Full wave diode bridge rectifier troubleshooting

Question

I am trying to make a full wave diode bridge rectifier, as the title suggests. I have a 120-12vac transformer that I'm using to supply the circuit. However, whenever I measure the voltage across the output of the bridge, I am still getting an AC signal. Whats even weirder is that I now get readings across the terminal on both DC and AC settings of my multimeter, and the VDC measurement is ~20V, which seems a bit odd... Anyways, I have no idea what is happening, so any suggestions would be fantastic.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

it's not surprising you are seeing close to 20Vdc at the output. The nominal voltage of your transformer secondary is 12Vac (which could be even higher ~14Vac). Now, the ac voltages you have are RMS. So the capacitor is going to charge up to the peak value of the sine wave, which is 12Vac*sqrt(2) = ~17V for a secondary output of 12Vac. If this were more like 14Vac at the secondary, it would make sense why you are seeing 20Vdc at the output.

Answer 2

I suggest you calculate the surge current on power up and see if the Cap ESR causes the diodes to blow. If Cap ESR = 0.1 Ohm then diodes will see initially a short circuit with peak voltage turn on with ESR*C =T fo depending DF or ESR of cap. .

Use the biggest bridge or didoes you have since peak current will be 10x average for 10% Vripple.

Bridge Caps rectify peak voltage which is 1.414 * rms and with no conduction load loss of 10% typ, your noload Vout =1.5 Vrated rms with 40% ripple .

• but you have a large cap of 1mF which may have an ESR of 0.1 to 1 Ohm similar to the tiny diodes, which you may get from specs for rising V/I when saturated.

** The initial cap energy must be transfer on surge pulses and conducted with series losses. If initial condition is Vcap= 0 and transformer output is say 18Vp into diode bridge , what is your surge ? Vp=(18-2Vf(diode))/ESR (diodes+cap+xfmr)** This may in the 50A range for a 4A supply. 1/2CV^2 energy transferred =18V^2*1mF/2=162mW.

• compute your diode losses and cap losses or choose bigger diodes, lower Cap ESR and use a soft start ICL NTC inrush current limiter.

Then ensure your 0V meter reference is always on V-.