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I intend to use a safety relay for my project, however I found industrial safety relay mostly work with 24 VCC and is very big, so I decided to make a safety relay based on my specifications. What I have learned yet is the safety relay have multiple input channel (series emergency buttons) with pair of the parallel relay in case of contact welded. HFA6 4H2D

I think to detect the emergency bottom status with different locations (cable length) can use the opamp to measure its current. There would be 3 possible states: short cut, open float, and connected. When it is connected the current may change or set for 20 to 60 mA, so I decided to use comparator opamp to detect whether the current is out of the preset scope and if it is correct to enable the relay coil. The following is my primary schematic for the safety relay however I couldn't figure out the whole connections. enter image description here

Two series emergency buttons are connected through the shunt resistor, the first opamp convert this current to voltage, and the second opamp compare it giant the upper and lower limit and then enable the serial BJTs to enable the parallel relays. There are two buttons for starting and stopping the motor my question is does this schematic logic work? Is there any IC for monitoring the current is the specific upper and lower limit? Any tips are really appreciated.

winny
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Hamid s k
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    Safety relays and op-amps in the one sentence has me worried. Have a read of my answers https://electronics.stackexchange.com/a/240352/73158 and https://electronics.stackexchange.com/a/314660/73158 to see if they help. It is not clear what "*I think to detect the emergency bottom status with different locations ...*" means. – Transistor Mar 05 '22 at 00:29
  • @Transistor , is not related to any PLC programing and just needs a safety relay with a start/stop button for a low power dc motor. Different locations mean different cable lengths than different resistors, that's why opamp should detect a range of I. So what is the best component to monitor the e-buttons status ? – Hamid s k Mar 05 '22 at 02:11
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    Why don’t the switches directly operate the relays? – Kartman Mar 05 '22 at 07:38
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    You seem to be using transistors to control the coils of the "safety" relay. This is generally a big no-no as you can't predict the failure mode of the transistors. Both can fail short-circuit leaving your relays permanently energised. You also have Q5 and Q6 arranged as a very poor AND gate. At best, with a single NPN transistor that will work as a voltage follower with an output voltage 0.7 V below whatever base voltage you get with the 1k driving resistors, R12 and R9. You may also find that once energised R12 provides enough base current to keep U1 on via Q6 base-emitter. – Transistor Mar 05 '22 at 16:55

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Safety relays have to undergo a rather thorough design, evaluation and testing process that costs a small fortune. Just the standards you need to read to know what to do cost over $1000 - just some PDFs, nothing more, and there's hundreds of pages of reading there. And then if you've never gone through such process yourself, you'll need someone to review your design documentation and test plan, because you'll inevitably miss something. Such reviews are not cheap either. Just understand that developing your own safety relay needs to produce well over a 100 pages of documentation that has to demonstrate that both your design process and test plan have addressed everything required in the relevant standards.

Of course it's entirely possible to get through this process even for someone who has never done it before, but it will be costly. And you can't sell the product until it gets tested by an independent laboratory, and quite likely the facility where you assemble this product will have to undergo periodic audits.

So, generally speaking, if you have not got several tens of thousands of US dollars to spend on development work, it makes approximately zero sense to roll your own.

Even just using a safety relay isn't cheap: you'll have to go through a process where you demonstrate first of all the need for a safety relay, and then make an argument that the relay makes your system "safe enough".

Kuba hasn't forgotten Monica
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  • tnx @kuba-hasnt-forgotten-monica for advice, but it is a hobby project and I get through a couple of documents and I find that most of those commercial products use pulse generators through all input channels and measure the current against the upper and lower limit, so I think It is possible to make a prototype using a couple of opamps to check the current of each input lines, however, I couldn't figure that out how they check all lines together – Hamid s k Mar 09 '22 at 02:47