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December 30, 2013

Ah Oscilloscope...

I was able to borrow a single probe oscilloscope from the Auroville ITI for a few weeks. There is nothing like an oscilloscope to understand transient signals. I was able to use it with the 10th graders at Udavi who had been working on time changing circuits and with the 5-7th graders in Isai Ambalam (IA) who had been working on understanding Energy and had visited a solar energy company in Auroville and encountered AC/DC, but didn't know what this really meant.

The primary demonstration I gave was of a voltage regulator (AC mains to 12V 'DC' output) - transformer with center tap, half-wave, full wave rectification and what happens when we add a capacitor followed by what happens when you put a load across the capacitor.
Having only a single probe I could not show them that a center tapped transformer has in and out of phase components at its output. This had to be inferred by the full wave rectification.

I was able to couple it with a cute experiment I discovered of taking capacitors of varying values, charging it by touching them to a 9 V battery and connecting it across and LED to see that the time for which the LED is on increases as the value of the capacitance increases.

The 10th graders made a couple of astute observations at the end of their class:
1) Sun - Yes, Sanjeev this makes sense. I always wondered why on turning off the power in some devices the LED of its charger is still on for some time. I think this must be because the LED works off the output of the charger which has a capacitor. Is that right?
2) Des - If the output with a load always has dips then we can't use this directly as a good DC. Should we create a higher supply and feed it to a chip like 7805 (voltage regulator IC we had used in class) and then use its output? This way as long as we maintain the ripple to be beyond 5V it will give a 5V output.

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