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How do I switch to Constant Current instead of Constant Voltage on RD6018?
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John B:
That and you have to trust (or measure) that the CC control loop limits the current fast enough to an LED alone, ie you can also add some series resistance for insurance. As above, try reducing the voltage too. No need to risk the output shooting up towards 5V if the Vf is only 1.5V.
Swainster:
In my experience (which doesn't include RD6018), bench power supplies can be slow to switch from CV to CC. I've blown a few LEDs by just setting the current and wacking the output enable button. On some supplies, you may even have to switch on the output before adjusting the settings, tempting you to connecting the LED while the supply is already at the compliance voltage - been there, done that. It's safer to turn on the output at a low starting voltage then manually ramp it up till it switches into current limited mode. 

BTW, the measured forward voltage depends on the current (and temperature, production batch etc), so your multimeter diode check is unlikely to hit the same value as the datasheet.
TimFox:
Also, some DMM "diode" modes will not go high enough in voltage for the shorter-wavelength (blue) LEDs that may need > 3 V.
sofakng:
Thank you so much for the help everybody.  I obviously have a lot to learn (and stop making basic math errors).

I have a follow-up question on the LED but I'm going to post that over in the beginner forum...

Thanks again!
Fungus:

--- 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.

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