Saturday, 31 January 2015

ac - Choosing capacitance for recifier



So I'm trying to use a couple of rectifiers. Specifically, the "NTE5326" and the "36MB120A."


I've tried following videos on how to calculate for a smoothing-capacitor but to no avail.


If you use these rectifiers can you please give the capacitance value for each, and show how you got your value?



Answer



Using Q = C * V, charge = capacitance times voltage, we take the derivatve with respect to time, to produce


dQ/dT = C * dV/dT + V * dC/dT


Then assume C is constant, making the 2nd term become zero, and assume dQ/dT = I, and we have


Icurrent = dQ/dT = C * dV/dT


schematic


simulate this circuit – Schematic created using CircuitLab



Now you need 3 variables: I (one amp), T for 50Hertz (50 cycle per second, 0.02 seconds) and V the ripple voltage looking like a sawtooth slow fall and abrupt recharge time of 1 volt


And rearrange the I = C * dV/dT to become


C = I * dT/dV


C = 1 amp * 0.02seconds / 1volt = 0.02 Farad or 20,000 microFarads


If you use full wave rectifier at 60Hz, thus 120Hertz recharge events per second, the math is


C = 1 amp *( 1/120seconds ) / 1volt = 0.00833 Farads, or 8,333 microFarads


If 0.1 amp load, and your regulator accepts 2 volts ripple, then


C = 0.1 amp * (1/120seconds) / 2volt = 416 uF (470 is a standard value)


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