Author Topic: Low voltage LM317 power supply power up  (Read 1327 times)

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Offline Tim TTopic starter

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Low voltage LM317 power supply power up
« on: August 18, 2015, 02:04:48 am »
Several years ago before becoming interested in electronics I built a low voltage power supply to be used as a diagnostic tool for quartz analog watch movements, the idea being to see how low a voltage under which the movement will still run. A movement in good condition should still work at about 0.8v, possibly lower.

Anyhow, the circuit I followed uses variable output from a LM317 going to the positive terminal on the watch movement while the 5v output from a LM7805 is fed into the negative, allowing for overall voltage to be as low as 0.2 or so and topping out about 3v.

The interesting part are these scope readings taken at  power up with no load, just the probe attached to the power supply output terminals, or output from the individual regulators. where voltage first goes slightly negative, then ramps up to a mid point (overshooting a bit), then ramps up a second time to get to the 3v target in this case.

Would this be a correct description of what is happening? As a newbie, it appears to me that it takes about 100us for the 317 to match the 5v from the 7805,  it then ramps up and settles down for about 400-600us to an overall output of +1.25v (the 317's reference voltage) where regulation kicks in. At this point the 317 then brings the final output up to the +3v (+8v from the 317 , – 5v from the 7805).

This might be old hat to many people but it is pretty cool to me.

thanks,
 


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