Relay Vs. Trans. outputs - What's the difference?

[308guru 0]

I've only used relay output FX's so far. What is the difference between relay and transistor? Besides an audible click Benefits/drawbacks/specific applications for each? Thanks for any info, Mike

[navillusi 2]

Apart from the obvious differences "Generally", relay outputs are used on AC circuits, usually up to 110/240v transistor outputs are normally used on DC circuits @ 24v. Other important factors are switching speed (trans faster) and number of operations before they pack up (trans more operations)

[toolman 0]

There is another output type you haven't mentioned and that is triac. Triac - Used for AC outputs only. Transistor - Used for DC only. Relay: Can be used for either AC or DC depending on the requirements. In general you will always use either a transistor or triac output. Relay outputs are best used when you need voltage isolation or for signal switching. I have also used them when leakage current is too high on the other module types. When you have high current loads then a properly sized solid state device will provide longer life than a relay.

[308guru 0]

So if I am doing very simple programs; pushbuttons, indicator lights, I should probably use a transistor type? Does that go for inputs as well as outputs? Thanks so far.

[toolman 0]

The program does not care if it is AC or DC. That is a hardware requirement. First make a complete list of all input and output devices listing the voltage and current requirements of each. Are devices AC, DC or mixed? Switches can be either one. Pick the modules by hardware requirements.

[panic mode 246]

as stated before, triacs are used only for AC and although leakage current might be concern sometimes, they are quiet solid state device with no moving parts and no wear. transistors are solid state devices used in DC circuits only. can be VERY fast, produce CLEAN output that doesn't require debouncing and it's cheaper. also it can be used to achive higher densities (cards with more outputs) than relays (so even cheaper). relays don't care if the circuit is AC, DC, PNP or NPN, no leakage current (ok, it's very small...) they usually have higher current rating than other output types, it is often used for interfacing when isolation is required etc. on the other hand it's bulky so you get fewer points per card, relay cards are tipically more expencive, don't produce clean signal (might require debouncing which uses PLC resources and lowers response time), produce noise, are subject to failure due mechanical wear, contact errosion, vibrations, dust etc. now, this doesn't mean that with every click of relay output, dollar goes down the drain or that rare visitor with pacemaker is going to have seasure. those are just some things to consider when selecting outputs for the application.

[Crossbow 294]

I wouldn't say a basic program with pushbuttons and indicator lights is better with transistor over relay. The things to take into consideration are: Number of operations (relays are mechanical and wear out) Speed of operations (relays and triacs are slow) Voltage and current draw of output load (relays are higher current devices than transistors) Leakage current (a transistor or triac is an electronic device and can allow a small amount of current through in the off position, which may cause some outputs (like low voltage LEDs) to glow faintly, where a relay is an actual open or closed contact. The type of outputs is really dictated by the application.

[larry818 0]

Always use transistor. Here's why. Relays can wear out, transistors never do. If you use transistor out to external relays, the relays are easily replaced. Most importantly, if you don't hook loads to the plc, it will live longer. I've had too many experiences where some electrician managed to short a plc output and kill either the soldered in relay or burn the trace off the board. Even with relay output PLCs I use external relays to switch the loads. Now I always use transistor out to a bank of external relays close by. The downside of this approach is all the wires from the plc to the relays. Lately I've been using the FX2NC plc (transistor out, ribbon cable connected) and make a pcb with all my relays socket mounted on it. I use huge traces that can handle the rated output of my dc supply (usually 5 amps). It ends up being quite clean.

[toolman 0]

Fusing is always a good idea.

[larry818 0]

The traces and relays in a plc are marginal for the rated load, so I would have to fuse each trace, with close rated speedy fuses, to blow before the trace does. Fuses love to blow, and no one ever has spares, so in goes tin foil or other suitable non-fuse-like material. I do use circuit breakers where ever possible. Currently I'm looking for speedy pcb mount breakers.

[Sergei Troizky 26]

Not really. First of all, output group fusing may not protect properly individual outputs, and fusing every output is very expensive and takes too much panel space. Looking for a blown fuse among tens will be a hard task also. For transistor outputs- fusing does not protect them from shortage. I never fuse transistor outputs. In some cases a current-limiting resistor is better choice. Concerning relay outputs. If they are used for isolation purposes, they typically commutate low-current control signals and do not require fusing by definition. If a relay output is used for a high-current circuit (such as a heater, contactor, lamp or motor drive), the load itself must be fused anyways. The relay output just must be rated at least for the fusing current. Therefore, the high-current consumers must be fused individually, not PLC outputs. IMHO, of course. Edited 19 Dec 2004 by Sergei Troizky

[Sergei Troizky 26]

Answering the initial question, I would recommend the following. Use 12-24VDC executables (relays, solenoid valves, indicators, contactors) and use transistor outputs for them. Use relays operated by transisor outputs when you need them for isolating, for AC load, or for analog signal commutation. Edited 19 Dec 2004 by Sergei Troizky

[highvoltpower 0]

On 12/15/2004 at 0:20 AM, mckeand13 said:

I've only used relay output FX's so far. What is the difference between relay and transistor? Besides an audible click Benefits/drawbacks/specific applications for each? Thanks for any info, Mike

A relay is simple electromagnetic switch activated by current, and a transistor is made from semi conductive material. Transistor also providing current gain. A relay voltage cause more mechanical contacts to close or open. But in transistor is a three terminal device that is configure to increase signal or electronic switch. Relays are slower than transistor – 50ms to switch and more. In my point of view transistor is better than relays, they both work same but the difference is transistor need low voltage for switching.