How to isolate RJ11? - Page 1
Forum section: electronics / beginners

author: sam1275
author_rank: Frequent Contributor
timestamp: November 22, 2014, 06:00:48 pm
content: Hello everyone, I'm using ADSL and the incoming line is RJ11 telephone line, which connected to a ADSL splitter, then modem and router.I want to know how to isolate the RJ11 line to avoid lightning or surge, just like using an isolated transformer in some industrial factory, or is the ADSL splitter already isolated?Thank you.

author: DmitryL
author_rank: Regular Contributor
timestamp: November 22, 2014, 06:09:47 pm
content: Why do you think that "isolating" will be good ? If you have 10KV surge it will happily pass your isolating transformer (also making use of capacitance between windings).BTW, all phone stuff like modems already has isolating transformer inside.Try googling "rj11 surge protector". Those ones that look like a small box with earthing pigtail should contain a bunch of TVS and gas discharge supressors.This is probably what you want.AFAIK nothing will help you from a direct lightning shot, you can find a lot of pictures with equipment turned into ashes

author: madires
author_rank: Super Contributor
timestamp: November 22, 2014, 06:23:54 pm
content: Quote from: DmitryL on November 22, 2014, 06:09:47 pmBut you have to look for DSL compatible surge protectors. The TVS' need to be hidden in a diode bridge to reduce the capacitance, otherwise the large capacitance of the TVS' would impact the DSL signal.PS: Most signal transformers used for DSL are rated around 2-4kV isolation voltage.

author: Tandy
author_rank: Frequent Contributor
timestamp: November 22, 2014, 07:41:55 pm
content: The RJ11 connector where the modem attaches is a straight through connection.You can attach a surge arrester to the DSL line, they are usually simple gas discharge tubes so not terribly effective but it may help. ADSL modems are so cheap I wouldn't worry about it, if you connect to the modem via ethernet then there are magnetics on both ends of the link modem and computer so damage to the computer is not that common.

author: SeanB
author_rank: Super Contributor
timestamp: November 23, 2014, 09:35:53 am
content: Common surge arrestors use a dual gas discharge tube across the input, with a short ( really short) thick wire leading direct to ground. Ideally a straight line down from the overhead line entry point to a big earth spike driven into the ground. Then you have a common mode choke ( wind the twin cables around a former of about 5cm diameter with 20 turns in the choke) leading inside, where you have another gas discharge unit connected to mains earth. After the inside GDT you have a pair of 22R 1W flameproof resistors and a standard telecoms semiconductor surge arrestor in a TO220 package with the tab cut off. Then you feed to the in house network.The inside unit is very common as a plug in phone arrestor, likely you have one that came with the modem, or a fax machine or phone answering machine. The outside unit is a common telecoms lightning arrestor unit, but not often sold to the end user, it is more often used in the exchange end as primary protection.Both will add to attenuation on DSL, but you will still have data passing through. My protection adds around 1dB extra to the line.

author: GreyWoolfe
author_rank: Unknown
timestamp: November 23, 2014, 01:36:23 pm
content: Quote from: Tandy on November 22, 2014, 07:41:55 pmIt may not be common but it does happen. A couple of years ago, we had a strike on the street in front of the house. All utilities run underground. The strike took out the modem, took out 2 ports on my router and the 2 computers connected to those ports. Thankfully, they were old computers that were given to me so it was no loss. The other 2 ports on the router were fine as was the wireless section. That strike also took out the DVR, an outside 2 meter antenna and the 60 feet of coax attached to it. The phone lines were unaffected, as was the TV connected to the DVR. Lightning does travel interesting paths.

author: SeanB
author_rank: Super Contributor
timestamp: November 23, 2014, 01:47:02 pm
content: Yes, interesting paths. My sister's laptop has a blown Ethernet port from lightning, but luckily i got a PCMCIA card that would fit the laptop and it has a network connection, as it does not have working wireless any more. Modem did not survive that hit, but the new one has the protection attached ( sis took it off to connect a phone to the line, so expensive lesson there as it killed the fax as well) and grounded.Now how to teach her that the clips do need to be pressed to release the cable, as all the sockets and half the cables are broken from her ripping the cables out when cleaning or moving stuff............

author: IvoS
author_rank: Frequent Contributor
timestamp: November 23, 2014, 02:23:58 pm
content: ...and, all low voltage wires such as coax, phone lines etc. entering the building should have grounds connected to intersystem bonding terminal.

author: G7PSK
author_rank: Super Contributor
timestamp: November 23, 2014, 05:12:34 pm
content: I always turn off and unplug during a thunderstorm, that's the only sure way to prevent damage. At one time the GPO as the telecoms were then would fit a bras plug and board if so requested, in the likely event of lightning you pulled the plug from one hole and inserted it into another which grounded the incoming line, I have not seen one since the 1950's though.