What things to think about when selecting a relay for controlling small valves?

Question

We are creating a novelty drink dispenser that controls a set of 8 pumps with a Siemens Simatic Step7 1200 PLC. The decision to use this PLC was based on our need to have a very stable prototype.

Now we are nearing a stage where we are going into mass production, and the PLC feels like overkill. It has many features that we don't use and it costs a lot. We would like to replace this with a simple relay-card.

We have limited experience with making electronics and we will most likely use a professional service for the development and production of this card. However given the importance of this card for our business, I would like to ask some general advice here first.

For example, what are the most important things to think about when selecting a relay for controlling small valves, and what features should we know about in a valve that can make our product better?

Of course, we want them to be able to open/close as many times as possible without failing, and when failing, never failing in an open state. Also if there is any way to detect wear/failure that would be awesome.

For reference, our valves are 24VDC 5W solenoids.

Answer 1

Short answer: don't use relays. One of our past projects was a liquor dispensing system and we drive the solenoids with TO-220 MOSFETs. I'm not aware of any failures in many years of operation.

Be sure to include the back-EMF diodes across the solenoid terminals.

Answer 2

The most important things to consider are the specifications which will determine life (which must be adequate) and to make sure EMI does not affect the rest of your circuitry. This is particularly important with a bespoke design with limited production because respins will be very costly.

When I did the controls for a highly configurable commercial espresso etc. machine some years back, the relays were selected to have a life of more than 10 years (there were PID boiler temperature controls, grinder controls, steaming controls etc.). A LOT of different parameters to be set. You can also consider solid-state relays at that current level, with careful design they should be quite reliable, but note that when they do fail (and everything eventually fails) they will fail 'on' generally and some system-level considerations may be necessary to override a failed relay if property damage could be an outcome. Sometimes a mechanical relay backup and a solid-state relay combination is called for.

If you do use mechanical relays, be sure to use adequate suppression on the solenoid valve coils (a diode or TVS at a minimum) and preferably use relays designed to meet European safety regulations as they tend to give better coil-contact isolation.

Answer 3

For those low power valves, I would consider switching them with transistors instead of adding intermediate relays (the relays will still need transistor switches) if possible. As Spehro mentioned, you can also use solid state relays, but those tend to be much more costly than a switching transistor. A common ULN2803A Darlington array device will switch 8 solenoids at up to (IIRC) 300mA each and it costs around $1.50 in single quantities.

To verify proper operation, you may consider a flow switch so if there is a pump or valve failure, it can be detected. One other thing to consider is duty cycle. Some solenoid valves will be rated by how long they can be powered before they must be shut off to allow cooling.