# Protecting circuit input against lightning - how to put all the pieces together?

I am designing a single wire (ground return) telephone for use in our hobby caving expeditions, starting out from a well known and successful design called Michiephone (http://speleonics.com.au/business/michiephones/images/mph_schem2.jpg).

Here is my input stage:

It is essentially a high input impedance voltage follower with an extra passband active filter. The telephones are powered from a single lithium ion cell, so in normal circumstances we expect not much more than 4.2 Vpp on the line...

Now very often the comm line would run from a tent camp on the surface to an underground camp in the cave - meaning that some few hundred meters of wire are in fact over the ground and exposed to elements. So I thought it would be great to add some basic surge/lightning protection to the circuit, since otherwise it might not survive even the mildest thunderstorm (... and we are exploring caves in the Alps, which means thunderstorms are frequent).

There is already some good material on the subject here on Stack Exchange. I read this and this and already know the building blocks of a lightning protection circuit: gas discharge tubes, varistors, TVS diodes, trisils, integrated TBUs. However it seems to me it is imperative that they are used together in a proper combination. And designing a good suppression network is where the challenge starts, especially for a designer like me (rated for 5V/100 mA).

Here is what I came with so far:

I selected the components reasoning as follows:

• The main energy absorbing element would be the GDT. The lowest sparkover voltage available seems to be 75 V (looking for example at this Bourns datasheet)
• The GDT takes time to switch on. Both this time and the voltage at which it switches on really depend on the surge profile. So I add a Trisil triac to clamp the surge voltage before the GDT turns on - or in case the GDT thing does not turn at all. I reckon the Trisil should be rated for a voltage exceeding the GDT sparkover voltage, say 120V (or is that too low?).
• Now, 120V is still a bit high. So I add a TVS diode array with a breakdown voltage of 6 V and a clamping voltage of some 18 V. And I just hope the input capacitor of the input stage holds. For a brief while these 18 V might be fine for the rest of the circuit - it takes a while to charge capacitors in the input stage through a 47k resistor, so I might hope these 18 V do not reach the IC at all.
• If I clamp the GDT/Trisil voltage to 18 V, it will prevent these parts from tripping. And that could quickly fry the diode array. So I add some reasonable amount of inductance between the diode and the GDT. And a wire-wound resistor to limit the current through the diode.

The question is: does this reasoning and the design make any sense?

There is a lot of information available online on many different transient/surge/lightning protection devices. But how do you - in general - connect all the building blocks?

I already ruled out MOVs in my application. Adding some 10 nF of capacitance to the circuit input kills its high impedance - which is the very idea of a single wire (earth return) telephone. Also the TBUs from Bourns may be a nice, integrated solution, however they come in DFN packages, which are rather difficult to solder in hobby designs.

I reckon the Trisil should be rated for a voltage exceeding the GDT sparkover voltage, say 120V (or is that too low?)

EDIT

You can't use a Trisil with a GDT because as soon as the Trisil fires, it clamps down to a low voltage and that will never allow the GDT to activate. Now, this may work fine on its own (and you might not need a GDT) but it'll never handle the surge power that a GDT can handle.

The GDT slow-strike voltage is 75 volts plus 20% = 90 volts. So if you use a regular TVS diode, it will allow the GDT to eventually take the brunt of the surge after initially clamping the surge to circa 100+ volts.

Next, you don't need C11 if that's all you have. If you have an output amplifier you need to reveal that because the input amplifier will survive without any further TVS devices.

• Thanks for pointing out that C11 is bogus! It is needed in the original design I quoted - presumably for removing DC bias when the circuit is transmitting. But the original circuit uses a clever arrangement to switch a single op-amp between receiving and transmitting, whereas I have separate Rx and Tx amplifiers and a Rx/Tx relay. So I indeed don't need it in my input amplifier! Commented Jun 30, 2020 at 17:30
• @Mateusz But what if you are transmitting when a surge arrives? Commented Jun 30, 2020 at 17:35
• @Mateusz - check the edit section. Commented Jun 30, 2020 at 18:29
• Thanks a lot! Now as I look at Figure 1 on the Trisil datasheet it makes perfect sense. So essentially you suggest a GDT and a (relatively) high-voltage transil. Well, lacking better ideas, for now I assume that if the surge happens during transmitting then it's bad luck... Commented Jun 30, 2020 at 19:37
• No, I'm suggesting use a Trisil (although I can't recommend one) OR.... use a GDT and a TVS. A transil and GDT won't work - the transil will always make the GDT redundant because it will always activate first on a medium to high speed surge. A slow surge (fairly unheard of) will use the GDT but it's a rare old event. Commented Jun 30, 2020 at 20:21