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I have a 220 - 14 V transformer, but when I hook it up to oscilloscope or DMM they show different values. DMM shows 11.8V and oscilloscope shows 17.2.

Ok, this is cheap Chinese transformer, so it can possibly be not accurate, but what the hell with readings? I know my equipment kinda low-class and low-accuracy, but how can it have such a big difference? What is the reason and what reading is more accurate?

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The oscilloscope shows peak values and the DMM shows RMS or average values.

$$\frac{17.2V}{\sqrt{2}} = 12.162237 V$$

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In addition to what Ignacio mentioned, basic DMMs are meant to provide readings for voltages that are not fluctuating. They update a few times per second (at most) and provide you with the average value. An oscilloscope, on the other hand, is meant for measuring very fast events. A scope is a way to focus on very specific events that happen in short durations.

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