What is the relay lifetime of a REX C100 PID controller?

The popular yet inexpensive REX C100 PID temperature controller typically contains a mechanical relay.

Mechanical relays will fail after some number of switching cycles. There are two types of relay lifetime specification relevant for these applications:

  • Electrical lifetime: How many switching cycles until the electrical characteristics degrade beyond the specification. Typically the relay will have increased on-state resistance when old.
  • Mechanical lifetime: How many cycles until the relay can’t switch properly any more due to  mechanical wear. Typically, after reaching its mechanical life, the relay will stick either open or closed.

My REX C100 (model 10G8888) contain Hongfa HF3FD/012-ZTF relays.
According to their datasheet, these relays have an electrical life of 50 000 cycles and a mechanical life of 10 000 000 cycles.

This means that if the REX C100 switches on and off every 12 seconds (like most standard Rex C100):

  • The electrical lifetime of the relay will be reached after 600 000 seconds, i.e. 1.74 days of continous operation.
  • The mechanical lifetime of the relay will be reached after 120 000 000 seconds, i.e. 3.8 years of continous operation.
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Often the best strategy is to design your circuit so you don’t depend on the electrical characteristics of the relay entirely, but your application will continue to work even with degraded contact resistance.

My recommendation is to design your application to tolerate 3-5 times the contact resistance and use only 1/3 of the rated current.

Note that the manufacturer’s specification should be considered to be just an estimation of your relay’s lifetime. The actual lifetime will vary between individual relays. If used properly, most if not all individual relays will reach their specified lifetime.

Note: Some REX 100 models have solid state relays. If you hear an audible clicking noise every time the REX C100 switches on your load, it’s a mechanical relay. The SSR-model REX 100 do not have an internal relay but need to be connected to an external SSR. These models switch much faster, approximately once every 3 seconds.