Choosing resistors to control pump power

[TullioK]

The setup I am near completing involves this pump (12V, up to 10A) which is controlled by either of 2 relays. The first is an uninterupted circuit with the 12V power supply, so the pump runs at full power. The second will operate the pump at a lower power. I'm looking to drop the voltage across the pump to about 8V so plan to put in a resistance of between 0.5 and 1 ohm. There will be up to about 30W dissipated by the resistor, so I'm reckoning on a pretty robust resistor (not one for 1p on ebay!). I also plan to arrange 4 (2 parrallel series of 2) and a smallfan to spread the heat and aid cooling.

I need help choosing the correct resistor, I have come across some ceramic ones on ebay which are rated at about 20W each but they're quite expensive. Looking for cheaper alternatives I found two others:

Resistor 1

Resistor 2

These are both rated at 5W but I don't understand much else of the specifications to make a judgement on wether they would be correct for my need.

Any advice on these or other products would be greatly appreciated, thank you very much,

Tully

[Mike90250]

if you use a resistor, you burn the power as heat Or you can run the pump at full speed for the same power consumed at low speed. (well, you might save a little power, but I'll bet the pump efficiency goes way down when you slow it down.)

[TullioK]

Thanks for replying. I know I will waste energy this way, but really what I want to achieve is just the option of a slower flow of water. I'm using the pump for a vegatable garden and won't always want max flow. I looked into voltage regulators but there wasn't much available at currents more than a couple of amps (also how does a regulator reduce the voltage without wasting energy as heat?)

Any further advice on the resistors would be useful, thanks

[4321loco]

Not sure if I'm close, but if the pump is more efficent at full power could you control output (flow) by adding a y or tee adapter on pump end, Then simply use a valve on one line and return extra flow back into water supply, when more flow needed simply close the return line.....It may sound like waste but may be easier on power then resistors and pump at low speed..

My guess, I'm under medication. Lol

[TullioK]

That is actually a good idea, unfortunately I am controlling the system remotely using relays so to use an electrically controlled y junction would be too difficult. I don't see any reason why your method wouldn't work though

[Naptown]

That is actually a good idea, unfortunately I am controlling the system remotely using relays so to use an electrically controlled y junction would be too difficult. I don't see any reason why your method wouldn't work though

What's so hard about controlling remotely or even automatically. There are valves that are electrically operated like on every washing machine or dishwasher ever made to name a few. They are called solenoid valves or zone valves which can be proportional. by reducing flow and head you will reduce power consumption still allowing the pump to run at full power but reduced load will reduce power consumed.

[TullioK]

Thanks for your idea and help Naptown, I'm trying the resistor method first as I have the control system to intergrate it with. I'll see how well this works both at the end of the hose and on the battery. I'll post the result once I've got it going

[cs1248]

Control

Never use resistors. Power mosfets and digital control of same is the way to go.
Resistors consume your power. Mosfets simply turn on and off power, consuming a miniscule amount of power. There is so much free info on the "net" about switching power I won't offer an answer to your rsistor question but I will advise you to educate yourself...With the internet it is free...wish it had been available when I was a kid. I'm 72 on Halloween and I just felt like venting.Good luck with your project!
Charles

The setup I am near completing involves this pump (12V, up to 10A) which is controlled by either of 2 relays. The first is an uninterupted circuit with the 12V power supply, so the pump runs at full power. The second will operate the pump at a lower power. I'm looking to drop the voltage across the pump to about 8V so plan to put in a resistance of between 0.5 and 1 ohm. There will be up to about 30W dissipated by the resistor, so I'm reckoning on a pretty robust resistor (not one for 1p on ebay!). I also plan to arrange 4 (2 parrallel series of 2) and a smallfan to spread the heat and aid cooling.

I need help choosing the correct resistor, I have come across some ceramic ones on ebay which are rated at about 20W each but they're quite expensive. Looking for cheaper alternatives I found two others:

Resistor 1

Resistor 2

These are both rated at 5W but I don't understand much else of the specifications to make a judgement on wether they would be correct for my need.

Any advice on these or other products would be greatly appreciated, thank you very much,

Tully

[TullioK]

THANK YOU. This is what I need to know, I've searched the internet for solutions but only came up with voltage regulators. I'll have to have a look around for something suitable. At first glance these mosfets look quite cheap which is excellent, I was worried they would be expensive. Nevertheless I know have my resistors so I may as well test the setup with them, but the power control is definitely the way to go thanks!