# Step down transformer reference list?

Discussion in 'Power Electronics' started by endlessparadox, Feb 14, 2018.

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Feb 14, 2018
Hello i am a very big beginner in electronics and i have a project to make a stabilised DC of 12v with 100 mA
I need help with the transformer because im looking for one everywhere and i can only find 240v to 110v or 240v to 12v i would like one that would turn 240v ac to 20v ac where can i find that?

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Sep 13, 2016
3. ### Harald KappModeratorModerator

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Nov 17, 2011
Why 20 V AC? After rectification and filtering this will give you approx. 28 V DC. Regulating this down to 12 V at 100 mA will dissipate (28 V - 12 V) * 100 mA = 1.6 W as waste power.
A typical linear regulator requires an input voltage 2 V ... 3 V higher than output voltage (this is called droput voltage). Therefore 15 V input are sufficient for 12 V output. 15 V input are available from a 12 V (AC) transformer output including 1.2 V loss in the bridge rectifier - provided you use a suitable filter capacitor after the rectifier. A rule of thumb is 1000 μF per 1 A of currrent, which means 100 μF are good for 100 mA. USe 220 μF or even better 470 μF and ripple will be very low, good enough for the regulator to create a stable 12 V output.

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Feb 14, 2018
I know i need 2 or 3 volt higher i just said 20 like this because i do not even know if 15 volt transformers exist

I am looking for a decent list or website where i can easily find components i need.

But a 240 volt ac to a 12 volt ac transformer is enough to give 12v dc output?

Last edited: Feb 14, 2018

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Feb 14, 2018
6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
230V is close enough. As would 220V or 250V.

The output voltage will vary a little too, but that's why they have a regulator

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Feb 14, 2018
I need to make it on paper before being able to test how can i predict the output voltage of the transformer if it is a 230v one?

8. ### Harald KappModeratorModerator

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Nov 17, 2011
Transformers are proportional devices. With input scaled from 230 V to 240 V, output scales by the same factor.

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Feb 14, 2018
Which means the transformer tolerate additional 10 volts?

10. ### duke37

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Jan 9, 2011
A small increase in voltage should be tolerated by the transformer, however, a 60 Hz transformer will also be stressed by running on 50Hz. You do not say what frequency you have in Mauritius or whether the transformer matches this.

Small tranformers have poor regulation so the output voltage is set high so that it comes down to the specified voltage on full load. This makes it difficult to simulate without the data.

11. ### kellys_eye

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Jun 25, 2010
No.

The increase from 230V to 240V represents around 5% voltage difference so the 12V output will increase to 12V +5% which is 12.6V - not worth worrying about.

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
And if you're doing that, also consider that the mains voltage might vary +/-10% under relatively normal conditions.

bushtech likes this.
13. ### bushtech

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Sep 13, 2016
Yep, your brow gets all sweaty when you realise that your power utility doesn't guarantee to give you the exact voltage.

14. ### duke37

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Jan 9, 2011
I looked up Maurtius and the information was 230V 50Hz, so why do you want a 240V transformer unless the supply is the same as in the UK where the standard voltage was changed from 240V to 230V to agree with the rest of europe. In practise, the tolerance was increased so no changes were made.

You could use a 12V-0-12V (24V centre tapped) with two diodes or a 12V single winding with a bridge rectifier. Chose what is simplest to build.
There should be plenty of standard transformers to suit.