I want to know for a constant led driver having voltage ranges of min-max, like 21-42VDC,150mA minimum and maximum how many LEDs can be connected in series.
Please help me regarding above matters.
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A single white LED tends to have a voltage drop of about 3 V. That means 'about' 7 to 14 single white LEDs. Without knowing how much tolerance your power supply has to load voltages slightly outside the stated limits, it would be safer to restrict that to 8 to 13 series single LEDs.
LEDs sometimes come packaged in multiple series units, so would have a much higher terminal voltage. Connect the LEDs you want to drive to the supply (making sure they are specified to handle the 150 mA) and measure the voltage.