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Old June 2nd, 2007, 05:20 AM   #1
Gromit
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Max distance fot transffering a 24V DC signal

Hello,

Could you please tell me what's the max distnace for transffering a 24V DC logic signal to a PLC? Let's suppose a 0.5mm2 cable.

Regards
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Old June 2nd, 2007, 08:07 AM   #2
danw
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The answer depends on the DI card and the sensor.

The 24Vdc DI card whose spec sheet happens to be on my desk at the moment has these specs:

ON voltage: 9.5Vdc minimum
OFF voltage level: 3.5Vdc minimum
Minimum ON current: 1.0 mA

If
- the power supply is 24.0 volts
- the sensor operates at exactly 1.0 mA (minimum for DI card)
- we assume the junctions & terminal blocks have no resistance (ha-ha),

then the wiring can drop 14.5 volts (24.0 - 9.5), yet still activate the DI logic on this card.

So how much wiring resistance (in ohms) will drop 14.5 volts at 1.0mA?

R = E/I
R = 14.5V/0.001A
R = 14,500 ohms

That's a long wire run.

You can look up the resistance of the wire gauge you provided and calculate how long a run (remember wire goes out and back) it actually is.

A sensor is likely to use more than 1.0mA. Increasing the current decreases the resistance needed to drop the 14.5 volts in the example, which in turn, decreases the length of wire. But it's still a long run.

If this is an actual issue, connect the sensor to the DI card, measure its sinking or sourcing current and find the minimum ON voltage from the spec sheet and do a calculation for your equipment.

Dan
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Old June 4th, 2007, 12:04 PM   #3
TNT
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Are you sure that is so simple!?

It's great I've found this post, I'm working in a 100m sensor-to-PLC right now.

Are you sure that is so simple?

For your information, and my PLC and sensor values:

Min 1 value: 13V
Sensor max current: 0.1A
So R = 11/0,1 = 110ohm

A 0.5mm2 copper cable has resistance ~ 0,17 ohm/m
So I can put the sensor ~ 300m far away (considering 2-ways cable)

Is this correct?

Thanks a lot
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Old June 5th, 2007, 08:24 AM   #4
panic mode
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why do you consider max sensor current if this is only going to drive PLC input?
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Old June 5th, 2007, 08:44 AM   #5
TNT
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You are right.
The current should be <<.
I don't have exact value for my sensor, but ~ 0,001A.

But, doing the calc again now I have 30km far away!
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Old June 5th, 2007, 08:49 AM   #6
panic mode
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it is not that simple but pretty close... also whenever you size something, you don't want
to cut it too close, always leave some margin (20-25% should be fine).

sensor has some voltage drop that must be taken into account (some 2-3V for solid state output
for example, read the specs of your sensor) and the sensor also has to use some power.
This means that if sensor is three wire device (such as most proxy or photo sensors) you will
have additional current that should be taken into account (usually just few mA).

Btw. nobody says that your 24V must be 24.000V.

so far i've been using 24VDC powered 4..20mA loops for example on cables which ware about
mile long. on servos, i have 250 feet (80m) long cables with bunch of connectors
and brake is 24V operated. Brake wires are #16 and the current is some 750mA.
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Old June 5th, 2007, 09:10 AM   #7
TNT
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Thanks for your reply.

I was trying read sensor without current loop.

I need to read just 100m sensor, with the calc above supposed with maximum current I get 300m. So I think I have a great margin.
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