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I'm working on something where I need to take ~10VAC @ 60hz and step it up to around 90VAC @ 60hz. Ideally a small step-up transformer should be fine, but I'm having trouble identifying the part I need through any of the major components vendors. Everybody seems to focus on listing parts that step down from 120VAC to some value... searching for something designed to go the other way has been an exercise in frustration. Can any tell me if there's a secret to identifying these things? Even the "parametric search" at places like Newark and Mouser don't let you do a simple thing like selecting the primary side and secondary side voltages.

And, FWIW, yes, I know I could use a step down transformer and just treat the secondary as my primary. But then you run into limitations in regards to how much current the "cheat primary" can carry, etc. I'd really like to find a part that's designed specifically to be a step-up, that has a ratio around 1:9.

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    \$\begingroup\$ You say that current is important, but fail to say what that current actually is. Also, why 90VAC? If we know what you're trying to do with this 90VAC then we might be able to give you a better answer than even you are expecting. \$\endgroup\$ – user3624 Nov 14 '11 at 20:35
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    \$\begingroup\$ I don't have an exact value for the needed output current, actually. I think a few milliamps will be enough, but I'm doing a bit of trial-and-error here... The reason for 90VAC, though, is because I'm trying to ring a telephone (that is, an actual old phone with a mechanical bell in it). \$\endgroup\$ – mindcrime Nov 14 '11 at 21:19
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    \$\begingroup\$ You need to specify desired power level. Saying what the application is will save lots of guessing and misunderstandings. Using a 110V:12V transformer rated at somewhat more VA than you need will work well enough in most cases. For the telephone application that you mention a 110:12 will be fine. Note that phone ringing is usually 16.666 Hz (from ancient memory) which will cause issues oif you use it or if you don't. There are ways aroind this. Tell us more. \$\endgroup\$ – Russell McMahon Nov 14 '11 at 21:57
  • \$\begingroup\$ Interesting, I could have sworn that I read that the frequency for ringing a phone varied between 16 to 60 hz earlier, but Wikipedia says it's 20hz in North America. Anyway, that's not a problem since I can make my oscillator generate whatever frequency. Basically, I'm building a telephone switch simulator. It's just a learning project, it has no actual application whatsoever. I just want to be able to connect two old phones and hear dial-tone, ring the phones, etc. I've got the basic voice stuff working if I go off hook, now my next step is to build a circuit that can send the ring pulses. \$\endgroup\$ – mindcrime Nov 14 '11 at 23:25
  • \$\begingroup\$ Eventually I hope to incorporate an Arduino or something, and actually read the DTMF tones, look up the appropriate "line" and the whole works. Basically a small 2 or 3 line telephone exchange to play with. :-) \$\endgroup\$ – mindcrime Nov 14 '11 at 23:27
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And, FWIW, yes, I know I could use a step down transformer and just treat the secondary as my primary

Get the datasheet for a stepdown transformer of about 10:1 or 11:1 and look at the difference in output voltage at no-load and full-load. You should be able to use it as a stepup transformer and use the voltage droop characteristics to determine roughly what it should yield under load.

It's fine to use a 120VAC 60Hz transformer at lower voltages (e.g. 90 on a reversed primary), but not at higher voltages -- one of the key limiting factors is saturation; the volt-seconds applied across either primary or secondary (scaled appropriately) can't be increased much w/o bringing the transformer into saturation. (Unless, that is, the transformer is overdesigned, in which case the manufacturer is losing money.)


(update based on comments)

20Hz? You need to includes things like that in your problem statement. If you want to use a step-up transformer, that would require a larger transformer than 60Hz because of the increased volt-seconds. You'd have to use a 60Hz transformer rated at least 270VAC on one side, in order to handle 20Hz power transmission.

Why don't you search for telephone ringer circuits? I'm not an expert but the usual way to generate odd voltages like this is to create the DC voltage you want, then switch it on/off with transistors, generating a square wave that is then low-pass filtered through passives (e.g. the phone network itself) -- that's how it's done for electroluminescent displays.

Unless the telephone spec says it needs to be, the ring generator doesn't necessarily have to produce sine waves. (this webpage cites several examples of commercial PBX equipment)

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