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Telco Surge Protection  

(For embedded modems)

Floyd Kling 





The information here MAY or MAY NOT help in your particular application.  It is prudent to define your needs before you endeavor to add protection for the modem.  Some surge events you are trying to protect from may actually compete with other events. Ensure your product is properly profiled and tested to your desired results. 


THERE ARE NO GUARANTEES HERE.  This information is for reference only.  Each design has its own nuances and results are not necessarily transferable.  I hope you find this useful but please do not put my feet to the fire if you do not get the results you are looking for.


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External Modems and especially Embedded Modem Modules are a very competitive businesses.  To keep cost and size to a minimum, manufacturers normally add only enough protection to pass the required agencies for Telco attachment devices like... FCC Pt.68, Industry Canada CS-03, CTR-21, with some UL/CSA. 


In some cases you will find modems that comply with higher level, UL/CSA, CE and even NEBS requirements.  These tend to be standalone boxed or rack mount modems that are expensive.  Not a great fit for embedded systems.


Embedded modems are then left to add external protection device(s) if more protection is needed.  The added protective components can be placed on the mother board (MoBo) as components or small modules.  You can even add external devices outside the host system i.e. in the RJ-11 cable.


Simply adding basic surge protectors across Tip/Ring and add common mode to Gnd on your mother board could help but but may not be sufficient for certain requirements.  Adding fuses/polyfuses, MOV's, thrystor's, neon bulbs, high wattage resistors and even various types of active components, will layer more and more protection as cost and size increases.


Internal Solutions:  - Stuff you can put on your Mother Board (MoBo)


LEVEL 1 Protection

As stated above, you can expend allot of size and money to add Telco side modem protection on your mother board. 


Most Embedded Modem manufacturers will at least add a 'Sidactor' on the modem module across TIP and RING that will actually offer quit a bit of protection... a common one is the P3100SB from TECCOR/LittleFuse (275V/100amp).

FYI... a 'Sidactor' is operates like two back to back Zener diodes - they turn on when a specific voltage is exceeded.

KEEP IN MIND, the key to any decent surge protection circuit is SPEED.  You want the circuit to clamp before the modem is subjected to the higher voltages/currents...  the TECCOR Sidactor boasts reaction speeds starting as fast as 1us.(Voltage dependent)

To add more protection for your embedded modem, on your Mother Board, across TIP and RING, you could add the more robust Sidactor P3100SC or P3100EC (updated 2/9/07) from TECCOR/LittleFuse.  It clamps at about 275v but offers a more protective 500AMP clamping. (note the xC designates the 500Amp, the SC is for SMT/500amp and the EC is ThroughHole/500amp)


We recommend the through hole(EC) if you have PCB area because you can get better pad spacing.  Remember the 3mm+ spacing rule for any metal to any metal.  Not enough board area?.. then use the SC (Surfacemount).


If you add a Sidactor across T/R on your MOBO make sure it clamps at the same or LOWER voltage than the Sidactor on the modem, else the Modem Sidactor will take all the current defeating the purpose of your MoBo Sidactor.


Here are data sheets for the P3100SC(275V-RECOMMENDED) and P2600(220V) <Available from Mouser>.  Be sure to use the SC(500Amp) version


More protection? - go to LEVEL 2


Add two more of these same Sidactors, ... one from TIP to gnd and one from RING to gnd.  Will take protection to the next level without too much expense if your MoBo has the space.  These are most effective in grounded systems.  If your system is not grounded, these may offer marginal help.


If you have the space, you can combine Level 1 and level 2 with a single part from Littelfuse... P3203AC



Even more Protection?... - try LEVEL 3


Adding series resistance in TIP and RING could help.

Simple way: add two each 5 to 10 ohm series resistors on TIP and RING.  Be sure to calculate your wattage needs... 1+ watt or greater may be needed depending on how much of a surge you want to survive.  In extreme conditions these could 'blow' like a fuse.

(Perhaps put the artwork for the resistor spacing in your mother board artwork and load a wire(0 ohm) or a cuttable trace to have the loading option later.)


More technical way: in place of the series resistors, would be two Resettable Fuses.  These would give the benefit of a auto-reset fusing - caution because they could be relatively slow (appx 1+sec) and may not give the protection the manufacturer suggests.


Here are links for Resettable Fuses  (Available from Digikey or Mouser):


Fast: (>.1sec <.3sec):   (CMF-SD35A or equal)


Slow: (>1.4sec <4sec)

Depending on your circuit, you may need the Bourns "FAST OPEN"


You still want even MORE protection?... there's always LEVEL 4


Adding 350V Gas Arrestors across T/R and T/Gnd and R/Gnd (usually helps with NEBS requirements)

Gas Arrestors could help dramatically.  Make sure to have the 5-10ohm resistors(1W+) after the arrestors and before the modem... see drawing


The Z series components are optional.  Be sure to consider the power they must release and leave adequate spacing for possible heat.


There is some question on some UL Tests


Adding a specially designed 'telecom fuse' in series with either tip or ring should satisfy this need. (not shown in the above drawing), but should be put in your artwork in parallel with Z1a.  I would suggest at least putting it in your traces and then you would have the facility to add the part if necessary without spinning another PCB layout.

Look at LittelFuse 4611.25EER (1.25A)


Note on Trace Routing.

Make sure the traces that contain the protection devices are routed to, and through, the protection devices and then to the modem... meaning... the traces should come from the RJ-11, (through the Resistors or PolySwitch - if any) to the Sidactor FIRST, then routed to the modem Pins... This may seem obscure, but could make a difference in protection.


DAA (Data Access Arrangement) trace spacing should target to be 3mm(.12") or greater - metal to metal from any trace or component inside the DAA area to any component outside the DAA area. If possible (and we don't often have this luxury) a 5mm(.20") min spacing would satisfy even the most aggressive agency people.


The Capacitors(C1x and C2x) are for EMI, they are not intended for surge protection.

If you have RFI caps (TIP to gnd, and RING to gnd) <and you should> make sure they are as close to the RJ-11 as possible with the gnd side of the cap attached to the front panel (if the panel is metal) If it is not metal or it is not possible to tie to the front panel, then make sure the grounding side of the RFI caps go to THE STRONGEST GROUND AVAILABLE. read this

Important Note on adding protection... I can't emphasize this enough... test, test, test.  Make sure you have a good understanding of your desired results and a clear definition of the testing procedure. (i.e. which standards are you trying to comply with?) "Protection" means different things to different people and different agencies.


"Protection" comes in many forms that are intended for specific elements... such as:

  • Safety of the persons/operators

  • Survival of the Equipment

  • Performance of the Equipment. 

"Protection" for one element, does not mean protection for other elements.



Can't modify your MoBo or Modem? ...Go External


You might try these external devices that mount on the wall or supplement the wall RJ-11 jack:

Here are some external protection devices that could do the trick for you.  They would mount on the wall, outside of box on the phone lines that feed the modem.

  • Fas-Tel-200T

  • Fas-31XT (same protection as the Fas-Tel-200T but has different mounting)


For High-Performance Lightning protection, look at:


The combination of the Fas-xxxxx... with the PPC-IS-SPTL is claimed to give the best protection for remote site lines that have maximum exposure. 


For professional consultation you can contact: for advice and sourcing of these devices.  US Toll Free: 800-543-8790  


NOTE:  These are only a few, there are others...

I am NOT affiliated nor endorse any products or services stated herein.

- Floyd



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Copyright © 1997-2011 Floyd Kling 
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