Section control relays 5V or 12V?

About to build some section control boxes, is it best to use 5V or 12V relay boards and are there an higher quality ones available?

I know AliExpress has plenty of cheap ones, but reliability over price is more important.

If you use a 5V relay card, you need to lower the 12V → 5V with which you use the coil of the relay. 12V can be supplied directly to the relays of the 12V relay card without lowering it.

I have also put the Arduino Nano, Ethernet Shield and switch below in the same box.

There are two ratings on a relay.

One is the trigger value that operates the relay.
If your using an arduino its easiest for this to be 5V

The other rating is the output it can control. Most Ali boards are rated 250V on the output so hard to go over. But the most important value on the output side is the amps it can handle. 10A is 10A purely resistive regardless of what voltage you are using.

Then you need to derate the Amp rating for both type of load and temperature. Operating solenoids that are an inductive load rule of thumb is /3, so a 10A relay can power a 3A solenoid and get its predetermined cycles before failure.

Temperature also degrades relays so it is best to run them in an off state, so most of the field the trigger side is not energized keeping them cooler.
For best reliability your 10A relay is now 2A. So keeping them cool energizing just for the short time they are needed helps longevity. This also is good for failure, as fail mode is section on.

All relays are as reliable as their ratings, 99% of them are made in china.


Dear Potato Farmer, thanks for the great info, 10A 15A values were always exaggerated, the /3 rule makes a lot of sense for inductive. And I can generally understand why relay boards are mostly triggered by 0 volts.
Not on ready-made relay boards, but in my own creations, I connect the EMF diode and the snubber to the contacts, because DC voltage in a relay or contact causes more arcing, which reduces the life of the relay contacts, but AC voltage creates less arcing.


Awesome video, really shows why flyback diodes and derating should be used on DC loads. DC can really weld. Partially must supported by the huge capacitor bank, capacitive loads look like derating could help as well.

Relays also try to prevent the arcing by opening very fast, and some are sealed with little to no air to try to prevent the plasma from being created.

Or get fancy and use solid state relays, no parts no arcs.

1 Like

Dear PotatoFarmer; What do you think of TLP3544? It can’t change the polarity of a valve motor, but it can be used in on-off operations, right? I was wondering, can the DC motor change the direction (polarity) be done with a relay external component?

Relay SSR 25mA 1.48V DC-IN 3.5A 40V DC-OUT 6-Pin PDIP

On-State Resistance 30 mΩ
Output Current 3.5 A
Output Voltage 40 V
Relay Type Relay
Resistance 60 mΩ
Termination PC Pin
Throw Configuration SPST

I use this board
It is much better than the others.

How better? The same China relays…

Show me a relay not made in China, lol. Lots of good American products are made in China like the iPhone, and flags.

Allen Bradley makes some relays in Italy but a small 1A one is $40. The rest are made in China to the required electrical specification with the same price level.

It could probably change the direction, but you still need pwm or the motor would fire up full speed.

I know, but they are the cheapest Chinese-designed relays available. The same as in all cheap ones
in relay cards.

The advantage is that you can use 5, 12 or 24 V input. I think it doesn`t matter, something between 5 and 24. The clamps are better than pins. It is easier and safer to fix the wires. And you can switch high or low level trigger.