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switching a 12v water pump

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Poleman

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Hi Guy's
I need a little help with a 12v system, I'm not that hot with electronics so any help or advice will be most welcome.

OK here goes
I have a water tank in my van the water comes from the tank to a 12v pump the pump is controlled by a 12v electronic controller and all works well.
I have added a heater to heat the water and this is were my problem began.

I added a relay between the 12v controller and the pump using the 12v supply, everything is working fine, the only trouble is I have to increase my water flow to activate the relay, the controller is just an expensive variable resistor, so to keep my water flow down I turn a knob on the controller and this has the affect of reducing the voltage on the relay.
These are the readings I have collected from my system.

12.36V across the main battery terminals
10.2V across the relay terminals with the pump running full speed
6.8V is the minimum power required to switch the relay (pump controller dialed to 3.5)
4.5V is what I have with the controller turned to 2 which is the minimum setting I would ever use.
So,.. I need a relay that will switch at 4.5V or below, but still be able to handle 10.2V -12v without burning out.

Failing the availability of a relay with this type of range to operate my boiler with, is there a power transistor or similar I could use to take the place of the relay.

I have drawn a diagram of what I mean in the hope it helps with my poor explanation.

**broken link removed**
 
A simple solution that comes to mind is to add a transistor to drive the heater relay. The heater likely takes a high current so it's easier just to let the relay handle that. A diagram is attached.

The transistor and diode part types are not critical. I just chose some that are available at Radio Shack but loose equivalents will work.

You can adjust the value of the transistor base input resistor to control the point at which the heater relay closes. Don't go below 300 ohms for a 1/2 watt resistor.

The transistor shouldn't require a heatsink but you should check its temperature, particularly at the low pump motor speed settings, just at the point where the relay pulls in. Warm is okay but if it gets too hot to touch then it needs a heatsink.

Heater Relay Driver.GIF
If you don't want to mess with electronics, you could also buy a solid state relay. You would need to know how much current the heater took and buy the appropriate size. Many are controlled by a current input and you just select the proper size input resistor to get the turn on point you want. They can be purchased from many electronic supply places such as Digi-Key. Of course, this is more expensive than the transistor option.
 
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Hi crutschow and the mad professor.
I'm away from the van most of the time and the contoler I have in my hand controls the flowmaster, so a simple switch is no good.

Many thanks for your replys it is most appreciated.
I think I may have explained it a little wrongly? In my diagram where you see the wire coming from the boiler, well this wire is at 3v and is just a trigger wire to fire the boiler up, all the boilers other wiring is completely independent.

My main concern is that the relay won't fire when I have the controller (flowmaster) turned down to give me the flow of water I need, when I adjust it to give me the flow of water I need the voltage across the relay coil is at 4.5v but when I increase the flow the voltage also increases and this can increase to 12v.
I need a relay or or something similar that would operate between these voltages?
I am truly sorry if its all a bit misleading.
 
So how do you turn on the boiler? Do you ground the 3V wire or do you connect it to 12V?
 
The relay is just acting as a switch and the wire you see on the boiler side is just a single wire cut and and then reconnected when the relay activates, everything works, I just need the coil side of the relay sorted out to operate between the specified voltages.
 
Poleman said:
The relay is just acting as a switch and the wire you see on the boiler side is just a single wire cut and and then reconnected when the relay activates, everything works, I just need the coil side of the relay sorted out to operate between the specified voltages.


You have a couple of options.
The most easyest and cheapest is to hook your relais up to 12volts...
and ground it using a high HFE transistor.

then you take as the base signal (input) the signal from your pump.

If the voltage on the pump is enable > 0,6Volts then the relais will be activated. (by adding some diodes you can tune it up. to the point you want)

what you say is possible but you will need a low voltage relais wich can switch a high current (your heater) and i think that will be much more expensive.

think about it!

1x BC547C
1x 10K ohm
1x 1N4001

and you are done!!

the plus from the relais can be joined to the heater.
The base of the transistor will be connected to the plus side of the pump inseries with the 10K.

then the Collector will be the negative side of your coil.
and the emitter will be ground.

as easy as.....

if you give more specs from the relais we can recheck the components...
 
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Many thanks TKS
I will certainly give this a go, as I said the wire from the boiler thats connected to the relay is at 3v only, so there is no need for switching high current as the boiler is Gas operated, the wire is just a trigger.

Many thanks for your help guy's you have all been very helpfull with some great info.
 
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