Question:Possible to use 2x12V transformer instead of 2x15V?

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ddt

Well-known member
Joined
Jun 14, 2004
Messages
274
Location
Prague
In the SSL project, the specified transformer is 2x15V, 20VA.
I was wondering if it is possible to use a 2x12V transformer instead, as I presume the voltage at the electrolytic capacitors will be slightly higher than 15V. Am I correct in this assumption?
Would a trasformer of a higher rating make a difference?
I am asking because a local store has 2x12V 25VA and 2x12V 60VA transformers in stock, whereas I would probably need to get the 2x15V transformer custom wound, as there don't seem to be any readily available in my area.
Anyway, sorry for the longish post, and thanks for any help with this!

ddt
 
A 12V AC transformer would give you close to 17V rectified. This leaves only a 2V drop for the regulators to work on, and this will be too little unless you use low-drop regulators.

A 2x15V trafo should not be hard to find !?!

Jakob E.
 
Thanks, Jakob.

The reason I was asking was because I saw the 12V trafos really cheap, about half the price of 15V ones...

Guess I'll have to look some more, then. :) Or do you think a regulator with a drop of 1V would be OK?

I think that in theory I could use any transformer with secondaries up to about 2x22V, as long as I don't exceed the maximum input voltage of the regulators, right?

ddt
 
A one volt drop is not enough to ensure proper regulation. I like 5Vs myself. You can get lowdrop Vregs, but you have to take into consideration that the Ac linevoltage can easily drop 10%, sometimes even more :evil:

22V may be too much also, depending on how much current you're pulling and if the Vregs are heatsinked. 18V may be okay, but I'd rather Jakob answered this.

A trannie for a project like that isn't going to be very expensive. I'd just order a standard 2x15V toroid and forget about it.
 
Also, I forgot to ad , the main flitering capacitors must be rated 2x the AC voltage. So even if you could use a 22V trannie, you'd better be sure the caps can take 44VDC! 50/63V caps are are also more expensive than 35V ones :wink:

I can see on the schematic that both 2x15V & 2x18V can be used. I'd recommend sticking with the 2x15V :cool:
 
Right, thanks for that!

Why make it more complicated than it needs to be...

:wink:

ddt
 
[quote author="sismofyt"]30VA would probably have better regulation, so it's a better choice :grin:[/quote]Thank you :D

1 question more. If i would like to put two boards in to 1 2U rack can i use the same power supply for both boards? Or is this not reccommended?
 
Two boards as in a dual stereo comp, 4 channels total? I'd think that 30VA could drive both without problems. They can only share the trannie, not any other PSU components, imo.
 
[quote author="sismofyt"]Two boards as in a dual stereo comp, 4 channels total? [/quote]Yes thats what i am planning to build.

Thank you for your assistance.
 
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