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### Topic: Transformer Power Supply Problem (Read 464 times)previous topic - next topic

#### SagarDev

##### Jun 19, 2017, 04:25 pm
Hey Guys,

I bought a 12-0-12 1A Center Tapped Transformer to build a 12V power supply for my projects.

I used a 2 Yard Mains cord for the 230V - 240V AC, a Bridge Rectifier and a 220uF Electrolytic Capacitor for the Rectification and smoothing.

I tested the Output DC Voltage with a Multimeter and the results were confusing and shocking:

Case 1: Tapping Connected to -ve terminal : V = 17V - 19V
Case 2: Tapping Not Connected : V = 40V - 42V

What is going on..? How do I repair this..? Is there any way to get 24V DC from a 12-0-12 Transformer..?

Note: Schematics attached...
Shakespeare's pen is an Electronics Engineer's Soldering Iron...

#1
##### Jun 19, 2017, 04:56 pm
You need a regulator now to bring the output voltage down to 12V.
A switching regulator is best - it chops up the DC-ish level and feeds it PWM-style into an output filter where it is smoothed again to make 12V DC.  If more current is needed, the PWM pulses are wider, if less current is being drawn the pulses are narrower.
Here's an example, the data sheet explains the method in better detail
http://power.murata.com/data/power/oki-78sr.pdf
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### jremington

#2
##### Jun 19, 2017, 05:23 pmLast Edit: Jun 19, 2017, 05:24 pm by jremington
Transformer winding voltages are AC, measured as root mean square (rms) values. The peak voltage is 1.4 times the rms voltage, so after rectifying and filtering, 12 VAC becomes about 16 VDC.

To get 24VDC, do not connect the center tap, use the transformer as 24VAC and a buck regulator as described above.

Do not use a bridge rectifier with the center tap connected as in case 1 of your photo. This shorts out one winding!

#3
##### Jun 19, 2017, 08:15 pm
The center tap is often connected to Gnd to make a +/- power supply.

See Dual Complementary Rectifier here

I used to do that for analog work when +/-12V or +/-15V supplies were needed for op amps and ADCs and perhaps DACs.
The 7812/7912 and 7815/7915 regulators could get pretty warm.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### MarkT

#4
##### Jun 19, 2017, 10:15 pm
I tested the Output DC Voltage with a Multimeter and the results were confusing and shocking:

Case 1: Tapping Connected to -ve terminal : V = 17V - 19V
Case 2: Tapping Not Connected : V = 40V - 42V

What is going on..? How do I repair this..? Is there any way to get 24V DC from a 12-0-12 Transformer..?
What's going on is you forgot to find out how to design a transformer power supply before buying the
(possibly wrong) transformer.

Things to remember:

1) open circuit voltage from a transformer winding are typically 10 to 20% higher than at rated full power.
2) AC voltages are rms.
3) No-one designs transformer supplies for a product any more because switch-mode supplies are
lighter, better, more efficicent and generally cheaper, but there are still a lot of design guides around.
4) mains voltage varies from place to place, so you have to be conservative with your voltage ratings, in fact
your transformer may be fine for purpose - that 40V rectified will become 33V under full load, you have to allow
for a couple of volts loss in the rectifier diodes, you have to allow for some voltage drop between the peak
voltage and the smoothing cap voltage, and you have to allow perhaps 2V drop-out voltage for a linear
regulator.

Perhaps you can see know why people use switch-mode supplies, the efficiency is generally a lot better
and they can compensate for different mains voltages automatically.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### Wawa

#5
##### Jun 20, 2017, 01:02 am
Ripple could be be a problem with a 220uF cap.
Rule of dumbs is 2200uF per Amp.
3300uF or 4700uF would be better.

Better dump that transformer based idea, and buy a switching supply.
Cheaper, much higher efficiency, minimal heat.
12volt/2Amp supplies are ~\$5 incl. shipping on ebay.
Leo..

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