Simple full bridge rectifer - output in reality not as expected?

Discussion in 'Homework Help' started by drdimmer, Mar 15, 2011.

  1. drdimmer

    Thread Starter New Member

    Feb 11, 2011
    I'm trying to build a simple a bridge rectifer like this to demonstrate to my brother the principle of diodes and roughly how AC can be converted to DC as he has just covered it in physics and he has a homework question on it so I though I'd make the circuit for him and test it and show science in "reality".
    What I thought would be a simple task hasn't turned out to be!

    I've constructed the circuit on breadboard, and I'm using a 47ohm resistor in parallel with a 10uF capacitor, and the diodes used are BYV27-200-TAP as the signal generator we are using is operating at 20kHz as in his homework question. These figures were derived from a ripple specification he was required to meet.

    The supply to the rectifier being 12Vrms = 15.57V Peak. The load resistor required would be Rl=15.57/0.321 = 48.5 ohms.

    Using I=C(dV/dT). where dt = 1/(2f) as its a full wave bridge rectifer, and dV being the ripple specification of 4% of the full wave, the capacitance is approximately 13uF.


    Having made up the simple circuit and built a model in PSpice to compare outputs, we put the sig gen on to the circuit and open circuit voltage of 20V pk-pk (limitation of 8W sign gen) input drops to around 8.44V pk - pk (due to potential divider effect of the circuit and the sig gen, with the circuits impedance being so small?).

    So I thought fine, I'll change the input in PSpice to 8.44Volts, keep the components the same and at least prove that the output of the circuit in reality would match that on PSpice. Boy was I wrong! Sticking a Digital Multimeter on the output of the bread board indictates an output voltage of 2.63V. This is suggesting that the voltage drop over the diodes is almost 2V!

    Can anyone think of a reason why this might be? All I can think of is the low impedance of the components used and power limitations of the signal generator are having some effect.

    And to top it all off, a future electrical engineer in the making has been put off electronics for life after watching me get no where!

    Any help would be greatly appreciated.
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    Actually, I don't think anything very strange is going on here. What type of generator are you using for the practical version? Many signal generators have a 50Ω output impedance, and so the output voltage would drop dramatically when feeding your circuit.

    You could try putting a 50Ω resistor in series with a 20V peak to peak voltage source in your PSpice simulation, to see if this gives a comparable result. As well as a straightforward loss of peak-to peak voltage, there would also be a change in wave-shape, assuming you had a sine wave in the first place.

    Your present PSpice result seems not unreasonable. An input voltage of 8.44V peak to peak gives 4.22V peak, so your 2.63V output implies a drop of 1.59V in the rectifier. A bridge rectifier has a voltage drop equivalent to two diodes in series, so each diode drops 0.795V. That is only a little more than you might expect for a typical Silicon diode at low current.