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Old 29-07-2009, 07:29 AM
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stephenb (Stephen)
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Cooling fan technical questions

I am testing a cooling fan for the 8"dob project and I need some advice regarding the fan's performance.

I purchased a typical chinese-made slimline computer fan 70mm x 10mm thick. It has a 5vDC operating capacity. It draws about 200mA.

I plan to operate it on 'AA' rechargable Nickel Metal Hydride (NiMH) from a small battery back mounted on the bottom of the mirror box. Although these batteries are rated at 1.2vDC (nominal), when fully charged these batteries are at a peak capacity of 1.45vDC, which I assume, they do not stay at this high voltage for very long when in use.

(1) If I use 4 of these batteries, at start up when fully charged, the total voltage of 5.8vDC. Is this too much for a 5vDC cooling fan?

Obviously after about 30 minutes operation, this drops to under 5V and steadies for a while until they begin to constantly drop further.

(2) If I use 3 of these batteries, at start up when fully charged, the total voltage of 4.35vDC. Will under-voltage put any stress on a small fan like this for a long period of time?


I have run 3 x AA NiMH batteries on this 5V fan and I got about 4 hours operation out of it.
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Old 29-07-2009, 08:23 AM
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erick (Eric)
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I doubt 0.8V over will upset the 5V fan. As you say, it doesn't stay there for long. Don't worry about under volts - fan just runds a little slower. I power a 12V fan from a little power brick that will deliver only 8 or so volts - all fine.
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Old 29-07-2009, 08:36 AM
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stephenb (Stephen)
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Quote:
Originally Posted by erick View Post
I doubt 0.8V over will upset the 5V fan. As you say, it doesn't stay there for long. Don't worry about under volts - fan just runds a little slower. I power a 12V fan from a little power brick that will deliver only 8 or so volts - all fine.
Thanks Eric for that. I've been doing a bit of searching on the internet about it. My gut feeling is that the 0.08 shouldn't be an issue, so thanks for confirming that.

I will run 4 x fully charged NiMH batteries today and see what duration the fan runs for.

I wonder if there is a simple data logger for computers to create logging graphs for test like this?
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Old 29-07-2009, 08:41 AM
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kinetic (Steve)
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Quote:
Originally Posted by glenluceskies View Post
I wonder if there is a simple data logger for computers to create logging graphs for test like this?
Good digital multimeters have PC connectivity and software for
data logging.

4hrs sound a pain in the bum though for duration.
If you went to D cell NiMhs or just a 6v Gel cell with a diode or
2 to drop it 1.2v, that would last days.

Steve
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Old 29-07-2009, 10:27 AM
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Quote:
Originally Posted by kinetic View Post
Good digital multimeters have PC connectivity and software for
data logging.

4hrs sound a pain in the bum though for duration.
If you went to D cell NiMhs or just a 6v Gel cell with a diode or
2 to drop it 1.2v, that would last days.

Steve
My Fluke 79 Series is an old model DMM which does not have that function, so I'll just run it and check the voltage at regular interval and log it in Excel or graph paper (the ol' fashioned way )

Steve, I'm not stressing about the duration only being 4 hours. It's a 8" Dob for brief sessions, with the young 'uns, and to pop in the boot of the car for a casual observing evening. I'd rather carry an extra set of 4 AA's and a quick charger. I want to keep any on-board electronics to a minimum, both in weight and complexity. If I was building a bigger Dob, things would be different.
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Old 29-07-2009, 04:21 PM
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stephenb (Stephen)
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I have spent all day monitoring the voltage drop on 4 AA NiMH connected to the 5v cooling fan. I manually logged the voltage at 15 minute intervals and created a chart from a spreadsheet. The chart speaks for itself....

For the first 30 minutes it goes like the clappers until the voltage stabilises at around 5v, then after 5 hours it starts to die, but I doubt I'd be still sitting on a stool with this 8" Dob observing after that period anyway.

I would assume this would change once it has the mirror mounted close to it, as the change in airflow can alter the load on the fan. But it was a good experiment to do.
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Old 29-07-2009, 04:26 PM
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DavidU (Dave)
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Make sure you get a ball bearing unit, much longer lasting and less vibration
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Old 29-07-2009, 05:27 PM
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stephenb (Stephen)
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Hi David, yes it is a ball bearing type, but it's a casual observing "grab and go" scope so I expect it to last a long time, and if it fails, they're cheap to replace.
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Old 30-07-2009, 07:58 AM
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Ive been toying with adding a fan to the back of my 10inch Newt.

I've never had anything to do with them, so, in the nicest possible way, - do you mount them to suck or blow?
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Old 30-07-2009, 08:06 AM
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LOL - They blow onto the back of the mirror, which, when I first heard about cooling fans in the Newtonian design many years, I thought was odd in a 'low to the ground' Dobsonian because I would imagine that this direction would blow up dust and dirt from the ground. This would not be a major issue to Newtonians on a higher mount. Obviously it's not a concern however.
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Old 30-07-2009, 02:58 PM
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Thanks Stephen,
the dust was why I was not sure.
There is a 3 speed 200mm antec gaming computer fan that looks like just the trick for his job.
http://www.megabuy.com.au/antec-big-...ck-p62408.html
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Old 06-08-2009, 03:11 PM
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I've just picked up a cigarette-lighter double adaptor, which I'll plug into a DSE 12vDC 3amp convertor, they threw in a small DC fan which I'll just have to add a cigarette socket to. I'm hoping that this basic setup will suffice for now
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Old 06-08-2009, 05:09 PM
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That'll do the trick, by the sounds of it, Ben.
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