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I've bought a relay board controlled over modbus from ali that contains 16x SRD-12VDC-SL-C relays to control my heating cables on the floor using rpi and nodered, my question is can I find a SSR pin compatible replacement for those relays? (I only use the 'NO' pin so NC output is not mandatory) After a couple of days of working I can see that I'll hit the 100k on/off cycles pretty fast, I want to have some information on what options do I have regarding potential fixes/replacements of those boards (besides replace to another SRD-12VDC-SL-C relay...)

Also if somebody have any other ideas I'll be happy to hear them out :)

EDIT System is "fail safe" that is, any damage of relay will not cause damage to my heating cables, or house, or anything...

I do not use RPI GPIO to control the relays, I control the state or relays using modbus RTU protocol

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  • \$\begingroup\$ What is the actual current? What is the consequence of a relay stuck on or off? \$\endgroup\$ Oct 6 '20 at 17:11
  • \$\begingroup\$ Currents are lower then 6A, consequences of 'always off' is that floor is not worm and 'always on' consequences are that the temp will go up to max (27C) then the 'safety system' will trigger, so pretty safe to fail \$\endgroup\$
    – Bartoszek
    Oct 6 '20 at 17:16
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You can consider SSRs (Solid-state relays). They're available in various ratings and qualities. You will need something of a heat sink at the 6A range. They can be driven directly with 3.3V or 5V logic and typically contain adequate isolation.

Photo from Digikey (note the safety approval markings):

enter image description here

Make sure your overtemperature system cuts off the actual power and not just the control signal- a frequent failure mode is stuck "on" permanently and ignoring the control signal.

Edit: There is no such thing as a pin-compatible SSR with those cheap relays, or most other kinds of relays, for that matter. Maybe some DIN-rail mount contactors have SSR doppelgängers that are more-or less compatible (probably with derated current capability).

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  • \$\begingroup\$ Thank you for answer. The 'Over Temperature' protection is an analogue circuit that enables another relay when temperature goes over 27deg physically disconnecting the heating, so failure or primary relay is safe. I can draw a circuit to illustrate this system if you want. But still, it needs to be pin compatibility :( \$\endgroup\$
    – Bartoszek
    Oct 6 '20 at 18:01
  • \$\begingroup\$ Your over-temperature protection should *disable the safety relay, not enable it, if it is to be fail safe. \$\endgroup\$
    – Transistor
    Oct 6 '20 at 18:31
  • \$\begingroup\$ Thank you for the comment. This is a thing that I might change in the future, the reason why I do it this way is to #1 avoid any state changes of OP in normal workload to reduce wear of the "protection" relay and #2 the power supply of OP unit is connected to the heating cables power line... so the only way to enable it is to enable the heating and leave it in this position for too long, then the relay of OP unit will enable itself and change it's position from NC pin to NO pin disabling the heating cables. OP -> overheat protection \$\endgroup\$
    – Bartoszek
    Oct 6 '20 at 19:03
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    \$\begingroup\$ Sorry, but there is no such thing as a pin-compatible SSR with those cheap relays, or most other kinds. Maybe some DIN-rail mount contactors. \$\endgroup\$ Oct 6 '20 at 21:30
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Use SSR-10DA, Its a 5V/3.3V DC input Solid state relay that can drive up to 10Amps (same as your SRD-12VDC-SL-C) and it has only NO point.

You can directly hook its input pin to Raspberry Pi and its output is separated from input optically.

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  • \$\begingroup\$ I do not use GPIO to control those relays, I use modbus, this device is an array or 16 relays so I need them to be a drop in replacement (pin compatible with the current relay) \$\endgroup\$
    – Bartoszek
    Oct 6 '20 at 18:09

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