24vdc coils on OB16 card

[ratcliffe_ic 2]

I was thinking of using an OB16 flex io card to drive some 24vdc pneumatic solenoids, the solenoids in question are 4w, this is OK for the current rating of the card but the caps are not fitted with fly wheel diodes. Can anyone give me any advice on whether this is a bad idea to do this?

[paulengr 44]

I haven't ever needed or used them, and haven't experienced problems with failed IO because of it. If it really becomes a problem, there are "low current" coils you can buy. Basically the coil contains a separate transistor that acts as a "pilot". It allows you to use say 5 A coils directly off a low power output without having at least a "visible" interposing relay (the coil has an internal interposing relay function).

[ratcliffe_ic 2]

Thanks for reply, i've just checked the contactors I'm using on the same cards and these are 3w but are fitted with a diode module for suppression. I'm thinking of fitting my own flywheel diodes to the solenoid caps just for piece of mind, any idea how I would specify one?

[paulengr 44]

Easy. The voltage rating should be at least 10 times the nominal voltage (or 240 volts in this case), and current should be at least equal to the current flowing through the solenoid, or 4 W / 24 V = 0.2 A. I think the next standard size off these specs is 250 volts, and 1 A. These are extremely cheap and readily available from any of your choice of electronics houses. I like mouser.com (better online catalog) but you can also use Allied Electronics, Digikey, or Newark. In practice diodes are so cheap that I usually grossly overspecify them to avoid any chance of them being damaged. A huge power diode that can handle hundreds of amps and thousands of volts for a rectifier for instance with screw terminals on top might run you $20-$30. Be aware that by adding a diode, it increases the time for the voltage to dissipate. Figure that it will increase switching times roughly 3 to 10 times normal. So if your goal of using DC is to switch faster than AC, you destroy the advantage with diodes. Edited 10 Mar 2011 by paulengr

[jfwoodland 0]

In addition to protecting your output device, the diodes will also reduce the noise contribution of the solenoids on the DC bus. This is especially important if you plan to power the solenoids with the same 24V that you use for PLC inputs. I once spent several days tracking down a 'program problem' on a brand new machine. I finally noticed that the erratic behavior always coincided with the retraction of a specific air cylinder. Watched the 24V DC control bus with a scope and saw the spike whenever that valve deenergized. If I paid real close attention to the lights on the input card, I could even see some of them flicker. Sure enough, the (unnamed) manufacturer left the diode out of the DIN plug on the valve. Added the diode, and the machine started working like a champ!

[Mickey 71]

Funny, had a similar thing happen to me. Every time an alarm horn was acknowledged ( shut-off) a near-by level transmitter would spike. Yes same 24vdc for both. Diode across the solenoid for the air-horn took care of it. We eventually installed a separate 24vdc power supply for all the analog transmitters. I just happen to be staring at the level on our SCADA when the operator came by to acknowledge the an alarm. Bingo "His Eyes Opened" "Darmok and Jalad at Tanagra" Edited 12 Mar 2011 by Mickey