# Power Supply Design 240VAC to 12VDC, what value smoothing capacitor do I need?

I am designing a power supply for a docking station, however I'm struggling to determine what value capacitor I will need? The transformer I am going to use is:http://uk.farnell.com/carel/tra12un100/transformer-240vac-in-12vac-out/dp/645400

I need a voltage regulator of 12V also and a current of 5A to power 2 separate circuits?

Update: I checked that link and it seems you want to use a 3 VA transformer to make a 60 VA supply !? You'll need a transformer of 75-80 VA.

First of all, a voltage regulator at 5A produces a lot of unnecessary heat.

Sticking to your design, I'll assume the voltage regulator has a drop of maximum 2V accross it. So it must be powered all the time with a minimum voltage of 14V.

So you have that transformer of 12 V RMS AC. After bridge rectifier you get $12 \cdot \sqrt{2} - 2 \cdot 0.7 = 15.57V_{p-p}$. This voltage should never drop below 14V. This means a maximum ripple of 15.57-14 = 1.57V. For even better reliability I'll approximate this to 1.5V.

Now the capacitor value is $C = {I \over {2fV_r}} = {5 \over {2 \cdot 50 \cdot 1.5}} = 0.033 F = 33000 uF$.

This high value makes your transformer unusable for this design. Now that you know how to calculate this, choose a transformer with higher output voltage so that the filter capacitor will have a smaller value.

• I am really struggling with this design! what design would you use personally, I am relatively new to design, any more help you can offer will be greatly appreciated. Commented May 18, 2014 at 14:33
• Get a SMPS kit like this suggested in the other answer uk.farnell.com/artesyn-embedded-technologies/nps63-m/… Commented May 18, 2014 at 14:48
• +1 for "just buy an SMPS", life is too short to reinvent every wheel unless there is a good reason (good reasons can include "for the hell of it"). Many minds far greater than ours have put a lot of effort into SMPS design so you can buy something safe, reliable, efficient and cheap from Farnell for 100x less than you'll spend in time wasted trying to whittle your own. If you just need some +12v use any old ATX PC power supply for peanuts, see dangerous prototypes website for how to wire them up. Commented Jun 17, 2014 at 18:17