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Below is my circuit which drive LEDs using 555 timer. ( Total 20 LEDs are connected in parallel to D20, I have shown only one here) enter image description here

I am getting ringing / overshoot for the current flowing through LED as below. enter image description here

Is there anything that I am missing here. I would also like to know why is it coming there.

When I added 1uF capcitor after R3 (470ohm), it disappears.

Thank you.

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  • \$\begingroup\$ When you said you added a cap, do you mean between that net and ground, or in series? If it's to ground then it's just low-pass filtering the output and smoothing the high frequency component. \$\endgroup\$ – Harry Beadle Sep 3 '19 at 16:00
  • \$\begingroup\$ Yes, making it low pass filter \$\endgroup\$ – Micro Sep 4 '19 at 8:14
  • \$\begingroup\$ @Lundin, It really helped. Thank you. \$\endgroup\$ – Micro Sep 5 '19 at 12:00
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On all real systems a step response also produces an overshoot.

The designer has to consider the trade off between rise time and overshoot: a faster rise time means more overshoot and but less overshoot normally means a slower rise time.

This is the decision you're making when you're adding the capacitor to low-pass filter the output. Adding this filter reduces your overshoot, but increases your rise time.

You can tune this RC filter to act to your specification. When simulating that can be done experimentally, or you can calculate a cut off frequency:

f_c = \frac{1}{2\pi RC}

You want this to be just above the maximum frequency you want your system to output.

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  • \$\begingroup\$ I have calculated max frequency and later added LPF there. Thank you. \$\endgroup\$ – Micro Sep 4 '19 at 9:22

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