interlock circuits

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Gibson486

Joined Jul 20, 2012
317
I usually see interlock circuits with relays, even on non industrial stuff ...

Is there a reason why we cannot use just logic gates? I am guessing that some people think relays are more robust and a better suited for stuff being plugged and unplugged (like a connector that goes to an external contact to fire off a relay) as opposed to some IC that you need to worry about ESD. Or maybe I have just never seen it, but people do it?
 

Kermit2

Joined Feb 5, 2010
4,163
If not carrying current of any significance, but just a signal voltage, so that contact welding is not a possibility. A relay is safer in this application, as it would fail in an "off" condition. An integrated circuit component would be much more likely to fail in a fashion that could leave the equipment energized when the interlock is opened.
 

kubeek

Joined Sep 20, 2005
5,657
It is possibly for historical reasons, i.e. it´s allways been done like that so it is easier to conform to the required standards, as oposed to proving that a new solid state equivalent si just as safe.
 

ErnieM

Joined Apr 24, 2011
8,011
I've followed some threads on a railroad that is near me my dad once worded for. The signaling was all relay based when installed near the beginning of last century, and while maintained well has recently been upgraded in many areas to modern stuff like microprocessors and the like.

Guess what works better in the rain? Or after a short to the 700 VDC third rail? Or after a lightning strike? Or after a nearby lightning strike?
 

MaxHeadRoom

Joined Jul 18, 2013
19,697
In most industrial equipment now the logic is done by programming in ladder etc, but the NEC and NFPA79 still stipulate a hard wired circuit for the E-Stop circuit, and also now becoming prevalent is Safety Relays with force guided contacts, already mandatory in many parts of the EU.
Max.
 

Kermit2

Joined Feb 5, 2010
4,163
90 + year old Tube radio with meshing plate capacitor and fish line drive on tuning knob. Still tunes across the radio band.

New digital broadcast car radio with optical interrupt style tuning knob. Stuck on 1610 AM and 107.9

Radio tuning has been Improved to the point of non functionality.
 

eetech00

Joined Jun 8, 2013
1,785
I usually see interlock circuits with relays, even on non industrial stuff ...

Is there a reason why we cannot use just logic gates? I am guessing that some people think relays are more robust and a better suited for stuff being plugged and unplugged (like a connector that goes to an external contact to fire off a relay) as opposed to some IC that you need to worry about ESD. Or maybe I have just never seen it, but people do it?
Back in the day, I used to design failsafe circuits for the railroad.
"Interlocked" generally implies that one circuits' operation prevents another circuit from operating.
You can use digital gates to do that. The railroad calls relays and circuits that are not required to be failsafe "non-vital".
Likewise, circuits and relays required to be failsafe are called "vital" circuits.

But....If it needs to do that in a "failsafe" way.....well then that's another issue altogether.
Its going to be more complex then "just using logic gates". There's probably going to be analog signal processing performed somewhere for self-check circuits, etc. The logic blocks themselves need to be periodically checked that they are functioning correctly before being "trusted" by the rest of the system. And the final black box output will still be needed to drive a railroad safety relay that connects to the wayside or signal equipment. Of course, its been about 10 years since I left that industry. Probably don't even need the safety output relays anymore.
 

ErnieM

Joined Apr 24, 2011
8,011
In safety critical applications software interlocks should always be used with hardware interlocks. See Therac 25
https://en.wikipedia.org/wiki/Therac-25
I have seen this taken to the extreme in one product we made here for a coast guard helicopter. Simple box, 4 illuminated buttons to send a state and respond to the master computers commands. What made it complicated was the requirement not to use any software in the design. No micro controller, no FPGA. Everything was done with random logic gates. Oh, and it was dual redundant too with two separate power supplies, two logic sections and two input and output sections driving one set of illuminated buttons.

It made for some interesting fixes when certain state transitions didn't quite match the spec, but ultimately produced a bulletproof unit.
 
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