Transistor or Relay outputs?

[gruntstripe]

Hi, What factors does one consider when deciding whether to use relay or transistor outputs?

[TimWilborne]

I usually go off of the speed that you need to turn the output on and off and the amperage needed to turn on the device connected to the output. It would also be dependent on the voltage of the device hooked to the output and the variety of voltages that are hooked to the output module.

[Colin Carpenter]

Normally go for relays unless there's a lot of high speed switching of low currents to be done. For example, I often use a small, cheap member of the FX family as a "pulse width magnifier" or pulse catcher for larger A or QnA PLCs .... easier than buying dedicated hi speed counter cards or messing with interrupts, so would tend to use transistor outputs here. Although most relay outputs are rated at 2 amps, if it's much more than 0.5 amps and is switching quite often, would tend to use interfacing relays, or at least fit flyback diodes if switching DC voltages. Anything above 1 amp, and I would definitely use an interfacing relay. Sparks eat contacts over time.

[Sergei Troizky]

Mechanical contacts are not good for DC (their life expectancy dramatically reduces). Transistors cannot switch AC. Make conclusions yourself. Edited April 12, 2006 by Sergei Troizky

[Crowbar]

I used to use mainly relay outputs for everything with an interposing relay for larger loads, now I'm leaning toward transistor outputs and interposing relays for general use and the transistor outputs by themselves only for fast switching needs. I should mention that most of my output loads are 120VAC. Edited April 12, 2006 by Crowbar

[panic mode]

there could be many reasons but here are some things one might want to consider: - robustness relays are tipically more robust or likely to survive occasional "disturbance"... - circuit type (AC/DC) transistor outputs are good for DC but relays can be used for either. this could be deciding factor if you have to live with mixed outputs (no room or budget for extra card). - circuit polarity (PNP/NPN) if you buy transistor output card (usually PNP) and find out later that customer standard uses wiring practices for NPN, you will be sorry for not going with relays. inputs on small plcs are usually universal for same reason (group can be wired for either PNP or NPN). - isolation transistor outputs are not isolated or potential free ("dry contacts"). - signal quality relays just don't cut it if quality of signal edge is important (contact bouncing). also relay contact errode and/or oxidize over time. - speed transistors can always outrun any relay. some applications are thus reserved for transistors (pulse train and PWM). relays are mechanical device and as such subject to wear, specially when switched often. - leakage current due smaller strucure, solid state devices have higher leakage current (which is bad) than relays. this is specially noticable on triacs (output cards with triacs are only used in AC), while leakage on transistors is hardly noticable. - operating current limit relays are tipically rated a bit higher (1 or 2Amp) than transistors (100-500mA). - audio noise when i was kid, i had a chance to enter "server room" of a telephone company with realy old system using TONS of relays. i was deaf for three days or so. this is usually not something people look at but who knows, maybe you are building something for TV show or movie... - size/density transistors take much less space so cards with 32, 64 or even more outputs are possible even in small package (see high density cards on Mitsubishi Q PLCs for example). - grouping relays cards are usually divided into more groups. more smaller groups allow for more diversity when outputs are powered from different circuits (permanent on for indicators for example and circuits controlled by e-stop only, e-stop and light curtain etc.). smaller groups of outputs mean bigger possible loads. for example you could have 32-pt card which has two groups of 16 outputs. each output can handle 0.5Amp but whole group still must not exceed 2Amp. if your outputs are loaded with some 330mA, you can have up to 6 outputs on at the same time (the other 10 outputs of the same group should be off). - price transistors are cheaper to produce. relay cards with same number of points are usually 10-20% more expencive. many people choose to use transistor outputs (better price, higher density) with interposing relays (one on each output). depending on application this is still probably an overkill (bigger cabinets overcrawded with graves, err. relays) but that's nothing new. lot's of people don't bother to do sizing or they are too afraid of failure (PLC failure!) since they are used to continuously replace things instead of investingating what the problem really is. For example: One of maintenance guys (new customer) was telling me that "those plc outputs are rated 5Amp or more but they still burn". He was pointing on relay outputs on a small PLC, so "he would prefer to see bigger relays just in case because after he did that on few other machines, problem was gone". We then measured current load and it was nearly 6Amp per output. He was shocked when we looked into manual (it was on his desk) just to find out it's max 2A. Additional interposing relay was obvious choice but he never bothered to read manual, he was used to swapping things out. I've purchased and installed at least some 300 PLCs over last 5 years alone (don't keep records on older stuff) so this must be well over 20000 outputs. I don't remember dead one other than few brand new ones that failed prematurely (within couple of hours) and ware exchanged under warranty. I do use relays too (and relay output cards) but I try to use them where they are better choice, not just because PLC output is not a relay type (ok, all our outputs are DC anyway). Well, that's me, your mileage may vary... (now you are warned) - availability it might happen that you will have to choose anything that will do job even if it's not best possible choice. - power consumption relays draw more power. make sure to check power resources of your PLCs power supply. different cards need more or less current from different sources (5V, 24V) and in some cases might determine I/O type (depending on resources of selected power supply). this is specially something to look after if you are upgrading existing project that uses small PLC (like MicroLogix1200 for example). - vibrations is this going to be on a mobile piece of equipment? - repearability do you want to be able repair bad output without replacing card? - temp.... (wife calls again , I better go now!)