# LED Blinking On turn on and off

I made a circuit for LED Driving (current controlling), based on this circuit. Everything works fine except that the LED is suddenly blinking when the circuit is turning on and off. How can I prevent this blinking?

I can't calculate the blinking current but I think it is about 10-20 mA which is very high for me and can damage LED.

• Based on this circuit? Show the real circuit diagram not something that may or may not be similar ("based on" isn't good enough). Show power rails voltages and op-amp types. Double check resistor values and report this information if different. Dec 16, 2019 at 10:10
• It's the behavior of the circuit, turning on/off the mosfet gate to keep the current constant. you can add an RC filter and a NPN transistor (darlington) to remove that "PWM like" signal from the circuit. Dec 16, 2019 at 10:12
• Is that your ACTUAL circuit? Same opamp. Same components? If not, please show your actual circuit. Dec 16, 2019 at 11:00
• That's a very special opamp - both expensive and capable in some areas and very very very bad in others. What is your LED (part number, link). Dec 16, 2019 at 11:04
• What current do you want? A much better simpler cheaper cct is possible for a CC LED driver. Dec 16, 2019 at 11:04

Without the actual circuit and component values, it is hard to assess the problem you are facing. However, I could think of a possible reason and another very unlikely one, and a solution for it.

Possible Problem

During turn-on $$\C_2\$$ starts to be charged, once the output of the voltage divider formed by the potentiometer $$\R_6\$$ is above the inverting input of the comparator, the latter will try to turn on the mosfet. At this moment an in-rush current will flow through the LED. If either the comparator is not fast enough (small bandwidth) or the feedback is too slow (maybe due to poor layout and parasitics), the control loop will not react fast enough to pull down the gate of the mosfet. If the battery cannot supply the necessary current, it will be drawn from the pre-charged capacitors $$\C_3\$$ and $$\C_2\$$ (mostly from $$\C_3\$$ because of its relatively lower $$\ESR\$$). This will in turn lower the supply voltage of opamp if there is enough resistance between it and the power supply, which might drive the opamp out of its operating point for a short time.

Possible solutions

1. Add a capacitor ($$\\approx 100nF ... 10\mu F \$$) between the output of the comparator and the source end of the mosfet. This will speed up the feedback.
2. Replace the opamp with a faster one (bigger bandwidth)
3. Replace the mosfet with one with a lower gate capacitance, therefore a faster response time.
4. Add a rectifier or schottky diode between the LED an and the capacitors, preventing the LED from drawing current from the caps.
5. In case you are using a long cable between the LED/laser and your circuit, try to reduce its length, since its parasitic inductance can form together with the upper capacitor $$\C_1\$$ a LC tank, which might lead to oscillations.
• Thanks for Reply, I test solution number 1 and 5 which were easier to test. I just eliminate the wires and add capacitor between Source and output of Comparator. But the point is by using 1000uf capacitor all the blinking on turning on and off eliminated. Is it safe to have such big cap ? can it damage LED or Laser Diode? Dec 17, 2019 at 13:31
• Update: after adding a 1000uf capacitor, I found that by applying 0.4 volt or more to the opamp input, then LED light change abnormally and by applying 0 Volt , the LED wont turn off at all Dec 17, 2019 at 14:01
• Are you really sure you added a $1000 \mu F$ ($1mF$)? This capacitor cannot be too big, otherwise it will hold the gate-to-source voltage of the mosfet high for as long as the capacitor is charged. Try using a smaller capacitance, which is just big enough to solve your blinking issue. Dec 17, 2019 at 14:34
• I am sure it is 1mf. How about 560uf. This also solve the problem. this cap (560uf ) also eliminate LED abnormal light on changing input voltage. Dec 17, 2019 at 14:43
• $560 \mu F$ sounds a bit too big for me, but then again I do not know the actual components you are using in your design. Dec 17, 2019 at 15:14