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I'm just starting to work with Transformers and while doing electronic experiments I found that I didn't know much. I'm trying to find an Audio Transformer with a ratio of 1:10 or 1:20 with an inductance of about 670 Ohms. After searching through the DigiKey website on Audio Transformers everything appears to have a ratio of 1:1. But there are specifications like 3.4kCT Primary, 4/8/16 Secondary.

What does the CT mean? And How do I calculate based on these numbers what the ratio would be or to find this type of transformer I suppose it's not a Audio Transformer any longer.

Can I find this transformer http://www.conrad.com/ce/en/product/739679/670-Audio-Transformer-110 at Digikey?

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  • \$\begingroup\$ welcome John Denver to EE stack exchange. CT means "center tapped". Your information is minimal, but that happens when you are just starting out. Why are you searching for transformer of 1:20 ? ? Sounds like you might be trying to match an amplifier to speaker impedance using transformer. Give us a little more information and we might be able to help. some audio amplifiers need a transformer to match to a speaker, others don't. Help us help you. \$\endgroup\$
    – Marla
    Nov 11 '15 at 1:33
  • \$\begingroup\$ Hi Maria, Thanks for message. CT answer is was super useful thank you. Really I'm just experimenting and want to change a signal 20khz signal by a ratio of 1:10 these part descriptions just have me baffled and searching didn't help. Is a Transformer with 600Ohm/1.2k the resistances in the stages that makes it actually 1:2? \$\endgroup\$ Nov 11 '15 at 2:21
  • \$\begingroup\$ John, as is usual, answers can be complicated. 1:10 means the turns ratio, and impedance will be changed by the ratio squared. The description when given in impedance (not really resistance) takes in a whole lot. For example, if a transformer were described by 600 ohm primary and 1.2K secondary (1200 ohm secondary) : this means that a 1.2k load (on secondary) would appear to be 600 ohms on primary. Look up transformer ratio and impedance transformation. \$\endgroup\$
    – Marla
    Nov 11 '15 at 2:35
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Audio transformers are normally specified in terms of impedances. Your 3.4kCT to 4/8/16 transformer is intended to have a primary impedance of 3.4 kΩ (center-tapped) when the secondary taps are connected to 4, 8 or 16 Ω, as appropriate.

The transformer itself does not actually have a particular impedance, but it does transform impedances. You can infer from these specifications that it has impedance transformation ratios of 3400/4 = 850, 3400/8 = 425 and 3400/16 = 212.5, respectively.

The impedance transformation ratio is the square of the turns ratio, so you can further infer that the turns ratios are approximately 29:1, 29:1.4 and 29:2, again, respectively, and these turns ratios are the voltage ratios you can expect to see.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I'm pretty sure ohms are proportional to turns squared, not to turns as you have drawn. \$\endgroup\$
    – Jasen
    Nov 11 '15 at 3:37
  • \$\begingroup\$ so, if 16 ohms is 34 turns, 8 ohms is 24 turns and 4 ohms is 17 turns and 3.4K would be 496 turns. (each half 248 turns) 248:24 may be close enough to 10:1 to be useful. \$\endgroup\$
    – Jasen
    Nov 11 '15 at 3:53
  • \$\begingroup\$ @Jasen: Quite right. Fixed now. \$\endgroup\$
    – Dave Tweed
    Nov 11 '15 at 14:15
  • \$\begingroup\$ ... and yes, the square root of 850/8 = 10.3, which is very close to the ratio you're looking for. \$\endgroup\$
    – Dave Tweed
    Nov 11 '15 at 18:26
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A transformer with 1:20 ratio is one with input of 240 v nd output of 12 v. Here how to calculate - Ep Ns = Es Np Ep being volts in primary Ns " turns is secondary coil Es is volts in secondary np= turns in secondary

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  • \$\begingroup\$ This is incorrect. 1:20 is a step up transformer and not a step down. Secondly 1:20 does not specify any specific input or output voltages. \$\endgroup\$
    – efox29
    Dec 7 '15 at 11:08

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