Choosing a Relay to Operate Solenoid Valve

Question

I'm working on a project in which a solenoid valve needs to be triggered when a sensor detects a passing object. I have the concept and general design down, however I need some help with the specifics.

Essentially, a photoelectric retro-reflective sensor will detect an object, send its signal to a relay which in turn will trigger a 24 Vdc solenoid valve.

• The sensor's signal voltage is listed as approximately Uv / < 3V and output current Imax is 100 mA

Sensor Datasheet

• The solenoid is 24 Vdc and 8W, with an inrush VA of 25 and hold VA of 14.5

Solenoid Datasheet

• 24 Vdc power supply is to be used

Question 1: What data do I need to consider (from that listed above or otherwise) when choosing a relay? Specific relay suggestions are welcome :)

Question 2: Once the relay has been chosen and provided its specs suit the sensor and solenoid, is any further circuitry required i.e. a diode for the inductive load etc?

As is probably evident from my questions I'm quite inexperienced in this area so sorry for any ambiguity but thank you, your help is much appreciated!

Answer 1

Assuming you are going to specify a DC coil, the only relevant number is the '8W' figure, which implies a nominal current of 333mA @24VDC.

Just about any power relay with 5A-10A rating will work, probably with a lifetime of many hundreds of thousands, perhaps millions, of operations, but you should put a diode across the solenoid coil so that the relay contact does not arc excessively when the contacts open.

Make sure the diode is connected correctly (reverse-biased) or you will probably damage the relay and the diode. A 1N4004 will work fine for this application.

You could also use a DC-output SSR if you wanted to avoid a mechanical relay- you would typically still need the diode.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

VA of 25 is only 1+ Amp at 24V but considering turn off arc, I would use 5A non gold plated contacts using an optoisolator driving 10mA from 3V (e.g. some require selection of series R for preferred IR current. ) with decoupling cap on 24V supply on Relay board for EMI reduction.

These are cheap (2$) and optoisolated as well in 5,12,24V types with many current ratings.

I suggest a 5A Relay but photo shows 25A ($2 on ee bae )

Or this multichannel one for future expansion/

It is not a 3V sensor but 5V.

THe sensor is 5V PNP <100mA pullup compatible with 5V Opto drivers for 15~20mA for high side drive with built in R on Relay cards that are PNP or NPN compatible.

http://www.ebay.ca/itm/OMRON-4-Channel-5V-Relay-Module-Opto-couple-For-Arduino-UNO-Mega-R3-AVR-ARM-/301286279825?hash=item46260fca91:g:RIsAAOSwgkRVVBzK

This one is better with many sources to buy.

Use suitable twisted pair for all drive pairs for improved EMI control. OMRON makes high quality relays.

Reverse diode clamp across output contacts are necessary to prevent high voltage impulse voltage with solenoid loads on turn off. (1N400x or equiv.)