PWM current LED PCB with STCS2A

[wdh]

Dear all, 

I received a LED PCB with 16 STCS2A constant current LED drivers in parallel, each powering 4 LEDs. Vcc=12V, for the PWM signal a 5V signal is used, which is supplied as follows: 24V PWM output of PLC is coupled to an optocoupler which is connected to a 5V power supply, the 5V output of this optocoupler is fed to the PWM input of the STCS2A drivers. When testing the system the 5V PWM signal shows high current values up to 1A, luckily this was protected with a glass fuse. What could be causing this problem? 

Happy to hear any idea's or useful things to test, the PCB is not my own production and I am trying to explain/solve the issue. 

Best, 

 

 

[Peter BENSCH]

Welcome @wdh, to the community!

well, almost understand what you mean by the description, but it's incomplete. Old rule: a picture is worth a thousand words, even if it is only hand-drawn. Just add a schematics.

Regarding your question: if I assume that the optocoupler switches the 5V to the PWM input like a high-side switch, the current may reach a maximum of 2µA (data sheet, table 5, EN, PWM leakage current). Even at 40V, a maximum of 120µA would flow, never 1A or more.

Is it possible that the STCS2A is defective or has the polarity reversed?

Regards
/Peter

[wdh]

Thanks @Peter BENSCH for the quick response! 

I think you understood the situation correctly but herewith a small sketch to clarify:

I checked the polarity of the STSC2A, which should be correct. It could be that it is defective, however it still able to light up the LEDs, only that draws too high current which leads to blowing of the protective fuse (1A). Any suggestions on how I can check if the STSC2A is indeed defective or not?

Best regards, 

 

[Peter BENSCH]

As already mentioned, PWM and EN are pure inputs and should not have any significant input current. You could approach the problem and measure the current to the PWM pin, possibly again with a brand-new device:

• 12V to VCC, 12V to EN, 12V via protective resistor of e.g. 1...10kohms to PWM
• 12V to VCC, 12V to EN, 5V via protective resistor to PWM

What does the connection from 5V supply to 12V supply look like?