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Old September 16th, 2020, 11:56 AM   #1
Archie
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Electrical Symbol on Light Curtain

Can anyone tell me what the upside T represents on this schematic of a light curtain?
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Old September 16th, 2020, 12:16 PM   #2
L D[AR2,P#0.0]
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I don't know for a fact but by implication it is the suppy to the light curtain. Maybe the symbol is trying to represent a vertically standing light curtain.. but then again there is no ground shown...
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Old September 16th, 2020, 01:46 PM   #3
kolyur
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I see this symbol a lot on European schematics. I believe it indicates the common/ground connection for a DC supply.

In your example schematic, the safety relay appears to be AC powered. Since the light curtain outputs are presumably 24VDC, there needs to be a return path which is why the DC common from the curtain is shown tied to the safety relay.
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Old September 16th, 2020, 04:31 PM   #4
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Quote:
Originally Posted by kolyur View Post
I see this symbol a lot on European schematics. I believe it indicates the common/ground connection for a DC supply.

In your example schematic, the safety relay appears to be AC powered. Since the light curtain outputs are presumably 24VDC, there needs to be a return path which is why the DC common from the curtain is shown tied to the safety relay.
Iíll second the above on the symbol. We use a lot of sew motors and on their encoders this is the symbol they use for 0v/earth.
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Old September 16th, 2020, 05:03 PM   #5
Gene Bond
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I believe it is a coded / pulsed DC signal, so the safety input recognize that the sensor is active and not bypassed... I've seen this as a problem from some safety laser scanners. There was a recent thread about this as well.
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Old September 16th, 2020, 07:16 PM   #6
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Quote:
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I believe it is a coded / pulsed DC signal, so the safety input recognize that the sensor is active and not bypassed... I've seen this as a problem from some safety laser scanners. There was a recent thread about this as well.
This is what I was also thinking since it is wired to S21. You would not want to ground the S21 since it typically is a 24V or or a pulsed output from the safety relay.
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Old September 16th, 2020, 07:20 PM   #7
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Kolyur and Dock have it right - this is a very common symbol for "ground", i.e. your zero volt connection.
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Old September 16th, 2020, 08:11 PM   #8
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This is what I was also thinking since it is wired to S21. You would not want to ground the S21 since it typically is a 24V or or a pulsed output from the safety relay.
The whole thing perplexed me every time I look at it. I am confident in the symbol meaning, whether or not that is the designers intentions, that’s another story.
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Old September 17th, 2020, 08:23 AM   #9
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From the MSR127RP manual
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Old September 17th, 2020, 02:35 PM   #10
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The symbol is called an "Up Tack"
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Old September 17th, 2020, 02:47 PM   #11
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It's not necessarily "ground", although it might be. That symbol is denoting the NEGATIVE rail of a DC power supply. The positive rail would be a normal "T" shaped symbol, but in this case they have two positive rails, so they are numbered instead.
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Old September 17th, 2020, 06:01 PM   #12
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So does this mean wiring the S21 and S22 to the DC Common does not cause a problem with the relay? If that is the case, what about the short circuit sensing?
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Old September 17th, 2020, 07:25 PM   #13
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Nope - see this diagram, also from the manual.

Normally your safety device has one contact between S11/S52 and S12, and one between S21 and S22. But in the case of a light curtain, you need to disable the "test pulse" checking and short circuit checking, because the light curtain does it's own diagnostics on that front. To do that, you connect S21 and S22 to zero volts, and the relay then knows that it's looking for two 24VDC inputs on S52 and S12.

Bit of an odd way to do it, but the diagram from the safety relay manual matches the diagram from your light curtain.


I don't use those safety relays any more; they're long superseded. I use the 440R-S13R2, which is a bit less creative with it's wiring methods.
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Old September 18th, 2020, 02:24 AM   #14
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Looks like it is a connection to the metal frame for ground.

https://www.edrawsoft.com/electrical-symbols.html

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