Picking a good Toroidal

[maxw]

I have an opportunity to purchase some toroidal transformers locally for a cheaper than normal price and I am just not sure what one to choose.

Normally I would search the forums and digest all the threads and make the decision my self BUT I have only a day or two to take advantage and I dont think I am going to be able to work it out for my self before its too late judging by the threads I have already looked at

I plan to make 2x REGULATED GCs. One with a pair of LM3875 and one with a pair of LM4780.

These are the choices I have:
18V-0-18V 300VA Toroidal Transformer
40V-0-40V 300VA Toroidal Transformer
50V-0-50V 300VA Toroidal Transformer

I know the supply voltage for the chips is 20-84V so I could use the 40V transformer but I know the voltage from the transformer changes after rectification and thats the part I dont understand (YET!)

So my question is which ones shall I choose and will one 300VA run a pair of LM3875s and will I need 2x 300VA to run a pair of LM4780s?
Thanks

[UrSv]

There's about a thousand threads on transformers for GCs so you should easily find the info you need.

Strictly speaking the optimal for the GC is considered to be a dual secondary rather than center tap transformer so in that respect none of the transformers you listed would be a perfect choice. If you choose to go ahead with these then the only choice you have is the 18-0-18 transformer as the others would give WAY to high supply voltages.

For 40 V you will get some 40*1.414 for each rail which is in excess of 56 V at no load. The maximum for the LM3886 is 42 V for each rail on dual rail supply or 84 V on single rails and in this case you're looking at 56 or 112 respectiely. The 50 V transformer is even worse obviously.

If you choose that 18-0-18 transformer and your loads are higher than 4 Ohms then I would say that one would run either the LM3886 pair or the LM4780 just fine.

[Nuuk]

Remember that you need a transformer with separate secondary windings if you plan to use the LM338 regulated circuit for your PSU.

Ideally, I would suggest 25 V secondaries for a regulated GC so saving yourself a few dollars on this special offer may not be a good idea!

[maxw]

Originally posted by UrSv
For 40 V you will get some 40*1.414 for each rail which is in excess of 56 V at no load. The maximum for the LM3886 is 42 V for each rail on dual rail supply or 84 V on single rails and in this case you're looking at 56 or 112 respectiely. The 50 V transformer is even worse obviously.

Thanks! that what I couldn't work out, the "1.414".

Originally posted by Nuuk
Remember that you need a transformer with separate secondary windings if you plan to use the LM338 regulated circuit for your PSU.

How do you know from what I wrote that they dont have dual secondaries?

Does it specifically have to say "dual secondaries"?

This is from the catalog and shows a diagram of the wire output (at the bottom):

[jaycee]

They are dual secondary transformers advertised. Usually if you write "30-0-30" you are referring to a centre tapped transformer, the correct way for a dual secondary would be to write 2x30V.

the 1.414 is the square root of 2 - you may have heard "Root mean square" applied to AC voltages. This means that the RMS voltage is the peak AC voltage divided by the square root of 2. When you rectify the AC into DC and smooth it, the DC voltage is (very near to) the peak AC voltage, so you must multiply the AC voltage given (which is always RMS) by the square root of 2.

[Nuuk]

How do you know from what I wrote that they dont have dual secondaries?

I didn't but wanted to warn you in advance.

[maxw]

Originally posted by jaycee
They are dual secondary transformers advertised. Usually if you write "30-0-30" you are referring to a centre tapped transformer, the correct way for a dual secondary would be to write 2x30V.

Thanks!

Originally posted by Nuuk


I didn't but wanted to warn you in advance.

And thank you!

I will get the 2x18V one just to get me going and look for a 25V later

[BlackDog]

Originally posted by maxw

These are the choices I have:
18V-0-18V 300VA Toroidal Transformer
40V-0-40V 300VA Toroidal Transformer
50V-0-50V 300VA Toroidal Transformer

Hi Max,

So I guess these are the Jaycar parts you're referring to???

I used 2 of their 2x18V 300VA transformers for my 6 channel GC based on BrianGT's 3875 kit,. It works very well, and I have no complaints at all with the power output.

The only place in NZ that seems to have the 22V or 25V toroids seems to be RS Components, but they are apprx. 50% more expensive...!

Good luck,

Steve

[maxw]

Originally posted by BlackDog



Hi Max,

So I guess these are the Jaycar parts you're referring to???

I used 2 of their 2x18V 300VA transformers for my 6 channel GC based on BrianGT's 3875 kit,. It works very well, and I have no complaints at all with the power output.

The only place in NZ that seems to have the 22V or 25V toroids seems to be RS Components, but they are apprx. 50% more expensive...!

Good luck,

Steve

Yes, thats right. Yeah RS have good ones but they aren't cheap.
Dick smith has toroids but they are being discontinued so they are cheap at the moment but there isn't a DSE in Christchurch that has them so I am getting Jaycar to match DSE's price

[tg3]

Originally posted by maxw

Thanks! that what I couldn't work out, the "1.414".

1.414 is the square root of 2.

Has to do with the conversion of peak voltage to RMS (root mean square) voltage.

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