker on each secondary winding to protect that side of things. So my first question is - does that sound OK?

Where I get slightly confused is on the primary side. If the transformer is 230V and 50VA I think it should draw around 220mA. If i use a 250mA slow blow fuse will that be enough to protect it? I know about the surge that occurs when a transformer is first switched on, and I've been told to use a fuse twice the amp rating of the primary winding. But I thought that was why you use slow blow fuses - to cope with this surge but still protect the transformer properly. So should I use a 250mA, or a 500mA fuse?

Any advice on this would be great as I don't know too much about transformers at the moment.

Thanks,

John.

 

Fuses are more complicated than that, you need to look at the time-lag ratings and the I^2t of the fuse vs. in-rush current and waveform, etc.

However, the empirical method is usually just as useful. Do you anticipate using 1.6A much of the time? First, install a fuse holder so you can easily change the fuse. Try the 1/4A fast-blo, if it is popping too often, change to a 1/4A slo-blo. Starting with these minimum sizes should reasonably protect your device, and by using the holder you can adjust upward accordingly.

I believe the VA rating usually applies to the secondary, however, it's all a matter of turns ratio and you still get the same numbers (50VA / 30V = 1.67A, etc.), which means if you properly parallel the secondaries you could get 15V @ 3.2A.

 

The most I'd anticipate using is around 1A but i'd like the extra headroom in case I wanted to use the controller on a larger gauge layout with larger motors, etc...

I've fitted the enclosure with one of those 3 pin kettle-type sockets with a fuse holder built-in so it's easy to change them.

Thanks for the advice, I think I need to look into the stuff you mention at the start of your post.

 

See attached for a couple of things that may help: