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How do I switch to Constant Current instead of Constant Voltage on RD6018?
TimFox:
--- Quote from: Fungus on December 07, 2022, 01:47:09 am ---
--- Quote from: sofakng on December 06, 2022, 09:49:52 pm ---What is the expected behavior of the following settings?
V-SET: 5.0v
I-SET: 0.1A (100mA)
The LED has a forward voltage of 1.5V so would the PSU reduce the voltage from 5.0V to 1.5V and go into Constant Current mode to limit the current to 100 mA?
--- End quote ---
Yes.
Except it's not called "constant current mode", it's "current limiting" mode.
A PSU with "constant current mode" doesn't require you to set the voltage to anything special.
--- End quote ---
A true "constant-current" source has a finite compliance voltage, and on a lab supply this can be set to limit the voltage into an open circuit.
For measuring a diode forward voltage, that compliance voltage must be set higher than that diode voltage will be.
For a bench supply with true constant-current mode, the voltage must still be set to a sufficiently high voltage.
WattsThat:
Do not rely on the power supply to control the current when testing LED’s, use a series resistor.
You have a supply that can deliver 10 amps or more, so excessive current (from too high a voltage applied) will destroy the diode before the supply can ever regulate down to 10 ma which is the maximum allowable continuous current of the diode. Sure, it can handle an amp but that’s only for 100 microseconds. I doubt the supply can even current limit that low, given it’s current output ability. The desired 10ma is incredibly low relative to the output capacity, 0.01/18 is only 0.05% of full scale output, a very big ask of the supply. This may be okay in an older pure analog supply but in a modern device where things are controlled by DAC’s, that low a value may well be impossible.
Since the datasheet says VFD =1.5-1.7 V, set your supply to 5 voltage and calculate the required resistor with R =E/I so 5-1.7 / .01 = 330 ohms.
I don’t know why the diode read 1.1 volts on your Fluke in diode mode, perhaps it’s been damaged by overcurrent? I’d expect it to be shorted if really fried but I don’t know if the Vf could somehow go down with over current, I doubt it. Someone here will know.
Martin72:
Finally the series resistor is the safest, no question.
Fungus:
--- Quote from: TimFox on December 07, 2022, 04:23:45 am ---A true "constant-current" source has a finite compliance voltage
--- End quote ---
Obviously you won't be able to connect up a 1MOhm resistor and ask for an amp. :-)
TimFox:
--- Quote from: Fungus on December 07, 2022, 11:16:01 am ---
--- Quote from: TimFox on December 07, 2022, 04:23:45 am ---A true "constant-current" source has a finite compliance voltage
--- End quote ---
Obviously you won't be able to connect up a 1MOhm resistor and ask for an amp. :-)
--- End quote ---
I think that's what I said. My good bench constant current supplies (old Keithleys) have a front-panel control for compliance.
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