Sign in to follow this  
Followers 0
MIKE31

AB output card

5 posts in this topic

HI what are the advantages /disadvantages of tran - src output card over the relay output card Thanks

Share this post


Link to post
Share on other sites
Transistors and triacs are semiconductors. They require much less power to trigger (less load on the backplane), they can act much faster than relay (transistor outputs on some cards can go to really high speeds) but they are less abuse friendly than relays. Transistors are used in DC enviroment only, while triacs are used in AC circuits only. Relays can be used in both AC and DC (of course depending on voltage and current rating) but since they are basically operating mechanically, they cannot last long when turned on and off frequently. Also if signal must be clean (for let's say counting purposes) or fast acting (multiplexing of LEDs for example), transistors are definitelly way to go. It really all depends on what type of load you have, isolation requirements etc.

Share this post


Link to post
Share on other sites
panicmode has pretty much covered it except for one aspect: If the load being switched is inductive, ie a contactor coil, when the circuit opens the coil's collapsing magnetic energy results in an induced voltage spike. If there is nowhere else for the voltage to go it results in an arc across the relay contacts, very much reducing their life and causing an RFI pulse that is a potent cause of intermittent faults in PLC systems. The normal cure is to install suppressor units across the contactor coils (although it is astonishing how many people don't bother). This eliminates the RFI issue and protects the relay contacts. After all if one relay on a 16or 32pt card fails it is potentially expensive to fix. On the other hand the great advantage of SCR's to drive AC contactor coils is that the SCR once triggered off continues to conduct through the AC half-cycle until the voltage across the SCR falls close to zero. At this point the SCR turns off and this action largely minimises the coil energy and any induced RFI issues. Thus if you are driving lots of largish contactor loads, SCR outputs are a good choice. The main downside of SCR outputs is that they can have significant leakage current and moe than a few techies have been tickled by a supposedly "off" output. On the other hand most IEC style contactors have a nice 24vDC Interface module that sits neatly on top of the coil, making it very easy to drive even the largest contactors with 24vDC and eliminates any safety concerns. The cost of a 32pt 24vDC output module is typically quite a bit lower and this offsets the cost of the Interface modules on the contactor. Overall if I have a clean sheet of paper, I like this approach best.

Share this post


Link to post
Share on other sites
In my experience, triacs will bleed volatage when troubleshooting if you lift the wire off of the card. You have to provide a load so you do not get a false reading. You can get pretty confused when troubleshooting and you think you have a bad card even if the output is off, you will still get power.

Share this post


Link to post
Share on other sites
Put very simply.... Relay -> slow switching, limited life Transistor -> fast switching, much longer life. Regards, Nick

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0