When LEDs are connected in series, what should I do if the total voltage drop is greater than the supply voltage?

sky999

When LEDs are connected in series, what should I do if the total voltage drop is greater than the supply voltage? [Copy link]

 

The VF of LED is 2V~2.4V, the current is 20mA. When 6 are connected in series, the power supply voltage is 12V. If a constant current source is not used, can a series resistor be used at this time?

maychang

[When LEDs are connected in series, what should I do if the total voltage drop is greater than the supply voltage? 】

The sum of the voltage drops of each LED is greater than the supply voltage, and the LED will not light up at all.

sky999

maychang published on 2023-5-29 16:29 [When LEDs are connected in series, what should I do if the total voltage drop is greater than the supply voltage? ] If the sum of the voltage drops of each LED is greater than the supply voltage, the LED will not light up at all.

Assuming the voltage drop is 2V, 6 of them is exactly 12V, isn't that possible?

maychang

If you connect a dry cell battery to a LED, it will definitely not light up. The red LED has the lowest conduction voltage among all color LEDs, about 1.6V, and the dry cell voltage is only 1.5V.

sky999

maychang published on 2023-5-29 16:29 [When LEDs are connected in series, what should I do if the total voltage drop is greater than the supply voltage? ] If the sum of the voltage drops of each LED is greater than the supply voltage, the LED will not light up at all.

If the voltage drop is 2V, and the 6 are 12V, will they not light up? I tried it and it seems to work

chunyang

sky999 posted on 2023-5-29 16:33 If the voltage drop is 2V, and 6 are 12V, will they not light up? I tried it and it seems to work

If it can light up, it means that the voltage drop is not strictly 2V, or the power supply voltage is slightly higher than 12V. But it is unreliable at this time, and the brightness is not enough.

chunyang

In order for the LED to work properly and achieve the desired brightness value, the supply voltage must be greater than the sum of the junction voltage drops of all LEDs and leave an appropriate margin, and then a current limiting resistor should be connected in series or a constant current drive should be used.

chunyang

If the OP's power supply voltage cannot be increased, the only solution is to reduce the number of LEDs connected in series.

sky999

chunyang posted on 2023-5-29 16:39 If the OP's power supply voltage cannot be increased, the only way is to reduce the number of LEDs in series.

OK, this seems to be the only way, thank you

sky999

maychang posted on 2023-5-29 16:32 Take a dry cell battery and connect it to a light tube. It will definitely not light up. The red light tube has the lowest conduction voltage among all color light tubes, about 1.6V. The dry cell battery...

OK, thanks

qwqwqw2088

VF is a 2V~2.4V lamp, it should not be white light

6 in series is a bit too much

It can be bright, but it is the kind of brightness that cannot be controlled by humans.

Do not let the lamp post operate at full current of 20mA. It is better to string fewer lamps and a current limiting resistor.

wangerxian

sky999 posted on 2023-5-29 16:33 If the voltage drop is 2V, and 6 are 12V, will they not light up? I tried it and it seems to work

You can measure the actual pressure drop and it should feel different from what you describe.

maychang

sky999 posted on 2023-5-29 16:33 If the voltage drop is 2V, and 6 are 12V, will they not light up? I tried it and it seems to work

[If the voltage drop is 2V, and 6 of them are 12V, will they not light up? I tried it and it seems to work]

If 12V lights up six light-emitting tubes in series, it is because the voltage drop of this type of light-emitting tube is 2V~2.4V under the rated current. However, when the voltage is lower than 2V, the current in the light-emitting tube is smaller than the rated current and the luminous intensity is not enough.

maychang

sky999 posted on 2023-5-29 16:31 Assuming the voltage drop is 2V, 6 is exactly 12V, isn't that possible?

The voltage-current characteristics of the light-emitting tube are similar to those of ordinary diodes. When the voltage at both ends is low, the current is extremely small (but a faint light emission can still be seen). As the voltage at both ends increases, the current passing through increases sharply. Therefore, the light-emitting tube should be driven by a current source. If it is a voltage source, a resistor should be connected in series to limit the current.

sky999

maychang published on 2023-5-29 18:39The voltage-current characteristics of the light-emitting tube are similar to those of ordinary diodes. When the voltage at both ends is low, the current is extremely small (but a weak light emission can still be seen). As the voltage at both ends increases...

I have 7 0.2W LED lights with a 3V voltage drop, and I have connected them in series with two 130R resistors in the middle, which is equivalent to one 260R resistor. I connect them to 24V, but a few of the lamp beads break almost every week. Is this a quality problem with the lamp beads, or did I connect them wrong?

maychang

sky999 posted on 2023-5-29 19:59 0.2W LED lamp, 3V voltage drop, I string 7, with 2 130R resistors in the middle, equivalent to 1 260R resistor, connected to 24V, but the lamp beads are almost weekly...

With two 130 ohm resistors connected in series, it is estimated that the current will not reach 60mA (the voltage on the 260 ohm resistor must be greater than 15V for the current to reach 60mA), so it is likely that there is a quality problem with the LED lamp.

maychang

sky999 posted on 2023-5-29 19:59 0.2W LED lamp, 3V voltage drop, I string 7, with 2 130R resistors in the middle, equivalent to 1 260R resistor, connected to 24V, but the lamp beads are almost weekly...

With this connection, have you ever measured how much current the LED passes through when it is actually working? Or instead of measuring the current, you can measure the voltage across the two 130 ohm resistors. Measuring voltage is more convenient.

sky999

maychang posted on 2023-5-29 20:15 With this connection, how much current does the LED pass through when it is actually working? Have you measured it? Or instead of measuring the current, you can measure the voltage on the two 130 ohm resistors. Measure...

I haven't tested it. OK, thank you. I guess it's also the quality of the lamp beads.

led2015

Add a current limiting resistor in the series circuit to protect the LED? Increase the supply voltage or reduce the number of LEDs in series?

吾妻思萌

It doesn't light up, and if it does, it's probably dim.