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# Transformer specs?

I'm looking to buy a toroidial transformer to maybe make a dual purpose power supply:

1. providing a clean high current (maybe 1.5 amps or so) +15V to power my Szekeres amp in one mode, or maybe a dual supply at +-15V for something else

2. power a virtual battery supply that'll keep NIMH batteries ~24V fully charged with a MAX712 circuit

How do I determine what specs of a toroidial transformer I should buy from a place like Digikey in order to supply enough voltage and current?

I noticed in Jan Meier's Corda power supply circuit he shows a toroidial transformer with 2 x 15V, 7VA. What does this mean? 2 15V secondaries and 7VA is the max voltage*current it has? Does that mean the current supplied if both secondaries are used is 7VA / 30V = 0.233 Amps?
If this is how it's calculated, that means I have to find a transformer that's 2 x 15V, 45 VA to get +-15V at 1.5 Amps?

Does the transformer have to have enough total secondaries voltage that can be combined to 30+ V so I can use maybe a LM317/LM337 combo to make the +- 15V, using a transformer that has maybe 20V and 12V secondaries? Or do I need something with at least 2 15V secondaries?

How do you determine the VA (VAC?) of a transformer if all a catalog gives you is a VCT of 88V, or secondaries of 44V as in this page:

http://www.allelectronics.com/cgi-bi...852&type=store

?

Thats alot of questions in one post! Can we narrow it down a bit?
Do you want to build the DC coupled Skeres? Or the capacitor coupled version?

There is no way to tell the VA rating of that transformer at Alltronics without measuring it, if they onlt give you a secondary voltage. You can make a very inacurate guess from the weight of the tranny.
I've already built the mentioned amps. I'm looking to maybe build a universal DIY amp power supply for myself.

But ignore that. The significant questions I had were about one topic, in the middle of the post. I'll relist them:

How do I determine what specs of a toroidial transformer I should buy from a place like Digikey in order to supply enough voltage and current?

I noticed in Jan Meier's Corda power supply circuit he shows a toroidial transformer with 2 x 15V, 7VA. What does this mean? 2 15V secondaries and 7VA is the max voltage*current it has? Does that mean the current supplied if both secondaries are used is 7VA / 30V = 0.233 Amps?
If this is how it's calculated, that means I have to find a transformer that's 2 x 15V, 45 VA to get +-15V at 1.5 Amps?

Does the transformer have to have enough total secondaries voltage that can be combined to 30+ V so I can use maybe a LM317/LM337 combo to make the +- 15V, using a transformer that has maybe 20V and 12V secondaries? Or do I need something with at least 2 15V secondaries?
Hey,

I have a Magnetek Transformer. It has rating, 28V CT @ 1.08 (14V AC @ 2.12A) effectively it is 30VA.

Mr. Meier's circuit does not require 1A output. May be something like .2A like you said. So 7VA would be reasonable. But for Szkeres' I would like more power delivered.

Tomo
Thanks, it's a lot clearer for me now. I was just using the Corda-spec'ed transformer as an example of something you'd see in a schematic.

I think I know what to look for now at Digikey.
Talema transformers (the ones at Digikey and which Jan uses) have dual separate secondarys. You can get e.g. 2X15V 7VA model for example which has 7VA *per secondary*. You can either parallel them for double current at 15V (14VA) or have them separate for two 15V supplies (+-15V) at 7VA. Primarys can also be connected for 110V or for 220V. And to top it all, they are low profile and are meant to be mounted on PCB directly which is incredible benefit if you're ever trying to put that stuff in a proper metal box.

Actually, markings on Talemas are very good, you get all the pinout and power information printed on the transformer. Also look at the digikey catalog page (available as link from any transformer order page there) and it will tell you more details including what fuse rating should be used.

The only problem is that Digikey is VERY frequently out of most popular values for Talemas. I once waited for 4 months to finally get transformer I wanted.

However, don't forget one thing: if transformer has 15V rating, that's 15V RMS. Peak voltage (top of the sine wave) is 15 * sqrt(2) and that's what DC voltage will be after rectification, plus transformers usually have 10% reserve to compensate for fluctuations in mains. So with 15V transformer the voltage at capacitors will be 25-26V!!! So you can't use 16V electrolytics caps! Yeah, if you load the transformer the voltage will drop and you also need a few volts for voltage regulator to work properly but I usually end up using 12V transformer to get 15V supply.
Hello,

Toroidal is pretty good for size, but then their not significantly smaller. Double Bobbin type trannies are also pretty good. And, they are also pretty good.

Jang's articles say that double bobbin/separate bobbin type have higher rejection for high frequncy noise in ac line. ...

Tomo
If a wiring diagram for one of these transformers shows 2 primaries next to each other, and 2 secondaries next to each other like in the diagrams in Digikey's catalog (http://info.digikey.com/T012/V4/662.pdf), how would one go about the primaries for a 110-120V AC input, and the secondaries for either serial or parallel configuration? (e.g. do the 2 middle wires get hooked together for a series connection, etc.)
Primaries in parallel for 110V, series for 220V, though I sometimes use only one primary. Secondarys, if you want series you connect the middle wires together as you said.

As I said, very versatile and practical transformer. The price is a somewhat high compared to ordinary transformers (about 2X). However the PCB-mount ability is for me personally by far the most imporant feature here.

For wiring in parallel, I just wire it so the current is flowing in the same direction, right?

Are the loops in the secondaries wound in the same direction? <-- duh, they should be if wiring the middle together is for series. Stupid me.
Try to find PCB layout of any project using a Talema transformer, like SDS amp or DAC. That way you'll be sure how to do it.
aos's not kidding re: sec voltage - these so-called 15V transformers are ludicrously overbuilt to ~26V... it's just gonna be heat. (although, it does drop alot if your load is 3/4th of the rated load, so don't over-spec too much)

>>2. power a virtual battery supply that'll keep NIMH batteries ~24V fully charged with a MAX712 circuit

Not to rain on your parade, but you can't. 16 cells is max per charge controller chip, so 20V is it. And when you get above 15V and also when you get above 10 cells, you have to compensate those MAX712/3 layouts liberally with pass transistors / level shifters. AND add to that, (depending on your charge current) those cells are gonna get pretty hot...

Actually, when you can't find any, just buy yours from aos... I know when he got so angry last year never being able to get them when he needed them, when they finally became available he bought like 50 for himself. hehahehae
Damn, ok I was being optimistic with 24V, but I can settle for even 18V (which I use now anyway), to power your #42 amp. I should double check those Max712 circuits I found to see if they could really accomodate my needs. Where can I find info about these pass transistors and level shifters?

Hmm, the circuit I was looking at is set for a max of 11 cells. Darn Do I settle for just a regulated and very filtered power supply instead? Maybe
Actually I only got 2. Those suckers are \$20 each and I am just a working guy (well at least I show up for work every day...).

There's a funny and embarrasing story to this, when I first got my 15V Talema I wrote a furious letter (well no, actually a polite one but with lots of techno talk and signed with my BSc. degree to add some weight) to Digikey telling them that the @!#\$#@ thing is damaged because I was getting 26V - and you bet my regulators in SDS DAC were scorching hot (and not just talk, I literally scolded finger a little). They were very nice and sent me another at their expense - and this one was the same. Sheldon Stokes in the meantime replied that he didn't expect 26V and is a little surprised his DAC is still working after 4 years (since capacitors are rated at 25V, but I guess as ppl said they can handle higher peak voltage than that). So then I wanted 12V transformer and of course they were out of stock. I had to wait 4 months to get one... er, 2. Lesson is learned and I have here a 2x18V transformer to use for 2x24V power supply for Kumisa 3.

But I DID get 10 EL2001ACN's this week and I'm not saying where from . Saving them for the ultimate portable DAC/amp. Probably CS8414 + PCM1704 + (Apheared AD823 + EL2001).
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