Relay or Transistor

Help please.

I am currently using a 10 relay output Crouzet plc. reading through the plc spec sheet I noticed that the relays are only supposedly good for 100,000 operations.

The relays in the plc are 8 amp contact rated but i am only using 0.2 amp, will this extend the life time cycles of the relays?

Some other plcs I have looked at purchasing are transistor output and are rated at 0.5 amp per output, (I notice they don't give life cycles?) would these type be a better future purchase?

Overview; I test doors / windows open & close etc, some of these tests are 50,000 cycles per test, this would very uneconomical and inconvenient if the controllers have to changed after a few tests!

New to this, hope its clear.

Cheers

The 100,000 operations sounds as a relatively low number, maybe the manufacturer is merely being conservative, so as to be on the safe side.

I dont know your PLC, but as comparison, Siemens ET200S relay contacts have a lifetime like this at 24VDC resistive load:
5A: 0.1 million
4A: 0.2 million
2A: 0.5 million
1A: 1.6 million
0.5A: 4 million
0.1A: 7 million

edit: For 24VDC maybe it is better with 24VDC solid state output, instead of relay outputs. For exactly the reason of 'unlimited' no of cycles. Does not cost more than relay outputs, on the contrary.

If you have low-current loads that cycle rapidly, solid state outputs (transistor or triac) are the way to go. If not overloaded, they will last much longer than relay contacts. The trade-off is that they have off-state leakage current, so you need to make sure the devices you're using aren't so low power that their minimum activation current is lower than the off-state leakage of the output. Also, it is very common to branch a surge suppressor in parallel to the output device. The reason for this is on inductive loads, when power is cut off, the coil still has some residual charge and for a very short time becomes a sort of generator. Without the surge suppressor to absorb that field collapse, power is induced back into the transistor and will shorten its life. Additionally, solid-state outputs will "fail on," meaning they will not turn an output off when they go bad. So if there are any hazardous outputs, you should make sure obviously to regulate the power feeding the outputs upstream with the proper safety relay.

Relay contacts have a higher amp rating, do not have off-state leakage current, and aren't as damaged by field collapse as triac or transistor outputs. But, the trade-off is the shorter life span.

So basically, outputs with a high duty cycle should be solid-state. If they are low enough current you can run them through the output directly. If they are higher current then you should use a heavy-duty interposing relay (which have much higher contact life ratings than plc relay outputs typically) with a solid-state output. Outputs with a low duty-cycle can run directly off the relay output.

Off-state leakage current of 5-8 mA is typical with AC outputs due to the noise suppression (Snubber). MOSFETs are used on DC outputs. Leakage is too small to consider. About .1 mA.

Use transistor outputs. Life cycle is infinite. You need to check for built-in suppression on the PLC output module, and on the solenoid valves. If one or both have it, no more suppression is needed. If neither has it, add a suppressor module, or use a UF4004 diode across solenoid or output.

When using relais you have to consider quality management of the test unit, wit transistors you dont have to worry about it,
please have a check on the relais (use a contact as input to see whether it is the relais or your door failing.

What type, or reference, of Crouzet's PLC do you use ?

Crouzet Plc

xiaolong said:

What type, or reference, of Crouzet's PLC do you use ?

Click to expand...

I'm using a 88 970 161 TYPE or RS No 536-9001


Thanks for your help everyone