AS3935: The Next Generation Episode 2

Hors de combat: AMS1117-3.3 LDO with crater and ejecta

In Episode 1, I isolated the modem ADSL disconnect relay module, and this setup survived about three months of storms, until this week. The system worked pretty well: the remote AS3935 disconnected the ADSL line from the modem a good 20 minutes earlier, so the problem area was limited to just the relay board. Instead of blowing everything up like my first rig.

Setup at time of strikes

Relay board with ESP-01S dismounted. No obvious burn marks this time.

However the ADSL line is still connected to the relay board pins, a lightning strike first took out the 12V to 5V DC-DC buck converter that served as the power module for the relay board. This caused the output to short-circuit to the input: 12V from the battery now appeared at the relay board power input.

Relay board after the second strike. Note the insulation tape over the relay pins

Now the AMS1117-3.3 LDO power regulator in the relay board is rated for 15V and while it crashed the ESP8266 CPU it probably  did not kill it. It did cause the 5V relays to run really hot. This set it up for the second close strike, just minutes later, which blew a neat little hole in the AMS1117-3.3.

It is very impressive what damage lightning can do. But there seems to be progress. The AS3935 lightning detector allowed an early modem disconnect, which probably saved the modem. And this time round there seems to be a lot less damage, probably because the lightning could not find an easy path to earth. The earlier 'disconnect' signal from the AS3935 module caused a second relay board to disconnect the battery charger from the mains. At the time of the second strike the system had been running off the 12V battery.

Since the strike also killed the 12V-5V DC buck converter powering the relay board from the battery, perhaps disconnecting the ADSL relay board from the buck connector might improve things. It does not really need to be powered up: I can use the relay 'Normally Open' pins to ensure the ADSL stayed disconnected. The relay board's 5V is really close to the ADSL pins as it needed to power the relay coil.

And I can make things a little more robust by using a 12V relay board to disconnect the ADSL relay board and to monitor for the storm's passing so that the modem can be reconnected when it is safe.

Diymore 4-channel WiFi Relay board with serial interface

And as a bonus the module PCB designer has helpfully cut a slot around the vulnerable relay output COMMON pin.

Note the 'U'-shaped slots cut around the relay COMMON pin

The power regulator is still an AMS1117-3.3 but now there is an added 78M05 linear regulator in series. This raises the maximum input voltage (ie the tolerance to surges) to 35V, from the AMS1117-3.5's 15V. Hopefully this is enough, but we are never really sure until the next lightning strike demolishes it. The relay coils are still exposed to the full weight of the surges but they are relatively tougher devices.

The setup is now thus:

System block diagram: the other 2 relays in the 4-channel module are used to disconnect the battery charger from the mains

Next Generation Episode 2 build: from top: 2-channel ADSL relay module, ADSL modem and 4-channel power disconnect module

Thus fortified, we await the next thunderstorm. Indeed I welcome the strikes, for it is looking like my 30-year struggle with lightning strikes may be coming to a close: fiber-optic broadband network has reached my front gate, and the service provider salesman will not be long after. Just when I felt like I am winning.

Fiber at the gates: the end for copper-based ADSL broadband is nigh

What is Ahab without his Moby Dick? Come, lightning and welcome. 

Happy Trails.

Ahab and his whale