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#41
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kony wrote:
ric wrote: kony wrote: I dont' know how much further that particular fan will tolerate a voltage drop so I can't help much. Past a certain point a larger resistor will prevent it from spinning up which obviously should be avoided. Placing a cap in parallel with the resistor seems to cure startup problems. "Cure"? To what extent? I mean, does it actually allow a lower working RPM then or just offset the overly high resistance? This is an interesting idea, do you have an example of what gain there will be for any particular fan? This is probably just plain dangerous to the rest of your system. It would depend on "giving a kick" at turn-on, provided that the power supply had a rapid rise time. If the fan ever gets stalled in normal operation the kick is not there, and the thing just sits and encourages the rest of the system to cook. -- "If you want to post a followup via groups.google.com, don't use the broken "Reply" link at the bottom of the article. Click on "show options" at the top of the article, then click on the "Reply" at the bottom of the article headers." - Keith Thompson |
#42
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On Wed, 06 Apr 2005 14:31:52 GMT, CBFalconer
wrote: kony wrote: ric wrote: kony wrote: I dont' know how much further that particular fan will tolerate a voltage drop so I can't help much. Past a certain point a larger resistor will prevent it from spinning up which obviously should be avoided. Placing a cap in parallel with the resistor seems to cure startup problems. "Cure"? To what extent? I mean, does it actually allow a lower working RPM then or just offset the overly high resistance? This is an interesting idea, do you have an example of what gain there will be for any particular fan? This is probably just plain dangerous to the rest of your system. It would depend on "giving a kick" at turn-on, provided that the power supply had a rapid rise time. If the fan ever gets stalled in normal operation the kick is not there, and the thing just sits and encourages the rest of the system to cook. What I'm suspecting is that if a cap were in parallel, a bypass for the power to the fan, it would just make matters worse because whatever the turn-on delay was already, adding more capacitance will only lengthen that delay. Ric wrote about it being parallel with the resistor though, so i presume actually paralleled to it, bypassing it on the same power lead. While that can be a crude but effective filter for some noise reduction, I'd not heard of anyone employing anything like this for fans. Sometimes it's not even, merely a matter of getting the fan to spin-up though, with some fans there will be pulsation below a certain RPM, actually making the fan louder than if it were running slightly faster-enough to eliminate this pulsation. Offhand I'd speculate that it pulsating occurs due to stronger (permanent) magnetic field in some fans than others. IE- It seems more common in = 120 x 32mm, large thick fans. |
#43
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kony wrote:
wrote: kony wrote: ric wrote: kony wrote: I dont' know how much further that particular fan will tolerate a voltage drop so I can't help much. Past a certain point a larger resistor will prevent it from spinning up which obviously should be avoided. Placing a cap in parallel with the resistor seems to cure startup problems. "Cure"? To what extent? I mean, does it actually allow a lower working RPM then or just offset the overly high resistance? This is an interesting idea, do you have an example of what gain there will be for any particular fan? This is probably just plain dangerous to the rest of your system. It would depend on "giving a kick" at turn-on, provided that the power supply had a rapid rise time. If the fan ever gets stalled in normal operation the kick is not there, and the thing just sits and encourages the rest of the system to cook. What I'm suspecting is that if a cap were in parallel, a bypass for the power to the fan, it would just make matters worse because whatever the turn-on delay was already, adding more capacitance will only lengthen that delay. No suspicion needed. It forms a simple high pass filter. Very straight forward. Once the cap is charged it has no further effect. The charging current, as the power comes on, supplies the initial start-up boost. -- "If you want to post a followup via groups.google.com, don't use the broken "Reply" link at the bottom of the article. Click on "show options" at the top of the article, then click on the "Reply" at the bottom of the article headers." - Keith Thompson |
#44
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kony wrote:
Placing a cap in parallel with the resistor seems to cure startup problems. "Cure"? To what extent? I mean, does it actually allow a lower working RPM then or just offset the overly high resistance? For fans that will spin at, for example, 5 volts but have startup problems, the cap acts as a short at startup, allowing the full 12v to reach the fan, then charging to the resistor's dropping voltage. I normally use a 10uF, 15v electrolytic. [Get the polarity right!] This is an interesting idea, do you have an example of what gain there will be for any particular fan? Not off hand. No gain in operational function. But it sure cures the "I have to help my fan start" blues. |
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