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I'm building a DMX transmitter, and follow the Texas Instruments guideline on IEC ESD, EFT, and surge resistant RS-485 circuits, having the following configuration at the moment:

Schematic of ESD and surge surpressor network consisting of MOVs, TBUs and TVS diodes as well as 2 MELF resistors

The MOVs clamp the ESD voltage to around 1kV, the TBUs block the ESD current to 1mA and the diodes clamp the voltage to 15-20V. That is higher than the maximum bus voltage, so the differential driver has to deal with the excess, which isn't a problem in transients but can lead to damage in case of surges, so two 10 Ohm MELF resistors limit the surge current. That is the reason for them.

Now, my problem is where to place the 120 Ohm termination resistor on the transmitting end of the DMX bus? Infront of the MELF resistors or behind, or even before the MOVs?

Or should I just take a 100 ohm resistor and put it with the 10 Ohm resistors in series so resistance is 120 ohms? In the later case before or after the MELF resistors, which is best for avoiding signal reflection?

I'm very lost currently, help is much appreciated, thanks ;)

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I would place it here:
enter image description here

That way you still get current limiting, without the rest of the bus 'seeing' something other than 120Ω. You could place a more ESD tolerant 120Ω resistor with a higher power rating farther up the chain, but if there is a spike with enough energy it could blow out the resistor first.

If this device is on an RS485 bus, you may also want a switch to remove the 120Ω, as you only need the terminators on the end of the bus.

enter image description here
https://www.ti.com/lit/an/slla272d/slla272d.pdf?ts=1724306198612

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  • \$\begingroup\$ Alright, will implement it there, sounds good! Also thanks for the hint with the switch \$\endgroup\$
    – Marvin W
    Commented Aug 22 at 16:42
  • \$\begingroup\$ Ah also, should I use a pulse-proof MELF resistor for the 120 ohm termination resistor too or is a normal smd resistor sufficient? \$\endgroup\$
    – Marvin W
    Commented Aug 22 at 17:08
  • \$\begingroup\$ If you put it behind the TVS diodes and other current limiting devices, those devices will take the brunt of the pulse, so you probably go with a regular resistor. \$\endgroup\$
    – Voltage Spike ♦
    Commented Aug 22 at 17:15
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    \$\begingroup\$ Since this is a DMX transmitter, by specs, it shall have the termination built-in as it will be at one end of the bus. Also true if this is RDM receiver, which it may be as it is also wired as receiver, but then it does not have proper biasing required by RDM compatible transmitter. \$\endgroup\$
    – Justme
    Commented Aug 22 at 18:11
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    \$\begingroup\$ @MarvinW You can ask a new question about that. Or look at the standard. \$\endgroup\$
    – Justme
    Commented Aug 22 at 18:35

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