Transformer

[adept]

Hey guys! I am making my own power supply because I have a lot of old junk regulators and stuff laying around and am finally DONE using batteries. Don't try to convince me to use a switch mode because I've weighed the advantages and have decided that an old linear type is okay for what I want. So basically, the last part I need transformer, but I can't find the right one. I need 10VAC at 6 amps for my power supply to function properly. Could some one either point me to a proper transformer, or a ferrite core of the right size to wrap around and instruct me in the number of windings, the shape, the wire gauge, etc.? (E coil or toroidal?? How am I supposed to know!)

[DrGeoff]

What's the tolerance on the 10VAC specification (eg 9VAC - 12VAC OK)?
What's the load regulation supposed to be (eg +/- 10%)?
What's the input voltage variation (eg +/- 20%?)

Something like a 12-0-12 160VA off-the-shelf toroid with the secondary windings in parallel will give you 12VAC at around 8A.

[adept]

The VAC should be close to 10 VAC. 9 or 12 is probably okay... I already took into account the input variation in the selection of 10VAC. That allows it to run at 110VAC and still work up to 125VAC. The load regulation should be close, but not exact. I have filter caps and linear regs in place for that... The transformer, did I mention, should also have a center tap that supplies 30VAC at at least 2 amps? Whoops. And could you elaborate on how I might use the 12-0-12 to accomplish close values? Would I have to re-coil the darn thing?

[DrGeoff]

Off-the-shelf toroids are available in 9-0-9 and 12-0-12 configurations. A 160VA toroid with secondaries in parallel will enable you to get a 9VAC or 12VAC output at 160VA.
No idea what you mean by 'a center tap that supplies 30VAC'. This is meaningless.
If you mean another tap that supplies 30VAC then it's becoming a specialty transformer. You could get one wound, but that's expensive. You might be able to add a winding over the first secondary winding to get the 30VAC, however at 2A you are pulling another 60VA, so you need to resize the transformer accordingly. A quicker and easier solution, if you have the room, is to get a separate 15-0-15 100VA toroid, which will give you a 30VAC 3A winding with secondaries in series. This might be the quickest and cheapest solution.

[Zero999]

Please post the full schematic.

What output voltage range do you need?

With a cheap linear regulator, a 10VAC transformer, a bridge rectifier and a monster 33,000µF capacitor the maximum regulated output voltage will be about 7V.

[adept]

I'm on a company computer right now, so I can't really post the schematic BUT, I do like the Dr's plan and am looking into it. I don't believe him when he says its the CHEAPEST... because the cores are really expensive. But I do think I would rather spend an extra 5 bucks on a toroid than have to unwind a darn transformer. I do want to ask where I can get a cheap transformer though. I don't want to spend 30 bucks a transformer, esp. if I am buying 2! I could always wind my own if it has to cost that much, just tell me where to get a cheap ferrite core and I'm your man! Thanks Doc for telling me that much.

[Zero999]

I'm not sure what you're getting at.

By toroid, he means a full transformer.

In your first post you mentioned ferrite cores but these are only used in high frequency switching regulators, mains linear regulators use a large transformer with a core made from electrical steel.

[DrGeoff]

That's why I suggested going for a second transformer rather than trying to alter or wind one.
The OP does not appear to have enough knowledge or experience to be able to alter an existing transformer with a reasonable degree of confidence.

As for cheapest solution, it depends on how you value your time. A 30 dollar toroid is cheaper than spending a couple of hours adding a new winding to an existing one. Besides, he'll need a larger toroid if he wants to add another 60-80VA winding to it. There should not be any need to unwind it though, just wind a new secondary over the top with the correctly sized wire.

[Zad]

How about using something like a "brick" generic laptop replacement PSU? They vary in output, but you should be able to find a 12V one at sufficient current.

[adept]

Hero999, do you mean like laminated silicon steel? And should I look at an E-I set of cores or a toroid? I started out thinking the E-I pair, DrGeoff told me to use like a toroid, and now I'm confused. Could either of you explain which shape I should use?
DrGeoff, would it be reasonable to use a microwave transformer; they have plenty of potential for what I want, already have the primary windings, and the really fine wire is pretty easy to take off. All the problem with one of those is that I don't know the exact number of windings on the primary, so I can't tell how many windings I need. And I apologize for my lack of formal education and experience with analog electronics: I'm doing my best though.
Zad, to answer your suggestion, its because I honestly don't HAVE one and there is no reason to spend 100 bucks on one when I have diode bridges, linear voltage regulators, resistors, zener diodes... to build one myself. The only cost is going to be the transformer... so hope that clears stuff up.
Questions for anyone: How can I calculate how much current will saturate my core? How do I know how many windings are in the primary of a microwave transformer(ohm meter maybe?)? Finally, how much extra heat is going to be made because of the transformer?

[Zero999]

The cheapest solution is to simply buy a transformer. You won't save any money making one yourself. The materials will cost the same or more than what you'd pay for a ready made transformer. If you want custom voltages you can buy transformer kits but it will work out more expensive than buying a ready made transformer with the correct voltage.

My local supplier sells 100VA toroidal transformers for £12.95, £15.54 including VAT which is $25.33.

Newark have a 120VA 15V transformer on promotion for $23.82 but I don't know what taxes/shipping will cost.
http://www.newark.com/multicomp/mcta120-15/toroidal-transformer/dp/38K4905

[Psi]

This isn't applicable to you, since your building a linear supply.
But in case it's not occurred to others. It's very easy to modify most switch mode power packs to output a different voltage (within reason) by locating the feedback resistors and changing the voltage divider.

Ya should be able to change the voltage +10% or -30% without having any problems. Ya just have to make sure you don't accidentally power it up with no feedback at all, they tend to blow up.

The hard part is normally getting into the powerpack, if its glued it can be tricky

[adept]

Thanks once again Hero, that's basically exactly what I needed! I just wish the one you showed me was 6A. However, I looked around that website and found what I needed, so thanks-a-whole-lot!

[Zero999]

The transformer I linked to has two 4A|secondaries which can be connected in parallel giving a total capacity of 8A.

[Reuben]

I used to build my own, but honestly buying a 5 amp digital power supply with presets was the best 200 dollars I ever spent.  Mine is a quakko that I got from mastech, but b&k make some very nice ones.  Extech makes some even cheaper ones.  I would advise buying one simply for the current limiting and preset voltages and currents.  Mine is very small and portable.  I love it.

[alm]

Keep in mind that I_DC = I_AC * 0.62 with a standard full bridge rectifier, so a transformer rated for 8A AC won't be able to deliver 6A DC.

[johnwa]

DrGeoff, would it be reasonable to use a microwave transformer; they have plenty of potential for what I want, already have the primary windings, and the really fine wire is pretty easy to take off. All the problem with one of those is that I don't know the exact number of windings on the primary, so I can't tell how many windings I need. And I apologize for my lack of formal education and experience with analog electronics: I'm doing my best though.

Microwave transformers are probably a bad idea - they are made too cheaply and run too hot. I have rewound them for use in welders before, but I would not suggest them for a continuous duty application that may be left unattended.

Keep in mind that I_DC = I_AC * 0.62 with a standard full bridge rectifier, so a transformer rated for 8A AC won't be able to deliver 6A DC.

Yes, in fact with a low source impedance and a large filter capacitor, it can easily be worse. I have got a graph of derating factors I can post if anyone is interested, but probably around 0.5 is reasonable for most applications.

[alm]

Keep in mind that I_DC = I_AC * 0.62 with a standard full bridge rectifier, so a transformer rated for 8A AC won't be able to deliver 6A DC.

Yes, in fact with a low source impedance and a large filter capacitor, it can easily be worse. I have got a graph of derating factors I can post if anyone is interested, but probably around 0.5 is reasonable for most applications.

I agree, it's just a rule of thumb, not sure how the 0.62 was calculated, probably with some standard filter capacitance (1000uF/A?) and transformer. The main point is that it isn't anywhere close to 1, and also significantly less than the 1/sqrt(2) one might expect.