Yes, Phoenix and Finder do have very small relays for interfacing but they are only single pole. I use a lot of 2 pole relays and therefore the Omron G2R are a better option for me. Horses for courses. I will not use the larger Finder relays due to the stupid little pins they use to plug into the base. Ham fisted electricians always bend then over. Have not really looked at the small ones due to the 2 pole problem.
Stephen, you mentioned leakage through a transistor keeping an output on. This is only very rarely a problem. In my original post SSRs were my beef, not transistors.
I find that the space I save, considering that there are usually many lights that can be driven through a transistor and usually far less relays, is a far better option than relay outputs. The cost is also generally lower at the end of the day. The higher the density of the card the lower the cost per point. Life expectancy of the transistor cards ia also far greater.
Relays on PCBs create heat which increases proportionally as the load goes up. You cannot have small cards and high capacity relay outputs, a fact of life. Once again, I much prefer the space saving and longer life of the transistor cards.
You would have to carry far more spare relay output cards on site to allow for the blow ups. Relays on PLC cards are typically soldered into the card and are not plug in. A pain for maintenence purposes. Much easier to use external plug in relays that are easy to change and, not always but in a lot of cases, have a greater air gap between the contacts. This equates, usually, to a longer life anyway. Add that to the ease of replacement in the case of failure and I will use them with transistor cards anyday.
Many years ago when Omron manufactured the old C20 PLC, they came with plug in PCB mounted relays. Yes, this goes back a year or 2. At least replacement of the relays was relatively easy. Nowadays I have not seen any manufacturer do this. Cost I guess.
Some manufacturers also have relays boards with leads and plugs to plug into the transistor card. Take up too much space, cost to much and many of them are relays soldered into a PCB anyway. Really defeats the purpose of usin external relays.
Think of the cost. We all whinge, from time to time, about the cost of PLC components now. You would probably double the price of the card.
By the way, if your brand of choice has transistor cards that will drive a 200ma load, Schneider now have contactors that can be driven direcly from the transistor outputs. They are designed to be driven straight off Schneider brand PLCs. If your transistor will only switch 100ma, you cannot use them. The transistors will not last. It appears that some manufacturers are at least thinking in the right direction, or they have a very large client(s) that has infuenced the design of these contactors. Yes, I am cynical. Spent too many years in sales and management of companies (25) and have seen how a large client(s) can influence the design and manufacture of products. A shame the manufacturing companies generally do not think about good design and usability for their customer base. It all comes down to money in the end.
Stephen, you mentioned leakage through a transistor keeping an output on. This is only very rarely a problem. In my original post SSRs were my beef, not transistors.
I find that the space I save, considering that there are usually many lights that can be driven through a transistor and usually far less relays, is a far better option than relay outputs. The cost is also generally lower at the end of the day. The higher the density of the card the lower the cost per point. Life expectancy of the transistor cards ia also far greater.
Relays on PCBs create heat which increases proportionally as the load goes up. You cannot have small cards and high capacity relay outputs, a fact of life. Once again, I much prefer the space saving and longer life of the transistor cards.
You would have to carry far more spare relay output cards on site to allow for the blow ups. Relays on PLC cards are typically soldered into the card and are not plug in. A pain for maintenence purposes. Much easier to use external plug in relays that are easy to change and, not always but in a lot of cases, have a greater air gap between the contacts. This equates, usually, to a longer life anyway. Add that to the ease of replacement in the case of failure and I will use them with transistor cards anyday.
Many years ago when Omron manufactured the old C20 PLC, they came with plug in PCB mounted relays. Yes, this goes back a year or 2. At least replacement of the relays was relatively easy. Nowadays I have not seen any manufacturer do this. Cost I guess.
Some manufacturers also have relays boards with leads and plugs to plug into the transistor card. Take up too much space, cost to much and many of them are relays soldered into a PCB anyway. Really defeats the purpose of usin external relays.
Think of the cost. We all whinge, from time to time, about the cost of PLC components now. You would probably double the price of the card.
By the way, if your brand of choice has transistor cards that will drive a 200ma load, Schneider now have contactors that can be driven direcly from the transistor outputs. They are designed to be driven straight off Schneider brand PLCs. If your transistor will only switch 100ma, you cannot use them. The transistors will not last. It appears that some manufacturers are at least thinking in the right direction, or they have a very large client(s) that has infuenced the design of these contactors. Yes, I am cynical. Spent too many years in sales and management of companies (25) and have seen how a large client(s) can influence the design and manufacture of products. A shame the manufacturing companies generally do not think about good design and usability for their customer base. It all comes down to money in the end.
