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# What Is the Difference Between LED Constant Voltage Power Supply and Constant Current Power Supply

Now there are many different specious statements about this problem. Some people say that in the volt ampere characteristics of LED, the voltage is fixed and the current is fixed. Therefore, the effect of constant voltage and constant current is the same. Some people say that when LEDs are connected in parallel, they should use constant voltage power supply, while when LEDs are connected in series, they should use constant current power supply; Some people say that because LED is a constant current device, it should be powered by a constant current source; It is said that constant voltage power supply should be used when using mains power supply, and constant current power supply should be used when using battery power supply. As for why such a request, no one seems to understand.

So, should we use constant voltage power supply or constant current power supply?

First, let's take a look at what kind of device led is. Because the brightness of LED is directly proportional to its forward current, and the structure of some led determines its heat dissipation, that is, power consumption. Therefore, most LEDs will give the rated current, such as Î¦ 5 is 20mA, 1W is 350mA... Etc., but this does not mean that the LED can only work at these rated currents, nor does it mean that the LED is a constant current device. For example, Cree's 1-Watt led and 3-watt LED are the same model. When the current increases from 350mA to 700mA, the power increases from 1W to 3W, so this led can work at any value between 350-700ma.

To deeply understand this problem, we must first know the volt ampere characteristics of LED.

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Volt ampere characteristics of LED

The Chinese name of LED is led, so it itself is a diode. Its volt ampere characteristics are very similar to those of general diodes. It's just that the curve is usually steep. For example, the volt ampere characteristics of a 20mA straw hat LED are shown in Figure 1.

Figure 1. Volt ampere characteristics of low power LED

If the dry battery or battery is used for power supply, a small voltage change will cause a large current change because of the nonlinearity of the volt ampere characteristics of the LED. In the figure above, for the LED with a forward current of 20mA when the power supply voltage is 3.3V, if three dry batteries are used for power supply, the new battery voltage will exceed 1.5V, three batteries will be 4.5V, and the LED current will exceed 100mA and will burn out soon. The same is true for 1W high-power LED. Figure 2 shows the volt ampere characteristics of 1W LED of a company, and the voltage of a 12V battery can be reduced from 14.5V to 10.5V when fully charged to almost discharged. The difference is nearly 20%. It can be seen from the volt ampere characteristics that a 10% change in the power supply voltage (3.4v-3.1v) will cause a change of 3.5 times the forward current (from 350mA to 100mA).

Figure 2. Volt ampere characteristics of 1W high power LED

2. Temperature coefficient of volt ampere characteristics

Up to now, many people think that the LED voltage is fixed and the current is fixed, so the constant voltage and constant current are the same. In fact, the volt ampere characteristics of LED are not fixed, but change with temperature. Therefore, when the voltage is fixed, the current does not necessarily change with temperature. This is because LED is a diode, and its volt ampere characteristics have the characteristics of negative temperature coefficient.

Figure 4. Series resistance can only reduce the influence of temperature, but can not eliminate it

4. Several LEDs are connected in parallel. Can you use constant voltage power supply?

Due to the discreteness of LED volt ampere characteristics, not only the volt ampere characteristics of LED with the same wattage produced by different manufacturers are different, that is, the volt ampere characteristics of LED of the same model produced by the same manufacturer are also different.

Figure 5. Dispersion of volt ampere characteristics of LEDs produced by different manufacturers and the same manufacturer

Obviously, if you use a constant voltage power supply of 3.4V, it is obvious that the current flowing through each LED is different, and the brightness of each LED is also different. Therefore, constant voltage power supply cannot be used.

5. After multiple LEDs are connected in parallel and powered by constant voltage power supply, can different series resistors be used to balance the current?

It's OK at room temperature, but it can't be maintained after the temperature rises. This problem is shown in Figure 6. The volt ampere characteristics of LEDs at room temperature are represented by solid lines. The volt ampere characteristics of the two LEDs are slightly different in slope. When using constant voltage power supply VO, different resistors can be selected to obtain the same forward current io. However, when the temperature increases, its volt ampere characteristics shift to the left, as shown by the dotted line. Because it is still the original constant voltage and the original resistance, the current at this time becomes I1 and I2. It's not equal to the original io.

Figure 6. The series resistance can keep its current constant at room temperature, but it can not keep current balance after temperature rise.

6. After n LEDs are connected in series, if the constant voltage power supply is used, the temperature effect (current increase caused by temperature rise) will be expanded by N times,

This is because after all LEDs are connected in series, the volt ampere characteristics of each LED are connected in series along the voltage axis

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Figure 6. Multiple LEDs are connected in series, which is equivalent to that multiple volt ampere characteristics are overlapped at the constant current point. After power on, the temperature rises and all volt ampere characteristics move to the left.

After the temperature rises, the N volt ampere characteristics shift to the left, which increases the current by N times. If the constant current power supply is used, the current can still be kept constant at Io after the temperature rises.

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