# Should I use resistor between relay and solenoid valve?

Hi,

I'm building automatic garden watering system using

First of all, relay module contains in itself transistor and kick back diode included, so no need for those.

I'm concerned about the current passed to valve (I'm afraid of burning it :)), so I have a few questions:

1. On a page I posted above for valve, it said current is 250 mA. However, on a valve I got its written 12V 4.8 W. How come that current is 250 mA? We know that I = P / u = 3.8 /12 = ~ 400 mA

2. Now, I know that valve and relay should have resistance, but I could not find it. In a page I posted above for relay module it says coil resistance if 70 ohm at 5 A. Does this mean that current that comes to my valve will be 12V / 70ohm = ~180 mA ?

3. Which consumer you recommend to measure a current with multimeter, maybe 12 V doide? I wanted to check which is the current that will flow to valve before linking it.

Any info on this topic is welcome. Thanks in advance

4.8W / 12V =400mA. Relay can do that OK, but you still need a kickback diode across the valve coil (1N4004).
Your 12V power supply can do at least 600mA, yes?
Don't try to power the 5V relays from the Arduino's 5V pin, you really need a separate 5V supply for the relay module. In hindsight, you should have bought a 12V relay module triggered by 5V.

That's a big problem with online sellers like Bangood, Allie and Amazon, the page is made by webpage programmers and "graphic artists" who know nothing about the product and sleazy marketers who know nothing besides the price (in my book, half a notch above politicians and lawyers). >:(
They WILL tell you important things like the color, you wouldn't want a hideous mustard yellow valve, would you.

JCA34F:

4.8W / 12V =400mA. Relay can do that OK, but you still need a kickback diode across the valve coil (1N4004).
Your 12V power supply can do at least 600mA, yes?
Don't try to power the 5V relays from the Arduino's 5V pin, you really need a separate 5V supply for the relay module. In hindsight, you should have bought a 12V relay module triggered by 5V.

That's a big problem with online sellers like Bangood, Allie and Amazon, the page is made by webpage programmers and "graphic artists" who know nothing about the product and sleazy marketers who know nothing besides the price (in my book, half a notch above politicians and lawyers). >:(
They WILL tell you important things like the color, you wouldn't want a hideous mustard yellow valve, would you.

Now I connected diode in place of valve Arduino - Relay module -> 1k resistor -> diode

However, when I measure current with multimeter before the resistor (output from relay), it is around 50 mA.
Since I'm relatively new to this, I guess current depends on a consumer? Led diode didn't pulled more than 50 mA. And for valve it should be around 400 mA ?

Or (it seems logicly) 50 mA is the current Arduino provide. If I connect a battery to VCC/GND on relay the output current would be higher?

A parameter to look for in applications like this is that the device is rated for "continuous duty." That means it is engineered to be energized for an indefinite amount of time without harm (within the limits of its other specs). The counterpart is "intermittent duty," which indicates that the unit must not be used above a certain duty cycle or it will overheat.
S.

aldm:

1. On a page I posted above for valve, it said current is 250 mA. However, on a valve I got its written 12V 4.8 W. How come that current is 250 mA? We know that I = P / u = 3.8 /12 = ~ 400 mA

2. Now, I know that valve and relay should have resistance, but I could not find it. In a page I posted above for relay module it says coil resistance if 70 ohm at 5 A. Does this mean that current that comes to my valve will be 12V / 70ohm = ~180 mA ?

3. Which consumer you recommend to measure a current with multimeter, maybe 12 V doide? I wanted to check which is the current that will flow to valve before linking it.

1. There is indeed a mismatch between the power figure printed on the solenoid and the claimed current. The difference is not huge and I would not worry about it provided the solenoid is rated for continuous duty.
2. Your project has two different coils: the relay coil and the solenoid coil. The relay coil is used only to actuate the relay switch contacts and has nothing to do with the current to the solenoid (other than turning it on or off). The solenoid current will be determined only by its coil resistance and the voltage supplying it.
3. Since this is a DC solenoid, the easiest way to predict the current through it is by simply using Ohm’s law. Measure the solenoid resistance, and divide into the voltage you’ll be supplying. That will be the real steady state current, and it will be more accurate and believable than either of the specs. As JCA34F stated, the relay should be able to switch 400 mA fine, but you will still need a flyback diode across the solenoid coil.
S.

Relay datasheet

With a relay power rating of 0.51W it should only draw about 100mA@5V

raschemmel:
Relay datasheet

With a relay power rating of 0.51W it should only draw about 100mA@5V

Except that its 0.36W coil, 72mA. You took the 48V coil power rating by mistake - its not
very well written datasheet. Its meant to be read as "0.36W (except 48V version is 0.51W)",
which agrees with the winding resistance values in the table.

Except that its 0.36W coil, 72mA. You took the 48V coil power rating by mistake - its not
very well written datasheet. Its meant to be read as “0.36W (except 48V version is 0.51W)”,
which agrees with the winding resistance values in the table.

“Close but no cigar…”
Thanks for the clarification.

MarkT:
Except that its 0.36W coil, 72mA. You took the 48V coil power rating by mistake - its not
very well written datasheet. Its meant to be read as “0.36W (except 48V version is 0.51W)”,
which agrees with the winding resistance values in the table.

So 72 mA is the maximum current that can flow to solenoid valve using this relay?

But is the current calculated based on relay’s voltage or based on external supply voltage I connect to relay (in this case 12V)? As I understood, the current flows through relay from external source to solenoid valve?
If the answer is yes, then I would also nee a transistor with a proper resistor to produce a valid current…

aldm:
So 72 mA is the maximum current that can flow to solenoid valve using this relay?

NO.
The relay is a switch. It controls whether or not current flows to the solenoid, just as your finger on a light switch determines whether or not current flows to a lamp. But it does not otherwise determine the magnitude of that current, just as your finger does not determine the magnitude of the current to the lamp. The current to the solenoid is determined by the resistance of the solenoid coil and the voltage supplying it, period. The voltage supplying it is routed through the relay switch contacts. You must not exceed the current rating of the relay contacts, but that should not be a problem here.

It sounds as if you are unclear on the concept here. It might be wise for you to post a clear diagram of your intended setup.
S.

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