Detecting 24V switch signal

[JeroenBK]

Dear all,

I have a PXI-1052 chassis with (among them) the following modules:

- PXI-6289 with a SCB-68 connection block.

- SCXI-1161 switch module.

What do I want?

I want to be able to switch solenoid valves that operate on 24Vdc.

With the SCXI-1161 module I can switch this solenoid but I want more!

I also want to detect if the solenoid is really switched (if there is a 24Vdc loop or not).

My intention was to use the PXI-6289 with SCB-68 (or SCB-100) to detect this signal.

The problem.

I have the switching part up and running but I can't get the control loop working.

Currently I'm testing with a 5V-10mA signal to see if I can get the 6289 working but I don't seem to be able to create a stable signal.

The next step is to find out if the SCB-68 is capable of handling 24Vdc.

Solution?

Yes please 😉

With kind regards,

Jeroen Kluft

[Dennis_Knutson]

You need to be asking if the 6289 is capable of 24V detection and the answer is no. The maximum range is +/- 10V. Is it not enough to know the on/off state you are sending?

[JeroenBK]

Hello Dennis,

Thank you for your reply. Thing is if I fysically disconnect (I unplug the connector) the solenoid I want to be able to detect this as well!

I've been thinking about a barrier or something to downscale the 24V to 5V (for example) but then I introduce an extra delay and thus error.

Does anybody know of a module/device which is able to handle 24V?

One other thing is I currently have a 5Vdc continue power supply on connectors 68(+) and 67(-) of my SCB-68. This should correspond with AI 16 of my PXI-6289 card (right connector).

I measure a signal of -5.8V. What is going wrong here?

[JeroenBK]

New info!!

I have a few more details to spare:

The PXI/SCXI card must be capable of receiving 24V with 500mA (max 1A, switch load).

I'm now thinking of the PXI-6515 which is capable of 500mA for one channel per port and maximum 4A for all (which in my opinion should allow the 1A switch load).

Am I thinking the correct way?

Also does anybody know if such modules are also available for cDAQ?

regards,

Jeroen

[Henrik_Volkers]

Here is a hack for the solenoids:

I would measure the current to detect an active coil. A simple way: Measure the current with a resistor to GND and if you selsect the value such that you get 3V , you can use this signal for a dgital input.

Usually a 24V valve will work with 21V, however you can put a 470µF cap parallel to the resistor.  Depending on the current the sense resistor migth get hot!!

If you use analog inputs for current detection, you can choose a resistor value to get 80mV ...  

If you want to measure 24V you should add a voltage devider build from two resistors.  For low impedance sources like 24V power supply a 10k/90k is fine. And if you don't take wirewound ones they work up to some MHz (SMD usually up to 1GHz) 

Please post a connection schematic 😉

and a link to the spec of your solenoid...

the -5.8V sound like an open or floating input with no bias path.

[JeroenBK]

Hello Henrik,

Thank you for your reply, it is helpful.

The only problem is that I don't have one type of solenoid but a whole range of different ones.

My job is to test all kinds of instrumentation to be used on our valves.

Below a list of U/I/P's of currently used solenoids:

24Vdc/400mA/9.6W

24Vdc/629mA/15.1W

24Vdc/187mA/4.5W

24Vdc/483mA/11.6W

We also have solenoids on higher voltages like 48V, 110V and 230V (both AC and DC in some cases).

The most ideal solution would of course be to be able to test them all....!

But to keep it to the 24Vdc ones first: If I could connect at least 2 and most preferably 4 of them with on each channel 1A it would be great.

[Henrik_Volkers · Accepted solution]

OK sounds like a test stand for solenoids...

Here are some schematics to drive a FET with a DO , for your application the first (left) one looks good, the IRF840 has about 0.84Ohm RDSon (there are better ones) and is good for 5-8A (cooling!) and can go up to 500V (DC)  ( How do you handle the turn off voltage spike?  RC-Snubber or free running diode?) The FET already has a diode implemented.

What do want to measure? solenoid current and supply voltage?  Accuracy needed?

Analyse the solenoid current at decreasing turn on voltage?   (you can detect the change of the magnetic flux in the current flow to detect a non acting solenoid (due to undervoltage) 

And there are SSRs (solid state relays) also with galvanic barrier , capable for AC and DC , together with some LEM current sensors (also with galvanic isolation) and industrial 'norm signal tranformers'  (also with galvanic isolation) you can build a quite robust  test stand, all controlled by your 6289..   for the 230VAC 😉

[Henrik_Volkers · Accepted solution]

 The HUF76423P3FS-ND is a logic level Nchannel MOSFET  ...  (60V 33A 35mOhm@5Vgs) ,   no driver needed 🙂