Author Topic: fx2424 output current  (Read 1940 times)


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fx2424 output current
« on: December 05, 2018, 03:59:37 PM »

I have an application where I need two more outputs which can handle more current than 250mA as possible with outputs 17-24
I have used all other outputs with higher capacity.
Is it possible to use two outputs in tandem to drive a single load or must I use a SSR off board to handle the load?
I am driving a 400 mA at 24vdc solenoid.
The solenoid does have snubber diodes for the inductive kick


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Re:fx2424 output current
« Reply #1 on: December 05, 2018, 09:03:53 PM »
I'd vote for some sort of external buffer or relay rather than attempting to parallel two outputs.

Paralleled MOSFET transistors share load currents much better than bipolar transistors.  There are some issues if you are switching loads at high frequency.  Because the driver IC handles 8 outputs you do have to worry if the load current for your solenoid and the other outputs will result in the driver chip over heating.  If the IC is socketed, you can replace the driver if it fries.

I am not certain if PLC outputs turn on/off at exactly the same time.  If there is a delay from output to output then only one of the two transistors will be on for a little while and this may cause some reliability issues.

Outputs 5-16 use discrete mosfet transistors for each Output.  These outputs could easily handle your solenoid power requirements.  Do make use of the GND terminal on the PLC output headers to try and shunt load currents back to the system ground point rather than having the load currents mix in the PCB ground plane.

Gary D
« Last Edit: December 05, 2018, 10:08:56 PM by garysdickinson »


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Re:fx2424 output current
« Reply #2 on: December 07, 2018, 08:11:04 AM »
Gary's suggestion is mostly correct - Thanks for the great explanation about the behavior between MOSFET and Bipolar.

What is the peak current draw by your solenoid? What is the longest duration that the solenoid will be energized?

The output 17-24 on the Fx2424 are MOSFET that can handle peak current of 1A but continuous current should be kept < 250mA. They also have short circuit protection so are reasonably robust.

If the solenoid are energized only briefly (a few seconds) and then turned OFF you probably should be able to parallel two or more of the outputs to handle the high current. But if you expect the solenoid to be energized most of the time then a higher current external buffer is recommended. You can construct one quite easily using P-channel MOSFET with the PLC's output driving the gate. (see below image that found on Google)

Tel: 1-877-TRI-PLCS


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Re:fx2424 output current
« Reply #3 on: December 10, 2018, 10:38:04 AM »
The solenoid takes 400mA and will be on for about 1 minute then off for 15 minutes or more. The exact off time is unpredictable but in general will have a pretty low duty cycle.

I would use the higher drive outputs on the board, but they are being used for other solenoids, I'm just two solenoids short of higher amperage outputs.

I will use an outboard driver or relay to increase the capacity and simplify support in the field to make the solution robust.