I have used relays as a switch in order to turn on and off things. This is how I do it:

enter image description here

My IC is my integrated circuit call it arduino. With a small current I am able to turn on and off things and I can now replace mechanical switches such as this one with a transistor.

Anyways now my question is will this also work when sending negative voltages? We have 2 different internet providers meaning we have 2 separate cat6 cables. Each provides internet. If the internet goes down from one cable then we connect the second Ethernet cable. I just learned that cat6 cables send voltages between -2V and +2V. Since the cat6 cable has 8 wires inside I was wondering what will happen if I do the following for each of the 8 cables inside the cat6 cable:

  1. Cut the cable at the middle.
  2. Connect one side to the collector of a transistor
  3. Connect the other side to the emitter of the transistor
  4. Connect the base to my arduino giving it a small current so that the switch is closed.

If I where to do this will there still be an internet connection? If there is an internet connection then I can use this mechanism to switch between providers instead of having to change the cables manually? I will probably not do this. I am asking for purposes of learning. I do not want to use a relay as they are larger and using 8 for each cat6 cable will be a lot.


The transistors will not be switching on and off at the frequency of Ethernet. They will be either on or off. They will only change states when switching between Ethernet cables. If they are not able to transmit negative voltages can I use 2 transistors in reverse direction like so:

enter image description here

That will represent the connection of 1 of the 8 cables of the cat6 cable. The base pin of the transistor is on the center. That will always have a possitive voltage to leave the switch on. Because there are 2 transistors in reverse direction it will now be possible to send negative voltages no?

Edit 2

This will represent one of the 8 cables illustrated with a better diagram:

enter image description here

Using something like this I should be able to send signals from both directions correct? I plan on leaving the transistors on until I want to change internet service providers They will not be turning on and off rapidly.

There is a problem though. I can connect the base to +5v of arduino but how can I share the same ground so that this diagram works?

  • 3
    \$\begingroup\$ you cannot switch ethernet in that manner ... a relay might work, but it is not for certain ... you would need only two DPDT relays (or one 4PDT), not eight \$\endgroup\$
    – jsotola
    Commented Jun 4, 2020 at 22:07
  • 1
    \$\begingroup\$ There will be no internet connection through that at all. Relays may not even work at that kind of high frequency. \$\endgroup\$
    – Hearth
    Commented Jun 4, 2020 at 22:18
  • 1
    \$\begingroup\$ @Hearth Not always. FET's create a resistive channel when on, with effectively no PN junctions at all in that path. They do have difficulty blocking AC though, because of the parasitic body diode in parallel, and so to fully replace a switch requires two in series, one to block each direction. BJT's can sorta be used in both directions also, but not as well as a FET. \$\endgroup\$
    – AaronD
    Commented Jun 4, 2020 at 22:49
  • 1
    \$\begingroup\$ That said though, the problem that I see is not the ability to connect or disconnect, but the parasitics of the construction interfering with the fast signals. The higher frequencies you want to deal with, the more critical the layout and construction become. DC and 50/60Hz AC are very difficult to mess up, but Ethernet signalling speeds are difficult to get right. \$\endgroup\$
    – AaronD
    Commented Jun 4, 2020 at 22:53
  • 2
    \$\begingroup\$ Read this datasheet -- 4-Channel DPDT ETHERNET SWITCH. You may not understand it all, but this is what it takes. And they've created a special chip to do it because it's challenging. That said, the transistor configuration you proposed would pass AC current with a certain amount of dissipation (the transistors would act like resistors). \$\endgroup\$ Commented Jun 4, 2020 at 23:59

1 Answer 1


Can a transistor send negative voltages when used as a switch?

Yes, they can. But for your application, the high frequency changes the equations to the point where you might have a hard time making it work. Here is a picture of validation of your idea, which works fine at low frequencies.

enter image description here


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