LEDs turn on without GND?!? CAT4101 CRAZINESS

I’m trying to dim two strings of 6 LEDs through a CAT4101. The schematic is attached. I have a 24V PSU supplying the LED power, a 5 V supply off that sam PSU acting as the input for Vin. Then I have the Arduino attached to ON/OFF PWM for PWM. And I know I have the correct resistor for RSET. Now this is where things get funky. When I bypass the Arduino by making the GND pin off the CAT4101 go to the GND of the PSU (rather than the Arduino) lights turn on to full brightness and everything is cool, but obviously no PWM. (Not weird yet) So then I detach the GND CAT4101 wire from GND PSU and attach it to arduino GND. Now I get PWM but the lights are dimmed to about half brightness even at 255.(Sort of weird) This is where things gets really WEIRD, I then remove the all GND (no Arduino or PSU GND), and a string of lights REMAINS ON!! WTF??? It must be grounding somewhere within the system?!?

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What about the 24V ground?

Nope. =(

There has to be a path to ground, that's physics. My guess is that is is going through the 5V to the arduino and then on to arduino ground and back through the mains tp the 24V ground. I think if you disconnect the 5V out of the chip the LED will go off.

You are right. :) What's the fix?? But I thought ground could be anywhere where current could distribute itself, like the ground...a less positive place (i.e. the anode).

By that I mean how can I get full brightness + PWM

When I bypass the Arduino by making the GND pin off the CAT4101 go to the GND of the PSU (rather than the Arduino) lights turn on to full brightness

What pin is this? Pin 1 of the CAT4101 is the Enable / PWM. Make sure you have used table 5 to identify the pins on the chip.

Taking that pin to ground should turn off the LED, taking it to 5V should turn it on. Do not go about arbitrarily disconnecting grounds that can damage the chip. If it turns it on then you have wired up something incorrectly. You should then be able to feed that pin with PWF from the arduino and get brightness to change over the full range by doing an analogWrite to the pin with values between 0 and 256.

Check the voltage between the CAT4101 LED pin and ground. If it is anywhere near 25 volts, you are probably getting an avalanche current through the transistor/fet to the Vin power supply.

While you are at it, check the 24v power supply. Is the output 24 volts? Or higher?

Many wall wart type power supplies are not regulated. They can output several volts over the rating at a lower current. If it is over 25 volts, then that is probably your problem.

@SurferTim naw it's 24V. I think a wire was crossed. So you were right Grumpy MIke. Now one string of lights is working and the other is just fluttering. Ugh. Whats wrong?? Did I break the chip to that string?? I mean when I switch grounds from Arduino to PSU, both light go on to full brightness!

Now when GND goes to the PSU GND lights go off ugh. All I did was switch out the chip controlling the LEDs that weren't turning on through the arduino!! What is up with this?? When I connect CAT4101 GND (All GNDs tied together) to the PSU GND all the lights go on. So I fell like there is something up with the arduino maybe.

All grounds connected together is good, do not deviate from this, doing so might damage components, you might have done so already.

Take the PWM input from each one in turn. Move the wire from ground to +5V by hand. If the light goes on and off then the CAT4101 is fine.

Next wire the PWM inputs to the arduino and write a small sketch to just switch them on and off with digital write. If any don't work change the pins and try again. You might find some output pins have been fried.

Finally drive the PWM pins with the analogueWrite and see if you can affect the brightness.