Solenoid Getting Hot and Not Functioning Properly

Question

Please forgive my ignorance on this schematic. I am new to using relays and solenoids so my diagram may be a bit inaccurate.

I am attempting to control a DC solenoid with the GPIO pins from a Raspberry Pi. The solenoid and other components are connected according to this schematic:

I am using the following components:

• Raspberry Pi 3
• 12V DC battery
• diode
• Relay SPDT Sealed
• N-Channel MOSFET 60V 30A
• 12V Solenoid

I am not sure that I have the relay pins labeled correctly in the schematic but this picture of the relay I am using illustrates which pins are connected to the other components and corresponds to the pins on the schematic:

The relay documentation can be found here.

I also have an indicator light attached to one of the other GPIO pins to determine when the signal is supplied to the circuit in order to open the solenoid. When I run the script to open the solenoid, the indicator light is on the solenoid does open but when it is off it seems like the solenoid is not completely closed as a little bit of water is leaking out. In addition, my understanding was that this solenoid would open when voltage was supplied (normally closed). However, when I put a voltmeter on the circuit in place of the solenoid the voltage jumps to 12V when the indicator light is closed. The current in the circuit when running is ~560mA. I also noticed the solenoid getting hot when hooked up.

Is there something I am missing here that would help me understand why the solenoid seems to be partially closed? Also, I am concerned about the heat as well.

After considering comments below, I am wondering if I could simply use this design to solve the problem with out the use of a relay.

Answer 1

According to the specifications, the solenoid claims a resistance of 26 Ohms. That makes the expected current 12/26 = 460 mA. However, there will be a tolerance on that 26 Ohms, and if your battery is a normal lead acid auto battery, the voltage can be rather higher than 12 V when freshly charged, so 560 mA is well within what could be expected.

12 V * 560 mA = 6.7 W. That's a lot of power for a package that size, I would expect it to get too hot to touch after a while. I would expect a component like that to have a limited time rating, perhaps 30 seconds operating in any 10 minutes.

Do you have it the right way round? Valves like this often only seal water pressure properly in one direction. Look for arrows or text on the plastic body. If it's the right way round, is it clean inside? Particles of debris caught in the sealing area could prevent it closing. If neither of those, it might be a dud.

The good news is that if the valve is drawing current, or not, under Pi control, then the FET and relay part of your circuit is working correctly.

However, your FET has quite enough grunt to handle the solenoid directly, without the need for the relay, if you wanted it to. Note that at present, the MOSFET is driving a coil, the relay coil. The valve coil needs more voltage (12 vs 5) and more current (about 500 mA vs 80 mA), but both are well within the specification of the MOSFET which is 60 V and 16 A. Make sure you retain the kickback diode D1 and place it across the valve coil as you've placed it across the relay coil.

Having just written 'retain D1' in the previous paragraph, I've just noticed that you haven't got a D2 across the valve coil. It would be beneficial to the relay life, to suppress arcing at the contacts due to the inductive energy stored in the valve solenoid coil. Relays and MOSFETs fail in different ways when this diode is left out. MOSFETs tend to fail all in one go, however note that the MOSFET you've chosen has a rating for 'unclamped inductive switching' figure 6, which means with small enough inductive loads, it doesn't need D1 to stay safe. Best to include it though if you don't know what the inductance of the coil it's driving is. OTOH, relay contacts tend to erode, and fail after a number of operations. As the load inductance and load current increase, the lifetime drops dramatically, you might get 200k operations at no load, and only 1000 operations at something that looks well within the relay rating. That 30 V DC 5 A contact rating will be for resistive loads only, those figures will drop a lot for inductive loads.

To answer the second diagram, this is how to connect it correctly. It will work just fine.

^{simulate this circuit – Schematic created using CircuitLab}

Differences to your sketch
1) Use it in common emitter, not common collector
2) Use a base resistor, this will limit base current to 3 mA at 3.3 V
3) Connect the ground to the pi ground
4) I've used the built-in schematic editing tool
5) I've put +ve at the top of the diagram, ground at the bottom, which is the way all engineers draw them

Answer 2

The datasheet is under Documents Tab in your link near bottom of page.
https://www.sparkfun.com/products/100

You may choose either NC or NO using a SPDT RELAY.

The Valve is NC for water when the solenoid coil is OFF connected to NO (pin 3) with power off and Drain to THrow pin 2 and Source to GND and other end of Solenoid to Vcc.

^{simulate this circuit – Schematic created using CircuitLab}

Schematics always show power-off condition so 2 to 4 = NC and 2 to 3 = NO
NC = normally Closed and NO = .... guess

with your schematic errors .. recheck but it won't work using 3 to 4

The relay is redundant as the MOSFET carefully chosen should be adequate to drive a solenoid, but using 12V open collector with pullup and careful power wiring direct from battery.

**Is 5V enough for Gate voltage? V GS = 5 V, I D =16 A YES

typical ROOKIE errors. no big deal...