Let's talk about several constant current methods of the popular TL431.
1, a single TL431 constant current circuit
As shown above, it is a schematic diagram of using a single TL431 constant current.
Principle: This circuit is very simple, using the 431's 2.495V reference for constant current, which also limits the voltage drop across the LED, but the advantages and disadvantages are equally obvious.
Advantages: simple circuit, few components, low cost, because the reference voltage accuracy of TL431 is high, R12, T13 only need high precision resistance, high constant current precision
Disadvantages: Since the TL431 is a 2.5V reference, the loss of the constant current sampling circuit is extremely large, and it is not suitable for a power supply with an excessive output current.
The fatal flaw of this circuit is that it can not be idling, so it is not suitable for external LED power supply.
The constant current point calculation of this circuit is believed to be known to everyone: ID=2.495/(R12//R13)
The power of the sampling resistors R12 and R13 is PR=2.495*2.495/R13). For low-power power supplies, the loss of this power is considerable, so it is not recommended to use this circuit for products with current greater than 200 mA.
As shown above, it is an improved schematic diagram using a single TL431 constant current.
Principle: This circuit also uses the TL431's 2.495V reference for constant current. The difference from the above circuit is that the voltage of the current sampling circuit is reduced. As long as the total design values ​​of R12, R13, and R14 are combined, the LED can be limited. Pressure drop
Advantages: the circuit is simple, the components are few, the cost is low, compared with the above circuit, the power consumption of the sampling resistor is significantly reduced, the constant current precision is high, and the fatal defect that the above circuit cannot be idling is overcome, when there are individual LED strikes When you wear it, you can automatically adjust the output voltage
Disadvantages: When the output is unloaded, the output voltage will rise, and the rising range is determined by the ratio of the current sampling circuit resistance to R12 and R13.
In fact, the real disadvantage of this circuit is that when the voltage drop consistency of a single LED is not high, the constant current point will change accordingly.
For example, the most common 12-string LED lights, the minimum voltage drop is about 35.5V, the highest return to about 37.4V (personal experience, of course, different manufacturers will be different), then the constant current accuracy will be 5% difference - 8%
3, two TL431 constant current circuit
The biggest feature of this circuit is that it can accurately maintain constant current in a certain range.
Connector brand: JST, MOLEX, HIROSE, JWT, DEPHI, TE, Deutsch, Yazaki, Sumitomo, these are very popular
Conector type: board to board connector, wafer connector
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