Electronic – Problem with capacitor after bridge rectifier


I wanted to make a simple power supply so I made a bridge rectifier with 4 diodes rated 6 amps, then I connected the bridge to a transformer (220 volts to 9 volts (measured 9.4 with voltmeter) 50 Hz) and when I measured the voltage after the bridge the output was 9.1 volts.

Then I added a 4700uF capacitor (50 volts) in parallel and when I measured the output voltage it was 13.8 volts but when I disconnect it, voltage is 9 volts again. I first thought that maybe the voltmeter is broken but when tested with an small motor it was actually running faster with the cap.

Is it pulling more pressure on the transformer to draw 13 volts or something? Because I'm really confused right now. By the way will it help if I use an smaller cap? Because that would increase the ripple voltage?

Best Answer

The answers already here are correct. I just wanted to add that since you are using a Full-Wave Rectifier (4 diodes), your ripple voltage is determined by:

$$ V_{ripple}=\frac{V_{peak}}{2fCR}$$

or equivalently, $$ V_{ripple}=\frac{I_{load}}{2fC}$$

Now, the thing here is that you want to keep the ripple voltage 'small.' A good number is within 10% of \$V_{peak}\$. From your post, I see \$V_{peak}=13.8{\mathrm V}\$

If you do the math, in order to keep the ripple voltage, say, 10% of \$V_{peak}\$, the maximum current you can draw from the rectifier is : $$ I_{load}=2(1.38{\mathrm V})(50{\mathrm{Hz}})(4700\mu{\mathrm F})$$ $$ I_{load}=0.65{\mathrm A}$$

That is the maximum current you want to draw if you want keep the ripple voltage at a maximum of \$V_{ripple}=1.38{\mathrm V}\$ or 10% of the peak voltage.

I saw in one of your comments that you were concerned about damaging components. You won't damage them as long as they are rated to handle the voltage/current they are being provided with. The capacitor should be fine (since it is 50V), but they may be a lot of current running through the circuit at startup (when the capacitor is fully discharged). So you want to make sure your diodes are rated to handle that much current, and also check the reverse voltage ratings for those diodes.

The reverse voltage across those diodes are theoretically the same as the peak voltage for a full wave rectifier with four diodes (in your case the diodes should handle more than the 13.8 volts you are getting at the output).