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I need to blink a LED with a LPC micro-controller. Enclosure is solid aluminum. LEDs will be on a separate PCB 20cm away from uC PCB and protruding from the metal enclosure. What would be the simplest reliable ESD/EMC protection strategy? I am considering transient suppressors, ferrite beads, resistors, but I am not sure whether all is necessary and at which side of the cable (maybe at both sides?) I should be protecting.

Thanks in advance!

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  • \$\begingroup\$ Not quite clear what "both sides" implies... Perhaps a block diagram or sketch of what goes where? \$\endgroup\$ Commented Aug 1, 2013 at 13:59
  • \$\begingroup\$ @AnindoGhosh they are asking if they would need ferrite beads on both sides of the cable or on both boards, instead of just one. Would two double the protection or would it be redundant. \$\endgroup\$
    – Passerby
    Commented Aug 1, 2013 at 22:26

3 Answers 3

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I had to deal with similar setup where an LED is protruding from a metallic non-grounded front panel. By shooting at the panel with an ESD pistol the shortest path for any spark would be over the metallic LED socket to one of the LED pins.

I and added a 5V TVS diode between right where the LED is connected to the PCB and the current limiting resistor, which should clamp any voltage spike directly to ground.

If your aluminum enclosure is grounded, additional protection may not even be necessary. But i don't know your exact setup.

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Neither LEDs nor microcontroller outputs have exposed ESD-sensitive MOSFET gates. If your LED box contains MOSFET drivers then you may need protection there; there are dedicated chips strictly for ESD protection, or you could examine discrete logic chips where the manufacturer has designed the chip for increased ESD protection (e.g. Fairchild Semiconductor's 74HC/HCTxx series)

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  • \$\begingroup\$ uC's output is a CMOS totem pole. Input is a MOS gate with some puny ESD protection. Problem is that even though the internal protection saves the gate it sinks the spike to uC's power supply inside the chip. Quite small discharges to enclosure generates secondary EMI (not even a direct discharge) and messes pin setup or makes uC reboot. \$\endgroup\$
    – Jurkstas
    Commented Aug 2, 2013 at 7:00
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I would use uC >zenner > transistor > clamp zenner > LED , but maybe it is an existing design already also it depends of the application of the device i/e automotive or industrial require a more robust protection than household.

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  • \$\begingroup\$ The device is household, but must be immune to ESD/EMI. For example no one would be very happy if a laptop would reboot when touched with a sparky fingers. \$\endgroup\$
    – Jurkstas
    Commented Aug 2, 2013 at 6:55

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