Silly question

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I am was looking at the details of a Allen Bradley plc output card. The rack is a 1756 Allen Bradley, the card is OB32 which
means its a discrete output. I was reading the I/O module specifications and I was wondering why this card at 5v has a current draw of 300mA= .3 Amps and at 24 v this card has a current draw of 2mA= .002 Amps. Shouldn't it be the opposite, you increase you voltage shouldn't the current go up instead of down?

 

[Tharon]

I think those are the power draws on the different voltage rails of your backplane. If you could link the specs page, it may help.

As far as Voltage/Current relation, Voltage rise = Current rise, on a resistive load. Other things happen when your load changes, like a motor, higher voltage, lower current (assuming same Horsepower).

 

[Mark Snodgrass]

Those are the ratings for the power draw from the backplane. The chassis supplies 4 different voltages to the backplane (1.2v, 3.3v, 5.1v, 24v). Those ratings are for ensuring you are not putting too much load on the power supply.

 

[[email protected]]

http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/360807/2284181/tab3.html
Attribute 1756-OB32
Outputs 32 (16 points/group)
Voltage category 12/24V DC source
Operating voltage range 10...31.2V DC
Output delay time
OFF to ON
ON to OFF​

60

​

​

μs nom/1 ms max​

200 ​

​

​

​

​

μs nom/1 ms max​

Current draw @ 5.1V 300 mA

Current draw @ 24V 2 mA
Power dissipation, max 4.8 W @ 60 °C (140 °F)
Thermal dissipation 16.37 BTU/hr
Off-state leakage current per point, max 0.5 mA per point
On-state voltage drop, max 200 mV DC @ 0.5 A
Current per point, max 0.5 A @ 50 °C (122 °F) linear derating
0.35 A @ 60 °C (140 °F)
Current per module, max 16 A @ 50 °C (122 °F) linear derating
10 A @ 60 °C (140 °F)
Surge current per point, max 1 A for 10 ms per point, repeatable every 2 s @ 60 °C
(140 °F)​

​

​

Load current, min 3 mA per point​

Oh so I should be considering both currents as the max and min amount of ampacity. ​

 

[bernie_carlton]

Oh so I should be considering both currents as the max and min amount of ampacity

I still think that you are considering these as having something to do with the output capability of the module. They don't. The module, to operate, uses power from the backplane. Every card does. Because the system power supply has only a certain amount of power capability for the various voltages, these numbers are used to judge whether a certain mix of cards in the rack will overload the system power supply. Again, these figures have nothing to do with the power that can be switched by the ouputs.

 

[Clay B.]

http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/360807/2284181/tab3.html

Attribute 1756-OB32
Outputs 32 (16 points/group)
Voltage category 12/24V DC source
Operating voltage range 10...31.2V DC
Output delay time
OFF to ON
ON to OFF​

60

​

​

μs nom/1 ms max​

200 ​

μs nom/1 ms max​

Current draw @ 5.1V 300 mA

Current draw @ 24V 2 mA

Power dissipation, max 4.8 W @ 60 °C (140 °F)
Thermal dissipation 16.37 BTU/hr
Off-state leakage current per point, max 0.5 mA per point
On-state voltage drop, max 200 mV DC @ 0.5 A
Current per point, max 0.5 A @ 50 °C (122 °F) linear derating
0.35 A @ 60 °C (140 °F)
Current per module, max 16 A @ 50 °C (122 °F) linear derating
10 A @ 60 °C (140 °F)
Surge current per point, max 1 A for 10 ms per point, repeatable every 2 s @ 60 °C
(140 °F)​

​

Load current, min 3 mA per point​

Oh so I should be considering both currents as the max and min amount of ampacity. ​

The highlighted current specs are the "consumption" current of the card at those voltages. These values are the amount of current from the PLC's power supply used by the card.

This is a limiting factor when building a PLC. Your PLC Power supply has an finite amount of power it can supply and if you excede this by adding to many cards then the PLC will not function.

The other current specs are the point current max value and the card current max value. The Point Current max refers to the maxium amount of current that one output can handle before damage occurs. This power is supplied external to the PLC. The Card current max is the sum of the current on all of the outputs and if that sum is exceded then damage to the card is possible.

The last is Surge. Anytime you hook a inductive device (solenoid, relay, etc) you will create a surge and this spec tells you the largest surge the card can handle.


Also you will note that you have a reduced current ability at higher temps. This can be a facot if your PLC is in a high temp area. This is called the derating value.

 

[Clay B.]

FYI... No such thing as a silly question... Silly is not asking and blowing something up...

First time I saw a spec sheet like that I had to ask several questions also... Nobody gets it the first time they see one. If someone tells you they got it the first time they read one they are freaking full of it.

 

[Timbert]

So here's a silly example

How many of these 1756-OB32 can you run with a single 1756-PA75 power supply?

The 1756-PA75 is multiple output power supply that outputs 1.2, 3.3, 5.1, and 24V. Different output voltages for different uses. The 1.2 and 3.3 are mostly used by the processor, while the higher voltages are used by the I/O modules.

The power supply has a capacity of 13A at 5.1V and 2.8A at 24V. Notice that each voltage output is capable of about 65W.
(13A * 5.1V = 66.3 and 2.8A * 24V =67.2W)

Each 1756-OB322 consumes 300mA from the 5.1V supply and 2mA from the 24V power supply.

The 5.1V power supply can handle (13A / 300mA) 43 of these cards. The 24V power supply can handle (2.8A / 2mA) 1400 of these cards.

So, with no other loads, a single 1756-PA75 could power 43 1756-OB32s; it doesn't matter that the 24V portion of the supply could handle 1350 more without a problem, you're limited by the capacity of the 5.1V output of power supply.

 

[matthew1]

Hello, looks like everyone here has answered you're question. This may help as well. The following link will take you to a download page. Scroll down to the Qualifiers and download one of them. (Iv only used the 1500) this will give you a visual example of how adding modules increase the load on you're backplane/power supply. Also the difference between discrete and analog modules power consumption. If you add to high of a current draw it will tell you of you're over current condition. This system if for a modular design not a rack, but the concept is the same.

http://www.ab.com/linked/programmablecontrol/plc/micrologix/downloads.html

 

[[email protected]]

Thanks a lot, you did answer all the questions, I understand now

 

[Charbel]

hello,

i found the above answers very usefull, however, something is not very clear to me. Why for different voltages (5.1 and 24VDC), we have different power (5.1*0.3A = 1.3VA) and (24*0.2 = 4.8VA. thanks!

 

[Mark Snodgrass]

The different voltage ratings are not for the same load in the card. There are separate circuits that each use a different voltage and have different load requirements. The 24v part of the circuitry requires more power than the 5.1v part of the circuitry does.