Transistor vs relay

[simonhodge 0]

If i am buying a mitsi fx-1n, i see that i can buy it as Relay, tansistor or triac output. Can someone tell me why i would buy one over the other. I have a bit of an idea but it would be great to get the correct answer. Thanks Sorry, should have done a search first i guess as i just found someone else who asked the same question and got lots of great answers. At least i was thinking along the same lines. Simon

[tragically1969 0]

It depends what you are switching, if you are switching an Amp or so you could go for relay or transistor and use an interposing relay. If you are switching a few mA or need speed for BCD or PWM functions etc then you have no choice and need to go for a transistor model. Personally i would go for transistor then use relays where needed if you need to switch a larger load.

[Crossbow 295]

Can't tell you what one to use since it depends on application needs. Relays switch higher current (AC or DC, up to 2 amps) but having moving parts, so can wear out and operate slower. Transistors are for lower current and DC only, but can be switched on and off rapidly. Triac is like a transistor for AC current.

[Sergei Troizky 26]

Transistors are capable of switching not that small loads- typical transistor output is rated 2A. Their limitation is DC only and the voltage, which does not exceed 30V for PLC outputs. Also. they spark through immediately on short circuit, those making impossible protection by conventional fuses. (From my experience, a 50W switching power supply overload protection fully protects FX1N transistor outputs from short circuits). Relay outputs can switch AC, including commutating analog signals without distortion. Their contacts may be protected by conventional fuses. They do not last long on inductive DC circuits because of excess sparking (arcing). Reversed diode in parallel with coil reduces sparking. Their mechanical life expectancy is ~1,000,000 switching cycles and contacts life under full rated load is ~100,000 cycles. Relays are out of game for high speed output instructions. Triacs are solid state relays for power voltages AC only. Being solid state, they also cannot be fully protected by fuses. On other hand, being such, their life is almost unlimited. As well as tragically1969, I prefer transistor outputs as most versatile, and add to them external relays or solid state relays when necessary. Edited 28 Oct 2008 by Sergei Troizky

[Mike Dyble 0]

Like other replies I would alway use transistor outputs, and then use them to drive relays, ssr's or whatever. That way all the PLC outputs are isolated from the field devices, and hopefully faults will only affect the output device. After all one damaged output transistor/relay/triac means a replacement output module, or even PLC if it a small 'shoebox' unit, where as a damaged relay, ssr etc can be replaced individually, and it dosnt require anyone to actually touch the PLC.

[Goody 5]

Actually, the repair of an FX plc with blown outputs (either relay or transistor) is not too difficult. The whole terminal blocks unscrew then the main unit comes away without a wire being removed. Getting to the output board is not too hard and then its a matter of de- and re-soldering in the new relay/transistor(s).