# EMC question

Discussion in 'The Projects Forum' started by badgerbk, Feb 5, 2009.

Feb 5, 2009
3
0
I have a simple LED circuit (2 strings of 2 LEDs each with a ballast R). Input to the circuit is from LED driver (60mA). There is no protection diode at the input/output due to load voltage requirements. LED reverse protection is provided through voltage divider as max reverse voltage is -12V. The LED has a max reverse voltage of -6V. If a cap is put from driver input to ground, will the circuit survive a -300V pulse lasting 0.05ms or is a reverse protection diode needed?

Thanks for the help.

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
If the cap is between the driver output and the limiting resistor, maybe. LED's don't like reverse bias, so a protection diode is a good investment protector.

Feb 5, 2009
3
0
A diode was present in the original design without the resistor network, however the voltage drop of the diode put the load voltage of the circuit over the max. The load voltage is used as a feedback voltage for fault indication. Another reason the diode was removed is so a small resistance (ballast resistor) could be used to balance the current division in each string instead of the forward voltage of the LEDs. Some of the voltage gained from removing the diode is now developed across the ballast R. The circuit needs to be immune to transient waveforms like the one described above. I have attached a sketch to help illustrate.

Input is always appreciated.

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4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Resistors in parallel with the LED's? That looks awkward to arrange. Do you need to exactly balance irradiance? I would think a couple of LM317's set up as current regulators would be easier to get good results.

5. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I've seen a similar scheme, with 4 or more diodes for a really simple bargraph (math is slightly complex though). Doubt that is what the OP is going for though.