Best kind of Relay for switching at 24VDC?

wlr3m4

Member
W
Join Date
Oct 2017
Location
Saint Louis
Posts
28
I've got a PLC that I'm going to read a signal from a source at 24-30VDC @ 2A but my PLC can only take 1A on its input signal. I want to get a Relay that will act between the signal coming in so I don't burn out my PLC, is there a common one that is recommended?

PLC is a Micrologix 1100 BWA by the way.
 
A digital input is not going to draw 2 amps. Most likely in the ma range.

What leads you to believe it's going to draw that much?

If you are trying to protect the input then fuse it.
 
Last edited:
Mickey is right. You are looking at this from the wrong direction.

Inputs are generally rated by max voltage. At or below the rated voltage the I/O card's internal impedance is going to limit the current to the safe level. Ohm's Law applies. For example, if an input has 2.7 kOhm impedance, at 24 VDC it will draw 8.9 mA.

Outputs are rated for both current and voltage. Switching more than rated amps will overheat the output and let the smoke out. Switching above the rated voltage will cause arcing on mechanical contacts or burn through of transistors or triacs. This too will let the smoke out.
 
I am looking at a Generator Controller PCB and the outputs on the board are listed as 24-30VDC @ 2A, I called the manufacture and they confirmed this and told me to mind the current. For reference: It is the DGC-2020ES from Basler Electric
 
You are confusing input ratings with output ratings. If you need an interposing relay for voltage reasons(you probably don't), then that coil current rating is what you should be concerned about. Digital inputs draw a few milliamps. You need to find out why there is a range on your board's outputs. If a digital output, then it should be based on a regulated on-board power supply. If a transistor or relay output, it might be based on what you put into it.
 
wlr3m4 said:
I am looking at a Generator Controller PCB and the outputs on the board are listed as 24-30VDC @ 2A, I called the manufacture and they confirmed this and told me to mind the current. For reference: It is the DGC-2020ES from Basler Electric
Click to expand...

The rating you reference is for outputs. In your original post you talk about reading signals and inputs. You need to differentiate between them - they are different, even if they reside on the same physical device.
 
I want to read the output from a Gen Controller that is listed as 24-30VDC @ 2A.

Will this or will it not fry a Micrologix 1100's Digital inputs with no relay between the device.
 
wlr3m4 said:
Will this or will it not fry a Micrologix 1100's Digital inputs with no relay between the device.
Click to expand...

No, if they are using the same voltage reference...

Current is a live circuit characteristic and its amplitude is determined by the 'consumer'...The 'consumer' of your circuit is the Input circuitry which's current consumption is usually at mA levels.

Of course, in a real world scenario, one would fuse the circuit for the lowest current rating of the circuit's components; 24 VDC Inputs fused at 50 mA are pretty common.

If your Outputs' voltage is different than the Inputs' one, you will need an interposing relay which's 'coil' will need to carry the same voltage rating as the Outputs; the Inputs' circuitry will be routed through the interposing relay's contacts which's states will reflect the state of the Output connected to the relay's 'coil'.
 
I think I see the source of your confusion.

The 2.0 A rating of the Gen Controller's output doesn't mean that it will always drive 2 Amps through the circuit. It means that the output can safely handle currents UP TO 2 Amps without failing. The actual current through the circuit is a function of actual voltage and actual circuit impedance.
 
Tom is right . The board output is a relay giving a volt free output . If you check the output terminals you should find a +V next to the output . You supply 24V to the board and wire the output to the PLC .
 
Tom Jenkins said:
I think I see the source of your confusion.

The 2.0 A rating of the Gen Controller's output doesn't mean that it will always drive 2 Amps through the circuit. It means that the output can safely handle currents UP TO 2 Amps without failing. The actual current through the circuit is a function of actual voltage and actual circuit impedance.
Click to expand...

+1

What we have here is a failure to grasp Ohm's Law.
 

Similar Topics

W
Best kind of Fuse for a PLC?
I've got a small project where we are moving a PLC from a non-enclosed setting to a cabinet and I want to put a fuse on the incoming 120VAC. I've...
Replies
7
Views
3,360
jvdcande
jvdcande
D
Studio 5000 best practice for multiple inputs controlling one output.
Compactlogix controller, program has 28 conveyors that use TON's to start the conveyors. The TT sounds a warning horn during start and the DN...
Replies
10
Views
482
James Mcquade
J
T
The best way to control 48 PID temperature control
I have S7 1512C controler for controlling 48 PID temperature loop, the output is PWM. Please I need the best, most efficient way to write the...
Replies
13
Views
592
tarekrslan
T
R
Best Ethernet/IP HART bridge to use?
I am going to need to use HART multi-drop in order to handle a series of Vega Radar units. There are a lot of options and I'm wondering what...
Replies
3
Views
251
joseph_e2
J
B
Best Practice - What controls recommended for a 2-position turntable
Out of interest, I'd like some thoughts on what would be considered best practice with regards to a 2-position turntable control scheme (see...
2
Replies
17
Views
1,124
Peter Nachtwey
Peter Nachtwey
Back
Top Bottom