# How to make a 12VDC pump run on 7VDC



## bmaverick (Oct 7, 2010)

OK, in this thread, ...  How to make my Fans 5v ...  it was talked about running fans from 12VDC to 5VDC.  Then somebody showed how to jumper the 4-pin Molex PSU plug to get 7VDC to run the fans with no problems. 

WELL, here's my question then, Can a pump like the MCP350/355 run with the 4-pin Molex jumpered like that without burning up the pump or causing any other issues with the PSU? 

Right now, I have a fan speed controller regulating my MCP350 pump to 9VDC.  The flow is good enough to keep the heat of the loop at bay.  

So, in the picture shown, replace the fan with the pump connection wired the same way.


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## JrRacinFan (Oct 7, 2010)

I wouldnt trust it.


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## PVTCaboose1337 (Oct 7, 2010)

bmaverick said:


> OK, in this thread, ...  How to make my Fans 5v ...  it was talked about running fans from 12VDC to 5VDC.  Then somebody showed how to jumper the 4-pin Molex PSU plug to get 7VDC to run the fans with no problems.
> 
> WELL, here's my question then, Can a pump like the MCP350/355 run with the 4-pin Molex jumpered like that without burning up the pump or causing any other issues with the PSU?
> 
> ...



Do you know how electronics work?  Plugging in to the 12v+ and 5v+ will do nothing.  You have to plug into EITHER the 12v or 5v slot, and then hitch it to a ground.


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## JrRacinFan (Oct 7, 2010)

PVTCaboose1337 said:


> Do you know how electronics work?  Plugging in to the 12v+ and 5v+ will do nothing.  You have to plug into EITHER the 12v or 5v slot, and then hitch it to a ground.



Actually I hate to say it.....

red is hot black is ground on a pump, so by connecting +12v to hot and +5v to ground you are essentially getting this, +12v - +5v = +7v


OH and I still wouldn't trust it.


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## PVTCaboose1337 (Oct 7, 2010)

JrRacinFan said:


> Actually I hate to say it.....
> 
> red is hot black is ground on a pump, so by connecting +12v to hot and +5v to ground you are essentially getting this, +12v - +5v = +7v
> 
> ...



Oh yeah seems to work, just tried it.  Did not know you were going to ground, just thought positive to positive.


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## angelkiller (Oct 7, 2010)

PVTCaboose1337 said:


> Do you know how electronics work?  Plugging in to the 12v+ and 5v+ will do nothing.  You have to plug into EITHER the 12v or 5v slot, and then hitch it to a ground.


Voltage is potential difference by definition. The difference in potential between 12v and 5v is 7v. The same thing applies to the 12v and ground. The potential difference between 12v and 0v is 12v. 

Plugging a pump into the 12v and 5v rails will indeed make the pump run at 7v. However, you're feeding a good amount of current back into the PSU. It was explained to me that something else *must* use *all* of that current. So make sure you have something using all the current that you feed back in.

Realistically though, I wouldn't do this because I can't be sure that all the current will get used. I'm going to use one of your MCP350 pumps (remember me?) in the system I'm building right now and I'm looking for a way to slow this thing down. I've had lots of ideas, but I think the best way might be to build a small PWM circuit. I'm not sure how to do this, so I was going to make a thread about it, but never got around to doing it.


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## sneekypeet (Oct 7, 2010)

In-line resistance is easiest


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## angelkiller (Oct 7, 2010)

sneekypeet said:


> In-line resistance is easiest


True. But the problem is that this is a 10W load. To reduce the voltage by 25%, the resistor has to dissipate 25% of the power, or in this case, 2.5W. Most resistors are designed for .5W max. I guess you could string 5 of these together.

Either way, resistance is pretty inefficient since all the power is lost as heat. Again, I'm in the same boat as the OP and my best idea was to build a PWM circuit. The power losses are much smaller.


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## sneekypeet (Oct 7, 2010)

sorry, I was thinking fans, didnt incorporate the draw of the pump


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## mlee49 (Oct 7, 2010)

Who wants to run their pump at 7V when you should be wanting to over-volt it to 18V


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## bmaverick (Oct 7, 2010)

mlee49 said:


> Who wants to run their pump at 7V when you should be wanting to over-volt it to 18V



Well, those that O-V to 18V need to cool down an OC or something.  There is a group in the WCing community that like totally s-i-l-e-n-t (shhhhh) computers 24/7.  Running the system a stock speeds and WCing with limited noise makes things very quiet.   

My Enermax fan speed controller can bring the pump down to about 9VDC.  And yes, it takes a bunch of heat doing it.


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## bmaverick (Oct 10, 2010)

OK, got another idea and tried it. 

My brother's 7" portable DVD player from Big Lots died on him.  It's 9 years old and seen a good life.   Well, the wallwart is a 9VDC 750mA.  I got to thinking, maybe it will run my pump.  

So, I got one power strip plugged into the wall with the wallwart and then my computer surge strip into that.  So, every time the pump strip is on, the computer will be on. Thus, the system will not have the pump off and the computer running.   

The walwart doesn't have the heat issues either.  It is very cool to the touch.


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## newtekie1 (Oct 10, 2010)

As long as the pump will start up at 7v and runs at 7v, and the computer stays cool enough for you, there is no problem with doing this.  You can even run it at 5v is you wanted to go even slower/quieter, again as long as the pump will start and run at 5v.


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## Deleted member 3 (Oct 10, 2010)

PVTCaboose1337 said:


> Oh yeah seems to work, just tried it.  Did not know you were going to ground, just thought positive to positive.



Positive is relative. +12 and +5 are relative to the black ground. Voltage is a difference between two values. So when using the +5 and +12 the +12 is actually +7 relative to the +5. That's how electronics work. Using negative voltages you can get up to 24v from a PSU.


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## robal (Oct 10, 2010)

Hi,

It's always best to use a good fan/pump controller, because it will start the pump at nominal voltage (12) and then reduce to desired value. It's safe and efficient.

However, because I'm cheap...  I use exactly the trick you mentioned here.
You can get +7V by plugging the pump between +12 and +5 rail.
It's way better than just +5V because it's more likely to be reliable (pump starts at 7V more easily).
Still, for MCPs +7V is below spec. (which is 9V), and it's not guaranteed to start at this voltage.
I use it for months now, but there's always small risk of pump not starting OK.


Cheers,


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## angelkiller (Oct 10, 2010)

bmaverick said:


> OK, got another idea and tried it.
> 
> My brother's 7" portable DVD player from Big Lots died on him.  It's 9 years old and seen a good life.   Well, the wallwart is a 9VDC 750mA.  I got to thinking, maybe it will run my pump.
> 
> ...


The only problem I see is that that DVD PSU is only rated at 6.75W and the pump draws 10W. Otherwise there is nothing wrong with this.

You may want to add a relay in there so when your computer turns off, the pump turns off also.


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