Basic Transformer Question

[Voltz]

I want to design a power supply as a first project in electronics, and I now want to start planning for parts but I'm confused about a transformer - these are all rated at 6VA - **broken link removed**
does that mean that a power supply based around this transformer can only supply 6W (and yes I'm aware that volt-amperes are not exactly the same as watts... blah blah power factor blah blah) but as a rough idea to how much power the power supply will supply

 

[MikeMl]

In a word, Yes

 

[Voltz]

In a word, Yes

Thanks, so do you by any chance know what I could teardown to get like a 300W transformer because I was thinking of a power supply along the lines of 30V@10A

 

[audioguru]

If you rectify 30VAC then you get a 42.3V peak voltage. The full-wave rectifier drops 2V and the filter capacitor output is 40.4VDC. When the current is 10A then the transformer must supply 423VA, not 300VA.
If the output is 30VDC at 10A then the load heats with 300W and the rectifier and voltage regulator heat with (42.3V - 30V) x 10A= 123W.

Some people tear down the transformer from a microwave oven. It is powerful but its full power rating is only for a couple of minutes. It will overheat if it runs a high power continuously. Maybe its magnetron fan also tries to cool the transformer.

 

[Voltz]

If you rectify 30VAC then you get a 42.3V peak voltage. The full-wave rectifier drops 2V and the filter capacitor output is 40.4VDC. When the current is 10A then the transformer must supply 423VA, not 300VA.
If the output is 30VDC at 10A then the load heats with 300W and the rectifier and voltage regulator heat with (42.3V - 30V) x 10A= 123W.

Some people tear down the transformer from a microwave oven. It is powerful but its full power rating is only for a couple of minutes. It will overheat if it runs a high power continuously. Maybe its magnetron fan also tries to cool the transformer.

Thank you very much for your corrections, I didn't consider any of that, The value of 123W dissipated in the voltage regulator seems like a recipe for swift melting, heatsinked or not, are there any regulator chips designed to work at that kind of power?

 

[tcmtech]

First off do you need 30 volts DC at 10 amps and does it need any high degree of regulation?

What exactly are you going to be powering?

If its just 30 VDC at 10 amps with no need for regulation then a common 24 VAC transformer with a full wave bridge rectifier would get you roughly 32 VDC after rectification.

Also 24 VAC transformers are very common and finding ones in the 300 VA range is easy being that 24 VAC is a common in industrial and commercial control applications.

 

[Voltz]

I want roughly those specifications but as I'm a newb and this is my starter project I'll scale back if necessary

 

[audioguru]

An ordinary 780x regulator has an output that is regulated with a maximum current of 1A. Its datasheet shows how to parallel output power transistors for higher current.

 

[Diver300]

30 V at 10 A isn't a starter project.

If you want a general purpose power supply, you don't need that much current. If you need 10 A, it will be to supply something large, so please tell us what it is.

 

[Hoiler]

You should consider a Microwave Oven Transformer - i know it has been mentioned before. Just whatever you do do not just plug it in unmodified! you'll get 2400V out of it - very deadly. Get a hacksaw and cut the secondaries off and wind a few turns around the core. Measure the AC output. Do the math. The reason i suggest a MOT is the power rating - around 1000W for a few mins, so 300W should be fine for however long you want to use it for.

 

[Diver300]

As microwave oven transformers are made down to a price, they might have quite high iron losses. In other words, they will get quite hot with no output load (or even output windings). If you can increase the number of primary turns, by just 5 or 10%, the iron losses will be considerably reduced.

 

[Voltz]

I don't really need 10A I just wanted to make something substantial enough that it invalidated just buying a cheap plug in power supply from maplins and using that instead