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Tech Forum

2019 Issue-6

Ripple Voltage Monitor

I need a circuit to monitor the AC ripple voltage on a 12 volt linear power supply? A digital display would be ideal.

Brian Lambdin
Plano, TX

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Mr. Lambdin inquired about a circuit to monitor the AC ripple on the output of a linear power supply.  The inference is that the frequency response of the circuit need not be excessive. He also wanted a digital display.

I found what ought to be a suitable device, in kit form, from Amazon, for $29.95. See https://www.amazon.com/Oscilloscope-Handheld-Pocket-size-Electronic-Learning/dp/B01LWK49W3/ref=asc_df_B01LWK49W3/?tag=hyprod-20&linkCode=df0&hvadid=309778489815&hvpos=&hvnetw=g&hvrand=18068906155916055584&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1018200&hvtargid=pla-465542683176&psc=1

Peter A. Goodwin
Rockport, MA

In a linear supply the vast majority of the ripple will be at the input frequency which will normally be the power line frequency (50 or 60 hz) or two times that frequency (100 or 120 hz.) A digital multimeter on the AC volt ranges is intended primarily for that frequency range. By blocking out the DC from reaching the meter any DMM set to the AC volt ranges should measure it fairly well. A "True RMS" type meter is best since the ripple probably will not be a sine wave.

To block out the DC part simply put a capacitor in series between the positive side of the supply and the positive input of the DMM. A value of 1 microFarad or more should work. If you use any type of polarized capacitor such as an electrolytic make sure to connect the positive end to the positive output of the supply. Of course, make sure the voltage rating of the cap is higher (16 or 25 Volts should be fine) than the supply. You may need to put a fairly high value resistor (say 10 K Ohms) across the meter leads (from the negative end of the capacitor to the negative side of the power supply.

William Cooke
Adams, TN