How to Calculate DC Ripple Voltageby Thomas James
In electronics there are two types of current: alternating current and direct current. Rectifiers are used to change from an alternating current to a direct current. Rectifiers are imperfect, and there is usually a residual oscillation, or "ripple," in the signal output from the rectifier. You can calculate this ripple using the printout of an oscilloscope reading of the signal output from the rectifier.
Place the end of one of the voltage probes of your oscilloscope at the output to your rectifier. Place the other voltage probe to a ground. Observe that there is a slight "ripple" visible in the signal displayed on the oscilloscope screen.
Press the print key on your oscilloscope. Retrieve the printout.
Hold the multimeter current probe against the output of the rectifier. Make a note of the recorded value of the current.
Multiply the frequency of the ripple voltage by the capacitance of the rectifier and make a note of this value. The capacitance of the rectifier will be recorded in the rectifier data sheet.
Divide the current value noted in Step 3 by the product of the capacitance and the frequency noted in Step 4. The resultant value will be the peak-to-peak ripple voltage of your circuit.
- If your oscilloscope does not automatically give you a value for the frequency of the signal, determine the frequency by taking the inverse of the period of the cycle. Measure the period of the ripple wave form on your oscilloscope print out and then find the reciprocal of this value. This will be the frequency of the signal.
- If your oscilloscope does automatically give you a value for the frequency of the signal, read this off the oscilloscope screen and do not print out your oscilloscope reading.
- The peak-to-peak ripple voltage is the difference between the voltage at the trough, or bottom, of the ripple and the voltage at the peak, or top, of the ripple in the rectifier output.
- As the frequency is just the reciprocal of the period, you can multiply the current by the period and divide their product by the capacitance of the rectifier to find the peak-to-peak voltage. It depends on whether your oscilloscope reports the frequency or the period of the wave form, but makes no difference to the final result.