Current sensing relay

Hello I have a 12 volt water pump (incorporating a pressure sensitive switch) in my caravan. When switched to the on position the pump will be idle but becomes active when a tap is turned on.

What I would like to do is have a small 12v solenoid activated when current is detected when the pump is running. So some sort of current sensing relay I guess.

The water pump draws approx. 6-8 amps when running.

The Solenoid draws about 320ma

Do you have such a devise it could be a combined current sensing unit or combined with a relay.

Thanking you in anticipation



1 Like

Hi Peter,

I might be missing something here… but why couldn’t you just trigger your pump and solenoid from from the same power supply (both triggered by the pressure sensitive switch?) Is it a wiring constraint? (i.e. are you wanting your addition to be non-invasive?)



Hi Peter, Welcome to the forum :smiley:

I can’t think of an all-in-one unit off the top of my head that would do what you’re after and switch a relay in response to the current measurement but you could certainly build a device to measure current then switch a relay using a microcontroller like an Arduino.

We have a range of simple current sensors like this one:

You could then read the analog value output via the sensor with a microcontroller with an analog to digital converter ADC, like an Arduino, and control a relay with the microcontroller output.
I’ve linked a tutorial for relay control here:

1 Like

Hi Peter
I am with James here. What is the solenoid going to do. If it is going to turn the water off how is the pressure switch going to detect a tap operation.

To my simple mind the only reason to monitor pump current would be to detect an over current situation like a jammed pump or an undercurrent situation such as the failure of the pump to start. Then the Hall device suggested by Trent (assuming current capability is OK) and an Arduino to monitor the situation would be good.
Cheers Bob

1 Like

A typical fresh water pump in a van or boat operates on the pressure differential between the inlet and output, so it turns on when the tap is opened and turns off after the tap is closed and the system has been brought back up to pressure. The indicator is used to warn that it hasn’t turned off if, for instance, there is a leak in the system or air in the lines. Usually, the pressure switch is not accessible from outside the pump, so some other means of indicating that the pump is still running is required, and a current sensor would do the job.


Good comment Jeff.
That would make for 5 conditions. 2 OK and 3 Faults:
1)Tap on, press switch on, water flowing, current normal. All OK
2)Tap on, press switch on, low or no water flow, current high. Jammed pump, fault.
3)Tap on, press switch on, low or no water flow, no current. Pump not started, fault.
4)Tap off, press switch on, no water (from tap), current normal. Faulty switch or leak, fault
5)Tap off, press switch off, no water (from tap), no current. All OK.

At least situation 4) requires some sort of sensor on the actual tap itself. The pressure switch would not indicate this fault as it would assume the tap is still on.

Still does not indicate what the solenoid is to do. The only thing I can think of is to shut down water supply in the event of a fault. Some sort of emergency access to water would be needed as any fault would need to be rectified before water is available.
Cheers Bob

1 Like

Thank you everyone for your interest. I’ll try and explain better what I am trying to achieve. I currently have a “Red Water” diversion valve fitted to my caravan. Basically this diverts water back into the tank until it reaches a pre determined temperature, only then does it allow the hot water to run out of the tap. This saves wasting water while waiting for the hot water to go through the system.
When connected to a mains outlet this system is bypassed by use of a tap. What I am trying to do is replace this tap with a solenoid. I could do this manually with a simple switch either wired separately or or in conjunction with the power switch which turns on the pump. This option would have the solenoid active when ever the pump was turned on ( even though it is not actually pumping water) although the solenoid would not use a lot of power. What I was hoping to achieve was a seamless activation of the solenoid activated only when water is flowing through the pump. (The pump is activated by the change in water pressure, current is passing through the wires to activate it, I was hoping that this current could trigger a response through some sort of current sensing gizmo, which in turn could activate a relay, which in turn would activate the solenoid and control the water flow through the red water valve.

Thank you for your patience, I hope I have explained it more clearly this time.


1 Like

Hi Peter
Don’t know how you have the pump on without pumping water but won’t go into that.
The current sensor mentioned earlier would detect that as long as the current drawn by the pump is within range of the sensor. You haven’t quoted this figure so one would only be guessing. This could be connected to a small Arduino or simple comparator circuit which operates a relay that in turn operates a solenoid. I don’t think an Arduino or comparator could operate a solenoid which is capable of turning a water valve on and off directly . Do you have a solenoid valve in mind. If some details are made available some advice could be offered maybe.
Cheers Bob

Thanks Bob, When I say turned on I guess I mean power is available to the motor awaiting it to sense a change in pressure, which activates the pump.
This is the solenoid I plan to use.

MV80 - Mini Solenoid Valve
An economical micro control valve with high flow and low pressure loss for use in drip, spray, and pop up sprinkler watering
systems. Ideally suited to residential use to meet standard water meter flows.
Body material Nylon 66 30% glass filled
Natural Nitrile elastomer diaphragm, or optional Viton diaphragm
Bonnet screws stainless steel 304
304 Stainless steel 40 mesh screen (on inlet side of valve)
WATERMARK approval ATS 5200.030-2004.
Solenoid encapsulant Nylon 66 (30%) Glass filled
Magnetic stainless steel grade plunger
Natural Nitrile elastomer plunger tip
Stainless steel 302 plunger spring
Voltage 24Vac 50Hz or optional 12vac
Voltage tolerance +10% or -10%
Power consumption 8 VA
Inrush current 320mA
Holding current 260mA
The valve will operate correctly with in the following conditions
Ambient temperature 2° C to 60° C
Relative Humidity 0% to 100%
Temperature of water 2° C to 80° C
Direction of fl ow markings on body (check before installing)
Minimum operating pressure 20kpa
Maximum operating pressure 1250kpa
Supply pipe velocity range 1.2 m/s to 6.5 m/s
Opening speed 1 sec
Closing speed 1 sec

Thanks Peter

Hi Peter

You do realise this device is AC voltage don’t you. You will not be able to power it with a battery.

This sort of current is way beyond any arduino or similar. You will need a relay to switch it or as it is AC a solid state relay may be OK. Note most solid state relays will not switch DC.

Thought that would be the case. However I am a bit harsh when it comes to inaccurate descriptions as they are usually very confusing. Guess it is my association with military type QA over my working years.

Pump motor current???
Cheers Bob


Hi Bob

I will be using the 12 volt version of this solenoid.

The pump draws approx. 8 amps when running.

I expect that I may have to use a simple 12 volt relay to activate the solenoid.

Hi Peter

You obviously did not read my last reply properly.
The data sheet for that solenoid (which you have copied) says this unit operating voltage is 24 or 12 VOLTS AC. You won’t be able to operate it with a 12VDC battery. As you are operating in a caravan I am assuming your 12V is a battery WHICH IS DC. NOT AC…I think if you try you may have smoke.

Correct. You will have to interface a relay driven by a bit of circuitry attached to your current sensing system.

Current sense: The Sparkfun hall device quoted earlier is 5A max. You will need more. I would go for a 10A unit. The breakout board system simplifies things a bit. Core have a selection available on the web site. Search “current sensor”. A 10 device would be easier to interface but Core or Sparkfun don’t seem to have one. Core have a 20A device which would be the maximum I would go for. As said 10 or 15 would be better.
Cheers Bob
It may be feasible to connect 2 of these units in parallel. You would only need to take an output from one of them. The other would act as a shunt to bypass half the current. You are not actually measuring the value of the current, only detecting weather current is present or not.


Thanks Bob,

I can assure you that the solenoid is 12 volt, might have been a misprint on the web page I copied. I have attached another for your info.

MV80 Micro Solenoid 12V DC NORMALLY OPEN 20mm Male Inlet/Outlet HR Products 50 lpm

This valve allows water to flow until the application of 12VDC which causes the valve to shut.


  • 12VDC coil
  • Body material Nylon 66 30% glass filled
  • Natural Nitrile elastomer diaphragm, or optional Viton diaphragm
  • Bonnet screws stainless steel 304
  • 304 Stainless steel 40 mesh screen (on inlet side of valve)
  • WATERMARK approval ATS 5200.030-2004.
  • Operating pressure: 20 - 1250 kPa

Thank you for your advice I will investigate the options you have given and hopefully be able to achieve the result I am after.


1 Like

Hi Peter
I googled MV80 and got onto the same web page you copied. It definitely states “24VAC 50Hz or optional 12VAC”. Maybe it should have read “or optional 12VDC” I have not seen the bloody device so am unable to comment. I have been just going on the info you have provided. If you somehow have known differently and kept it to yourself then so be it. You just said

without clarifying the AC or DC question.
I just stated a fact. If you use 12vDC on an inductive 12VAC device you will get smoke.
Cheers Bob