LED light flickering problem, help

alianyyj

LED light flickering problem, help [Copy link]

 

The circuit is as shown below. Its purpose is to provide constant current power to 12 3V LEDs. The problem now is that the LED light is visibly jittering when it is on. The AC gear of the multimeter tests that there is no fluctuation at 36V. There is an AC voltage of about 0.06V at both ends of R17.

I tried: 1. Connecting capacitors 104, 105, and 106 in parallel to both ends of CN1 had no effect; connecting a 10uf electrolytic capacitor also had no effect;

2. Adding 103,104 to the R16 position has no effect;

3. Changing the resistance value of resistor R14 to 5.1K or 47K is useless;

Please give me some advice, thank you.

alianyyj

Among the five samples, one is shaking slightly, one is shaking quite obviously, and the other three have no visible shaking.

maychang

alianyyj posted on 2020-10-19 18:52 There are five samples, one is slightly shaking, one is more obvious, and the other three are not visible.

Disconnect the right end of R14 from the microcontroller I/O port, and connect the right end of R14 directly to 5V to see if it still shakes.

chunyang

You cannot use 36V to power 12 3V LEDs. You must increase the voltage or reduce the number of LEDs to provide enough voltage drop space for the constant current source circuit.

chunyang

If you must use 12 LEDs and the power supply voltage cannot be increased, then change the LEDs into two groups, connect each group with a current-sharing resistor in series and then connect them in parallel, and then use a constant current source circuit to drive them.

qwqwqw2088

The 36V power supply needs constant current to prevent the LED light from shaking, which is actually stroboscopic

There is no need to string 12 lamp beads together

Calculate the constant current value of the LED.

To ensure that the current of the LED lamp beads can emit light normally, a current limiting resistor can be added if necessary

alianyyj

chunyang posted on 2020-10-19 19:23 You cannot use 36V to power 12 3V LEDs. You must increase the voltage or reduce the number of LEDs to provide enough voltage drop space for the constant current source circuit.

The actual voltage drop of 12 LEDs on the board is only 31-32V. The LED current is 18ma.

alianyyj

qwqwqw2088 posted on 2020-10-19 20:08 36V power supply needs constant current to make the LED light not shake, in fact, it is strobe. There is no need to string 12 lamp beads in a string. Calculate the constant current value of the LED, ...

36V comes from the mains through a back-up step-down circuit, so it is not possible to make a constant current

alianyyj

maychang posted on 2020-10-19 19:23 Disconnect the right end of R14 from the MCU I/O port, and directly connect the right end of R14 to 5V to see if it still shakes.

I tried connecting to 5V directly without going through the microcontroller, but it still shakes.

well_kim

Cancelling Q4 will definitely not flash

qwqwqw2088

alianyyj posted on 2020-10-20 08:12 36V comes from the mains through the back step-down circuit, and it is impossible to make a constant current

The LED light string flickers because the current is not appropriate

How big a circuit should be for the LED string?

alianyyj

qwqwqw2088 posted on 2020-10-20 09:57 alianyyj posted on 2020-10-20 08:12 36V is from the mains through the back step-down circuit, and it is impossible to make a constant current for the LED light string...

The customer required the LED current to be 18mA. It was previously constant voltage (36V power supply directly connected with current limiting resistor, three-stage tube drive). However, due to differences in LED forward voltage drop and power supply voltage at 36V, the output current of the board was in the range of 17-22mA, so constant current was used.

chunyang

alianyyj posted on 2020-10-20 08:07 The voltage drop of 12 LEDs on the actual board is only 31-32V. The LED current is 18ma.

Then you have to change the constant current circuit to reduce the voltage drop.

chunyang

alianyyj posted on 2020-10-20 08:12 36V comes from the mains through a back-up step-down circuit, so it cannot be made into a constant current

The appropriate solution depends on where else 36V is used and what the requirements are for voltage accuracy.

alianyyj

chunyang posted on 2020-10-20 14:19 The appropriate solution depends on where else 36V is used and what the requirements are for voltage accuracy.

36V is also used in one circuit to step down the voltage through the voltage regulator tube to power the relay; the other circuit is stabilized by a 12V voltage regulator tube with a series resistor, and then stabilized to 5V through the ht7550 to power the microcontroller and other circuits.

alianyyj

alianyyj posted on 2020-10-20 14:31 36V is also used in one way to power the relay through the voltage regulator tube; the other way is stabilized by the 12V voltage regulator tube with series resistor, and then stabilized to 5V by ht7550 to supply the microcontroller and other...

The 5V is then filtered through a second-order low-pass filter to power a small signal detection module, with a current of microamperes.

alianyyj

well_kim posted on 2020-10-20 08:44 Cancelling Q4 will definitely not be a flash

The reason for this is that the current difference is too large. See the 12th floor

chunyang

alianyyj posted on 2020-10-20 14:31 36V is also used in one way to power the relay through the voltage regulator tube; the other way is stabilized by the 12V voltage regulator tube with series resistor, and then stabilized to 5V by ht7550 to supply the microcontroller and other...

Then you can increase the voltage of the 36V power supply appropriately. You must consider the discreteness of the junction voltage drop of LEDs from different manufacturers, different batches and at different operating temperatures to ensure the normal operation of the LEDs under any circumstances.

However, as far as your power supply solution is concerned, the efficiency is too low. A more appropriate solution is to reduce the 36V power supply voltage to meet the device with the highest voltage demand in the load circuit. Other circuits can be powered by a secondary switching power supply. Do not use any voltage regulator tubes or linear voltage regulator circuits. The LED can be connected in two series in parallel.

qwqwqw2088

alianyyj posted on 2020-10-20 13:55 The LED current required by the customer is 18mA. It was previously a constant voltage (36 power supply directly connected in series with current limiting resistor, three-stage tube drive), but the board produced was due to the LED ...

LEDs need constant current drive to reduce flicker

To calculate and configure the current limiting resistor

12 LEDs, each with 18mA, do not string them together, use string first, then parallel, and then current limiting method

alianyyj

qwqwqw2088 Published on 2020-10-20 14:49 LEDs need constant current drive to reduce flicker. You need to calculate and configure the current limiting resistor. 12 lamp beads, each 18mA, do not string them together, use...

The biggest problem is that the LED part cannot be modified. The customer made a universal LED board suitable for their various products.