Posted by Tomi Engdahl on 11/16/2005, 4:09 pm, in reply to "Alternate to PWM" You can control the current using programmable current source. Basicly you have a D/A-converter that gives out voltage signal and then convert the different voltage values to current with suitable electronics. Bsicly you look the current through LED as voltage over some suitable measuring resistor. Compare that to D/A converter output with aid of opamp. The output of opamp drives the transistor that gives current to LED from power supply. The downside of the basic current source system is that there is easily considerable power loss on the transistor and in the current measuring resistor. The same idea can be converted to switch mode power supply current source. In this case you have a suitable coil and diode on the circuit. And the comparator has some hysteresis. Now when current though LED is lower than set minumum (in the beginning zero) transistor is switched on, it stays on until current had reached the set maximum in that tiem transistor gets off, power geds fed to LED through energy stored on coil, until current drops below the set minumum and the cycle is tarted from beginning. This is kind of PWM smoothed with coil+diode circuit, PWM feedback controls itself so that the current is right.
Link: ePanorama.net
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A LED an be dimmed well with PWM or by controlling the current through it.
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