Test constant current LED Driver, without an LED - Page 1
Forum section: electronics / projects

author: ajp8868
author_rank: Regular Contributor
timestamp: February 16, 2023, 08:45:51 pm
content: I'm wondering if it's possible to test a constant current led driver without a working led?I have a dmm with current testing

author: inse
author_rank: Super Contributor
timestamp: February 16, 2023, 08:57:29 pm
content: Substitute the LED by a resistor R=Uled/Iled

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 16, 2023, 09:18:27 pm
content: Well if you have a DMM, just set it to current measurement with an appropriate range and place it in circuit as if it were the load. Nothing more to it.Note that if your LED driver has some integrated safety feature such as short-circuit detection, it may trigger it, depending on the internal shunt resistance. Conversely, depending on the range, the shunt resistance may be too high. Read your DMM specs and select the range appropriately. If this is a highish current driver, and so the DMM range will select a low shunt resistance, AND as I said, IF the driver has a protection feature that would detect short-circuits, then inserting a resistor in series, with an appropriate value, will do the trick. In that case, calculate the power dissipation and choose an appropriate resistor.

author: georges80
author_rank: Frequent Contributor
timestamp: February 16, 2023, 09:23:14 pm
content: What topology driver? A boost driver will most often go 'direct drive' if the output voltage (your DMM on current range for instance) is lower than the input voltage. Boost drivers typically have a DC path from input to output when not boosting.Choosing a resistor that provides the appropriate "Vf" at the chosen drive current is likely a good choice to play it safe. And obviously the resistor rating should handle the power being dissipated.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 16, 2023, 09:33:59 pm
content: Good question, can you tell what kind it is from this picture? It takes a 12v DC supply in.How do I calculate the correct resistor? I'm not sure what was meant by R = Uled / Iled (assuming Iled is led current?)

author: georges80
author_rank: Frequent Contributor
timestamp: February 16, 2023, 09:47:37 pm
content: Link to the driver? What is the output rating?Resistor = LED_Voltage / LED_Drive_CurrentBasic V = IR formula...

author: ajp8868
author_rank: Regular Contributor
timestamp: February 16, 2023, 09:56:57 pm
content: It's an OEM part so no datasheet or ratings unfortunately.The LED it originally paired with was 50w, rated for 5v.Ah, I wasn't sure if the U was a typoI can get some more pictures of the board and find the main chip number

author: georges80
author_rank: Frequent Contributor
timestamp: February 16, 2023, 10:01:44 pm
content: Well, if it's 12V in and meant for a 5V Vf LED (weird LED to have 5V Vf...), then it is more than likely a simple buck driver (versus say a buck/boost).With a buck you would likely be safe using your meter on the amp range. But, still, a load resistor would be the safest 'just in case'.Of course the easiest would be to put an actual high power LED (3V white LED) onto the output as a load

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2023, 11:25:24 pm
content: Quote from: georges80 on February 16, 2023, 10:01:44 pmThat sounds wrong to me. I've not seen an LED with a forward voltage of 5V. It might be a constant voltage output and the LED has a built-in resistor, or just that it's specified for driving LEDs with a forward voltage up to 5V, as a way to inflate the power rating.

author: georges80
author_rank: Frequent Contributor
timestamp: February 16, 2023, 11:49:31 pm
content: ^ well, it says it's a LED driver (based on the labels) and it also has a current sense resistor to the LED+, so at least some evidence of likely being current regulated.It has 50W on the PCB label. Not sure where the OP got 5V. And yes, 5V as a Vf seems 'strange/unlikely'. Bit more info would helpIf U1 (appears to be the switcher chip) has a part number, that will help determine if it is a LED driver chip.Given the 1.5uH inductor, the switcher IC must be running pretty high frequency.