Wiring panel: current

Thursday, October 3, 2013

Constant Current LED Drive

Most LED driver circuits use a series resistor to control the current through the LED. For applications needing a few LEDs, this is optimal. However, for applications needing many LEDs, this becomes extravagantly inefficient and it is tempting to keep the voltage drop across the resistor as small as possible. That leads to poor control of the current. ICs such as the MM5450 and its relatives and the A6275 and its relatives provide constant current outputs so that the current through the LEDs is well controlled even though the voltage drop across the circuit doing the control is acceptably small. However, the difficulty with these circuits is that because they contain many constant current drivers crowded into a relatively small package, unless the supply voltage is small, they become too hot and can destroy themselves.

This problem is not easy to solve. The solution is to maintain a small voltage across each constant current source. In this circuit, this is accomplished by REG1, the LM317L, which provides a bias of about 1.5V ±5%. Each transistor works as an emitter-follower, presenting the A6275 inputs with about 0.9V. Vled, the LED supply voltage, needs to be high enough to ensure that there will be at least 0.5V across each transistor but it is safe to allow significantly more than this and the supply need not be well regulated. The transistors can be general purpose NPN types such as BC548 and a single LM317L will easily supply a total LED current of at least 1A. A6275s are made by Allegro.

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Saturday, April 6, 2013

Ampere or Current Booster

Small and very useful circuit, Can be built on a veroboard

Volt regulators such as the LM708, and LM317 series (and others) sometimes need to provide a little bit more current then they actually can handle. If that is the case, this little circuit can help out. A power transistor such as the 2N3772 or similar can be used.

The power transistor is used to boost the extra needed current above the maximum allowable current provided via the regulator.
Current up to 1500mA(1.5amp) will flow through the regulator, anything above that makes the regulator conduct and adding the extra needed current to the output load. It is no problem stacking power transistors for even more current. (see diagram). Both regulator and power transistor must be mounted on an adequate heatsink.

Circuit diagram:

Ampere or Current Booster Circuit Diagram

Parts:

R1 = 1R-2W
R2 = 10R-2W
C1 = 35v-470uF
C2 = 35v-470uF
Q1 = TIP2955
IC1 = 78xx Regulator

Source :http://www.ecircuitslab.com/2011/06/ampere-or-current-booster.html

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