What will happen if I connect a 120 Ohm resistor to the outputs of a constant current LED driver designed to provide 0.600mA (24 - 38 volts)?

According to Ohm's law this resistor will allow a current of ~0.300 mA to pass at 38 volts. Will the driver work fine under these conditions? Or will it try to reach its target (0.600mA), heat up and burn my house?

enter image description here

  • 1
    \$\begingroup\$ Why would you connect a 60 ohms resistor to the output of this LED driver? LED drivers are for driving LEDs. It's a 600 mA driver to 60 ohms x 0.6 A = 36 V across the resistor. Power will be: 36 V * 0.6A = 21.6 W so that better be a resistor that can handle 21.6 W! The driver will be fine as 36V, 0.6A is within its rated output range. The resistor will get HOT though. \$\endgroup\$ – Bimpelrekkie Nov 1 '19 at 8:01
  • \$\begingroup\$ @Bimpelrekkie, I messed up a bit. I meant 120 Ohms resistor instead of 60 Ohms. The reason I'm asking is that I'd like to know if a LED driver can be used as a constant voltage source (at its upper voltage limit). \$\endgroup\$ – Eugene Epifanov Nov 1 '19 at 8:08
  • 2
    \$\begingroup\$ @OP - Depends on the driver architecture but most likely no. I would strictly discourage the thought of using CC driver as a CV PSU. You are clearly driving it out of spec. \$\endgroup\$ – Whiskeyjack Nov 1 '19 at 8:21
  • 1
    \$\begingroup\$ if a LED driver can be used as a constant voltage source The answer is simple: NO. A LED driver regulates the current through the load so even though the voltage might be constant when the load is constant (like a resistor), when you draw more current the voltage will drop. \$\endgroup\$ – Bimpelrekkie Nov 1 '19 at 9:12

A constant current driver will raise the voltage until it is able to force the mentioned current through the load. Voltage can't rise to infinity because of circuit limitations/configurations.

Your driver will want to push 600 mA irrespective of load connected to it. In your case, a 120 ohm resistor, it needs to set the voltage to 120x0.6 V = 72 V.

72 V is clearly out of possible voltage range. Now this out of spec operation is handled differently by differently designed drivers.

Case 1 - Driver will max out at 38 V and push 38/120 A = 316 mA of current continuously.

Case 2 - Driver will try it's best to reach 72 V but will fail and reset. This will keep on repeating. If you put an oscilloscope, you will see a PWM sort of curve. ON time represents the driver trying to reach 72 V mark, OFF time represents the reset duration where driver is waiting for the anomaly to go away before it tries again.

  • \$\begingroup\$ I suspect "Case 2" is the most likely scenario. Thank you a lot! Now everything is perfectly clear. \$\endgroup\$ – Eugene Epifanov Nov 1 '19 at 8:28
  • \$\begingroup\$ Yeah, case 2 is usually the case. I work with LED drivers on a regular basis and 99 % of them behave as case 2. I came across one which was behaving like case 1. I loved that design. \$\endgroup\$ – Whiskeyjack Nov 1 '19 at 8:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.