RJ12, RJ45, RJ50: max current per pin? - Page 1
Forum section: electronics / projects

author: DiTBho
author_rank: Super Contributor
timestamp: December 13, 2020, 09:13:12 pm
content: hifor a crazy project, I haven't yet decided, but I am thinking about the use of a RJ12,RJ45 or RJ50 for connecting a couple of devices, but in this case, since these device are 5V, the connector would need to carry up to 400-500 mA as max current to a couple of MPUs.Is it possible?I haven't found clear documentation about that, and I am not sure about how much current can be carried by each pin in such RJ-connectors.Thanks!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 13, 2020, 09:22:32 pm
content: I push up to 1A through a wire pair ( 24Vdc and GND return) of CAT-5 and RJ45 connectors.For "Power Over Ethernet" which is an offical standard I think the current is a bit lower, around 800mA or so.A good and simple approach is to use some higher voltage, and a small SMPS module in each node. This does not only reduce the current through the cable, but the SMPS also compensates for the voltage drop due to cable resistance.Another part you have to watch out for is the bulk capacitance on the nodes. Even a 100uF capacitor (without additional current limiting / shaping) will have a very high peak current if you plug it to a cable that already has 24Vdc power on it. This results in sparks and damage to the contact surfaces of the connectors.

author: DiTBho
author_rank: Super Contributor
timestamp: December 13, 2020, 09:26:12 pm
content: Thank you a lot! So my specs (max 500mA) are satisfied. That's a great great newsI need more than 8pins on the connector, I can build a RJ50 cable with 10 pins, but do you know if there is a "panel" RJ50 connector?I can only find RJ45 panel connector.

author: Pinkus
author_rank: Frequent Contributor
timestamp: December 13, 2020, 10:05:11 pm
content: As devices powered by Power over Ethernet (PoE) will have to use these connectors too, just check the technical details for PoE. More here for example:https://en.wikipedia.org/wiki/Power_over_Ethernet

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 14, 2020, 04:49:08 pm
content: Quote from: DiTBho on December 13, 2020, 09:26:12 pmDo not forget that the connector contacts are not the weakest point, there is also a thin wire and not very strong connection of the knives with the wire in the connector. Wurt Electric offers two types of PoE connectors: 350mA (pn 74982104400) and 600mA (pn 74984104400) both THR. Note that this is the current for a pair of contacts.

author: DiTBho
author_rank: Super Contributor
timestamp: December 14, 2020, 05:53:47 pm
content: Quote from: S. Petrukhin on December 14, 2020, 04:49:08 pmOK, the pn#74984104400 is perfect since it carries more than the 400-500mA I need.Now I have a reference, and I only need to find a RJ50 cable and panel-connector. I need 10 wires, of which, two are Vcc=5V and gnd, others are for digital

author: Siwastaja
author_rank: Super Contributor
timestamp: December 14, 2020, 06:11:29 pm
content: The actual name for the connector is "modular connector". Yes, the name sucks, using generic adjective isn't a good idea for naming things, but just FYI to help search. Further piece of the name is 8p8c, or 6p4c, or 6p2c, depending on how many contacts the plastic housing has, and how many are actually populated with the contacts.Now <0.5A definitely isn't a problem of heating too much, so the connectors easily handle it, but seeing you use 5V, the question is how much voltage drop you can accept? For this, you need the sum of connector and wire resistance. If the wiring is long and you use off-the-shelf cabling designed for communication not power transfer, make sure your devices receive enough voltage - during peak consumption as well - capacitors are good to only less than a millisecond!

author: langwadt
author_rank: Super Contributor
timestamp: December 14, 2020, 06:23:53 pm
content: Quote from: Doctorandus_P on December 13, 2020, 09:22:32 pmthat's why a USB device is limited to the equivalent of 10uF on Vbus

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 14, 2020, 07:07:33 pm
content: Quote from: DiTBho on December 14, 2020, 05:53:47 pmI haven't seen 10 RG50 wires in 20-30 years.Most likely, you can only count on 4 pairs - 8 wires.See the PoE power transmission diagram. You have been advised to do this more than once.You don't have to use Ethernet and PoE, but the principles are pretty good.

author: DiTBho
author_rank: Super Contributor
timestamp: December 27, 2020, 11:10:49 am
content: So, I bought some connectors, but the carrier hasn't yet attempted to deliver my items, and probably they will be delivered in 2021.This is an example of "panel connector" of which I will measure the voltage drop on its pins.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 27, 2020, 05:52:29 pm
content: Quote from: DiTBho on December 27, 2020, 11:10:49 amDo you want to check the voltage drop on the pins of this connector at high current?

author: DiTBho
author_rank: Super Contributor
timestamp: December 27, 2020, 07:08:50 pm
content: Quote from: S. Petrukhin on December 27, 2020, 05:52:29 pm5V with a 500-600mA load connected through a 10 meters long cable

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 27, 2020, 07:42:16 pm
content: Quote from: DiTBho on December 27, 2020, 07:08:50 pmThe active resistance of UTP is about 1Oat 10m = 2in both directions.You will lose about 1V on the line and spend 0.5W to heating the wires.

author: DiTBho
author_rank: Super Contributor
timestamp: December 27, 2020, 08:43:13 pm
content: Umm, too bad. I though it was less ohm/m.OK, I have already built two DC/DC units. 5V to 48V, and 48V to 5V.With 48V on the line, it will reduce 1:10

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 27, 2020, 09:14:26 pm
content: Quote from: DiTBho on December 27, 2020, 08:43:13 pmThis option is better! But you can use a PoE chip for these purposes to do it nicely and carefully. For example: SI3402-B-GMR on the consumer side, but I don't remember which chip is put on the power side. This chip contains a DC - DC component with a built-in MOSFET.

author: fcb
author_rank: Super Contributor
timestamp: December 29, 2020, 07:15:03 pm
content: Quote from: DiTBho on December 27, 2020, 07:08:50 pmDid something like that awhile back - had to have 12V, current was roughly fixed.In the end we used a two cores for the 12V (supplied from a voltage regulator driven by a PWMDAC). And another two cores to feedback the voltage at the other end (sort of 4 wire measurement), one of the jobs for the sending end microcontroller was to measure the returned voltage and then adjust the PWM DAC to compensate for cable losses.It worked really well. The thing that kills RJ45 contacts (and most connectors) is sparks/arcing when connecting. So if you've got a bunch of capacitance on the 5V 'receiver' watch-out.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 30, 2020, 07:32:03 am
content: Quote from: fcb on December 29, 2020, 07:15:03 pmAnd PoE also protects against this - power is not supplied immediately, but after load matching.