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Old March 13, 2009, 10:55 AM
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undervolting didnt work for the record, put a noctua in line adapter in there, neither of them started the pump...
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Old March 13, 2009, 11:59 AM
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Most pumps have a minimum startup voltage, and some actually won't work below 10-11V. The average in-line resistor for a fan probably has way too much resistance to be used on a pump motor - you're probably leaving the pump the equivalent of only a couple volts to work with, and definitely risking burning out the resistor.
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Old March 13, 2009, 02:58 PM
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specs state an 8v startup voltage, and no worries, neither of the noctua adapters drop it to a "couple volts", I don't think any fans start at less that 5...

12v is 12v, pump or fan don't matter, current draw could cause the resistor to burn out, but no matter what you hook up to it there will be the same voltage drop over the resistor.

but yeah, I thought it might burn out the resistor, but I also didnt care much, not using them for anything else anyways (worked fine when tested on fan after)
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Old March 13, 2009, 04:39 PM
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Actually, the Noctua adaptors use resistors to get their voltage drop. Noctua P-12 fans only pull about 1W of power, whereas that pump is pulling about 9W. Furthermore, a Noctua in-line adaptor has about 100ohms of resistance, which isn't a big deal because the fan's internal resistance is about 150ohms, which means that the majority of the voltage still gets used at the fan. That pump has an internal resistance of only 36ohms, meansing that the in-line adaptor is getting about 3/4 of the given voltage, leaving only 1/4 of the voltage for the actual pump, or about 3V from a 12V line.

On the other hand, you're not too likely to burn the resistor out in the process, since the resistor will drop the current flow so much.
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Old March 13, 2009, 05:01 PM
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Quote:
Originally Posted by donimo View Post
Will this damage the pump at all? I know its ok with fans, but what about pumps?

It is a DD CPX-pro, I have it on a foam pad with the rubber mounts, it hums above all the fans and vibrates the whole desk...

also the "3-pin connector" seems to be a waste of time, no RPM reading at all...

I have this pump and ya the rpm thingy is screwed! However I've found this pump cold starts off 5vdc every single time. No probs. I run it at 5vdc and there's ~1C difference to 12 vdc, but I'm only running one block (CPU).

Edit: I don't see any way it could harm the pump. The biggest deal would be starts at low(er) vdc. Overall, I feel it's a great pump.
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Last edited by kirk_d; March 13, 2009 at 05:10 PM.
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Old March 13, 2009, 07:12 PM
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Quote:
Originally Posted by MpG View Post
Actually, the Noctua adaptors use resistors to get their voltage drop. Noctua P-12 fans only pull about 1W of power, whereas that pump is pulling about 9W. Furthermore, a Noctua in-line adaptor has about 100ohms of resistance, which isn't a big deal because the fan's internal resistance is about 150ohms, which means that the majority of the voltage still gets used at the fan. That pump has an internal resistance of only 36ohms, meansing that the in-line adaptor is getting about 3/4 of the given voltage, leaving only 1/4 of the voltage for the actual pump, or about 3V from a 12V line.

On the other hand, you're not too likely to burn the resistor out in the process, since the resistor will drop the current flow so much.
I corrected myself.

With your numbers you would get this:

12v into a cicuit containing 2 resistors R1(100ohms) + R2(150ohms), so the current would be 0.048A, and the voltage seen by the fan (R2) would be 7.2V

with the pump it would be a total current draw of 0.088A and the pump would see 3.2V

so you are right, I was just having a bad day, blame 7 months of a baby waking me up 6 times a night and leave it at that

of course most of your numbers are a bit off and measuring the current draw of a motor is more than just its "resistance", but I get your point all the same.
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Last edited by donimo; March 13, 2009 at 08:13 PM.
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