# What will happen if I connect a resistor to a constant current LED driver?

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?

• 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. – Bimpelrekkie Nov 1 '19 at 8:01
• @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). – Eugene Epifanov Nov 1 '19 at 8:08
• @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. – Whiskeyjack Nov 1 '19 at 8:21
• 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. – 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.

• I suspect "Case 2" is the most likely scenario. Thank you a lot! Now everything is perfectly clear. – Eugene Epifanov Nov 1 '19 at 8:28
• 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. – Whiskeyjack Nov 1 '19 at 8:47