Do I need additional hardware to control solenoids?

[jozaeta]

Currently we have an NI MID 7602 motor drive and a SCB-68 connector block connected to an NI PXI-7334 motion controller. I need to control several solenoid valves. I have never dealt with solenoids before, so do I need additional NI hardware to be able to control these solenoid valves? My professor and I assumed that we could use the digital I/O interface (through the SCB-68) to control the solenoids, but I have a feeling a vital component is required.

[K C]

Hi,

You have to be careful with driving solenoid directly with a digital output. You can easily damage the output.

The digital output port of a PXI-7334 is TTL (5V) and can source or sink 24mA. If you want to drive a, lets say, 5V / 20mA solenoid then it is no problem. You must connect a diode over the solenoid coil to protect the output driver.

If you want to drive a solenoid with a greater voltage and/or current you can use a solid-state device to drive the solenoid.

Do you already have the solenoids ?

[rallyfan]

Hi,
I don't use PXI cards but I already had to deal with solenoid valves. In my case they need an awful lot of power (400mA on 6V). Because of this I built an circuit with relays and an extra power supply. For this I took 2 relays. One to change direction of power and one to switch the valve on and off. The relays are controlled with Fieldpoint DO.

[jozaeta]

The solenoids I am using are LEE INKX0503450A inkjet valves. On the product information, a pulse voltage of 12V was shown. Since the description says pulse voltage higher than 5V, I assume that I will have to use a solid state device to use the valves.

I need to drive seven valves, acting independent from each other. Could someone recommend which solid state device I should use to power and control the valves?

[tbob]

Use 5V relays and an extra power supply as rallyfan suggested.  Wire the DIO output to the relay coil, other side of coil to ground.  Relay common contact wired to power supply (set to 12V).  Relay normally open contact wired to solenoid coil, other side of coil to ground.  Set the DIO high (5V) for a moment, then low (0V).  This gives a pulse to the relay, which gives a 12V pulse to the solenoid.  If you need really fast action, a simple switching transistor circuit would do, or use a solid state relay device, available from most electronic supply stores (newark, mouser, allied, digikey, etc.)

[LV_Pro]

And as KC mentioned you will, in most instances, want a diode connected across the each of the DIO driven relay coils. This diode is connected so that it does not conduct when the coil is driven. It is there because the collapsing magnetic field of the coil, when it is de-energized, can induce surpisingly large voltages back into the driving circuit, in the same manner that the spark coil in an automobile ignition system works and the diode effectively shorts this reverse emf.

P.M.

Message Edited by LV_Pro on 10-17-2005 03:39 PM

[tbob]

You can buy relays with the suppression diode already built in.  When using that type, you have to pay attention to the polarity when connecting.  In this case, the output lead of the DIO goes to the cathode of the diode.  The other side gets ground.

[billings11]

I am using some relays from crydom, www.crydom.com, that do a good job with solenoids.  Just make sure you put a flyback diode.  Remember inductance means current can't change instantaneously, do if you don't put in a diode across the coil you will turn off the relay but the current will keep going full blast for a couple of microseconds.  Now you turned off the relay - so it changed to a very high resistance, except the current can't change as fast as the resistance because of the high inductance of the solenoid in series with it. Ohms law applies and think about the power that will get dissipated across the relay.  Make sure you get a few extra relays so you can blow up a couple for a fun experiment.  In this manner you can save a lot of money on fireworks at new years.

Message Edited by billings11 on 10-17-2005 04:09 PM

[tbob]

billings11:  Your description is slightly innacurate.  When the voltage to the relay changes to 0 (or any lower value), the magnetic field collapses and induces a voltage in the relay coil, which then acts like a power supply for a very short moment.  Without the diode, the current feeds through the DIO (or whatever is driving the relay) and back to the opposite end of the relay coil.  This damages the DIO (or whatever is driving the relay) because the current is being backfed into the device.  The relay coil usually does not get damaged.  The DIO is the device that takes the damage.  With the diode, the collapsing magnetic field turns the coil into a power supply with the correct polarity to make the diode forward biased.  Then the diode takes most of the current, and the DIO is spared.

[billings11]

tbob, I was talking about the solid state relay that I recommended he use between the DIO and the solenoid.  The solenoid's inductance would cause current to continue to flow after the solid state relay is turned off.  Without a diode this could blow up the solid state relay - just like a switching power supply would blow up its switching FET without a flyback diode.  I wasn't suggesting that the coil would be damaged - just the solid-state relay in series with the solenoid.   I was assuming his DIO couldn't push enough current to enable the solenoid on it's own - hence the solid state relay.   Perfectly accurate!

Message Edited by billings11 on 10-17-2005 04:30 PM