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I'm doing a university project and we're not allowed to use mains. Hence, I planned to use a 9V input and step it up to 24V.

However, I couldn't find any transformers online which have a 9V ac input; all I found are designed for mains (220V/110V).

I'm using a switch mode circuit to convert the 9V dc to 9V ac, then step it up to 24V ac and use a rectifier circuit to convert it back to dc.

So my question is what will happen if I bought a 80Vac "input" to 24V ac "output" transformer and used a 9V ac as the input?

I know the voltage will lower in the secondary because the voltage is much lower in the primary. If that is so, how can I calculate the difference?

Also, I'm using a voltage regulator (0 to 40V input) -> (24V output) after the rectifier circuit. Any solutions? Thanks in advance.

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    \$\begingroup\$ If you don't have enough background knowledge to answer this yourself, it's a good thing you're not allowed to use mains! Do some basic reading on what transformers do, and tell us what you think will happen. \$\endgroup\$ – Brian Drummond Feb 19 '16 at 12:10
  • \$\begingroup\$ Hey Brian ! you're correct I don't have good understanding about transformers. What I know using 9V on a 80Vin and 24Vout is the out put will decrease a lot because the secondary winding is related to the primary winding. However, I'm not sure about the safety operations as I asked Mr Olin below, "80V in and 220V out, 220/80 = 2.75, hence for 9v its 24.75 will that work ? If so could it damage the transformer?" this is my only concern. Thanks ! \$\endgroup\$ – Abdulaziz Alissa Feb 19 '16 at 12:21
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If a transformer is rated for 80 V in and 24 V out, then its output will be 24/80 = 30% of its input for any input that doesn't saturate the core. Therefore, 9 V in will yield 2.7 V out.

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  • \$\begingroup\$ Thanks a lot !, so if I use a step up transformer which has a 80V in and 220V out, 220/80 = 2.75, hence for 9v its 24.75 will that work ? If so could it damage the transformer? \$\endgroup\$ – Abdulaziz Alissa Feb 19 '16 at 12:15
  • \$\begingroup\$ @Abdu: The math is correct, and it won't damage the transformer. However, the no load current drawn from the 9 V input may be quite high. \$\endgroup\$ – Olin Lathrop Feb 19 '16 at 13:45
  • \$\begingroup\$ Note also that transformers are symetrical - if you have an 80 V in/24 V out transformer, you can connect it "backwards", feeding 24 V in to the "24V out" side to get 80 V out of the "80 V in" side. \$\endgroup\$ – Peter Bennett Feb 19 '16 at 17:01
  • \$\begingroup\$ Thanks Olin Lathrop and Peter Bennett, I really appreciate your answers. \$\endgroup\$ – Abdulaziz Alissa Feb 19 '16 at 18:48

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