transitor vs relay

Hi, I have no real experience with creating electronic circuits but I am familiar with the fundamentals.

I need a very simple circuit to create a loopback on a switchport, so there is no "load". The loopback just needs to connect pin 1-2 and 3-6 of the 8pin connector and it probably needs to be low resistance (I'm actually not sure how low but I'm guessing < 5 ohms). So far I've only been able to achieve about 80 ohms with a transitor. I've tried adjusting resistors but can't get it lower. I'm guessing this has something to do with saturation but since most transitor circuits deal with amplifying, I'm not finding relevant info for my case.

Can this be accomplished with a transitor circuit or do I need to use a relay. If relay, can anyone recommend a part number for something readily available. It needs to be as small as possible.

Thanks

Ron

What is the open-circuit voltage across the two wires before closing the switch?
How much current flows through the switch when it is closed?
What is the polarity?
Does the polarity change from +_ to -+?

Depending on your answers, mosfets may work.

A transistor doesn't have a resistance, it has a voltage drop. A relay's contacts have almost no resistance at all, but it will wear out sooner. I would recommend using a solid state solution, but make the feedback using FET (Field Effect Transistor) they have resistances instead of voltage drops, but they are slightly trickier to wire up. It would help if we knew what pin 1, 2, 3 and 6 were.

WELCOME TO ELECTR-TECH-ONLINE
while the resistance of normal transistors when switched ON with base bias, is little higher.
If you need very low resistance while ON it is perhaps possible with power mosfets
like IRF540 at voltages below 50V DC and IRF 830 etc at higher voltages like 230V DC
you need to check the maximum current carrying capacity

and drain to collector withstanding voltage, and ON resistance between Drain and source.


a special caution is MOSFETS are voltage driven and many might need 9 to 12V DC as Gate drive. These gates of Mosfets must be terminated by some resistance otherwise the devices turn ON due to induced voltages on the gate pins and / tracks that connect to gates.

It's a standard ethernet switch, so it's a digital signal of course. I'm pretty sure it's 0v to +5v, I'm looking for the exacts specs though... will post them. Here is the pin info.

TX=transmit data
RX=receive data

Pin 1 - TX+
Pin 2 - TX-

Pin 3 - RX+
Pin 6 - RX-

I have 25vac power supply for the circuit, amperage needs to be low as possible but 100ma should be no problem. Of course the smaller the better.

Well the electrical signals used in Ethernet proved very interesting and confusing. Fortunately, modern 100Mbit and Gigabit switches use the same signal, -1v / +1v for data and 0v / +2.2v for other signals like the NLP (normal link pulse).

Since the circuit I need only switches on and off every 20 mins or so, I'm thinking my best bet is a relay since the Ethernet signaling has to be clean and undisturbed. It would be nice to use solid state if possible though.

Any thoughts would be appreciated.

Ron

I suspected that the signal(s) you are trying to switch are bipolar, which means that transistors and standard FETs are going to need special circuits to pass bidirectional currents. I think it is far simpler to do this with a relay. If you are concerned about power to hold-in the relay, could you use a mechanically or magnetically latching relay?

Ah, a latching relay, yes, that may be useful. Thanks for suggesting that.

Also, at the risk of looking silly, but since I haven't given up on a solid state solution yet, is it possible something like an op-amp might work? Then the amplified signal could be turned on and off. Seems that would be the same thing I'm trying to do, or is that going to get complicated?


Thanks

A DC solid-state relay designed for bidirectional switching and rated for 1A or more should work.

A schematic would be nice but it was mentioned about a bi directional switch. Whats wrong with a triac? Its possible maybe?

Dragon Tamer said:

A transistor doesn't have a resistance, it has a voltage drop. A relay's contacts have almost no resistance at all, but it will wear out sooner. I would recommend using a solid state solution, but make the feedback using FET (Field Effect Transistor) they have resistances instead of voltage drops, but they are slightly trickier to wire up. It would help if we knew what pin 1, 2, 3 and 6 were.

Click to expand...

You mean an bipolar junction transistors (BJTs) don't have resistance, it has a voltage drop. And field effect transistors (FETs) have resistance. They're both transistors.

MrDEB said:

A schematic would be nice but it was mentioned about a bi directional switch. Whats wrong with a triac? Its possible maybe?

Click to expand...

What's wrong is that they have about a 1V drop across them when on. Triacs are mainly used for switching power, not low level signals.

Perhaps we forget that, the derivation of the word"transistor" is from transfer -resistor,and that the resistance between collector and emitter would vary in accordance with base-emitter current.