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Cooling fans on a Skywatcher 200P


Wbarkingmad
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No i mean to suck air out of the bottom of the tube

Graham

Never tried reversing the flow but pushing the air up the tube works for me. Downside of pushing is early dewing if you run too long. Downside of sucking air out is pulling dust onto the mirror.

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OK, I've managed to find a supplier who will either laser or water jet cut 3mm aluminium 'donuts' to do this job, I've ordered mine for a 200P which, I've calculated on using a 92mm papst fan as they are a top brand, good priced, run quiet and are very happy to run slow on lower voltages. SO, for a 200P that's a 215mm circle with a center hole of 89mm. If anyone wants one, let me know. Cost is approx £10 + p&p

EDIT

Found another fan type I'm happy with and have ordered, it's a "Be Quiet Silentwings 92mm"

http://www.ebuyer.com/product/169230

Edited by TopHouse
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I added a moinster fan and a variable speed control on mine. To be honest it never creates vibration even when running at full tilt. Normally I let it run for maybe 15 minutes while the cope is being set-up and the mount aligned - seems to he; get the scope to thermal balance a bit quicker but I couldn t swear to it.

The final thing has a mesh over the fan to stop the fingers being mashed up. I didndt fit it initially as I consiodered the risk low - after having my finger tips buzz cut I changed my mind and fitted a grill :blob10:

Heres some pics.....

post-14805-133877480248_thumb.jpg

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A_B, your setup most closely mirrors what I had in mind. Where did you get your speed controller from? Being an electronics engineer I should make one but probably will end up braving Maplin. Is that a 125mm fan? Either way, its the next job after making a solar filter.

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That's a very neat controller Mel, and guess who hadn't thought of a grille. Is there not enough rioom to pop a hole in the ally and have the potentiometer built in so to speak?

My idea was to make it just like yours but put possibly a switched pot in situ at the side of the fan, and also put a panel mount socket on there for the power lead to just plug in. In fact that way, no need for the switch, just pull the plug. Until i get a power tank of some sort i was also going to see how long the fan will run off a rechargeable PP3 battery and then if it's acceptable, fit one of these on the plate too :-

5x PP3 9V Battery Holder Box with Flying Leads on eBay (end time 15-Sep-10 17:33:44 BST)

ideas/opinions?

Edited by TopHouse
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I use a variable voltage mains transformer from Maplins. it goes from 3v to 12v. at 12v it's great for initial cool down and I then switch down to 4.5v and this just keeps the fans turning slowly to maintain a light breeze up the tube and across the mirror.

it cost about £10 from memory.

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I need to get my NiMh PP3 charged and connect it to a fan and see how much run time it gives me. I'm not a fan of trailing wires to be honest.

You might find a PP3 struggles to keep a fan running for too long. Having spent some time with radio controlled cars in the past I'm considering a 7.2V NiMh RC car battery pack, say around 2 to 3 Ah. This should be good for quite a long period. I guess it depends on whether you don't mind charging regularly.

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My fan is 90mm its way oversized for some good reasons -

1/ It means at low revs it can still push a lot of air - low revs means less vibration

2/ I got all geared up and set aside time to do the job only to find Maplins had sold out of smaller fans :blob10:

The fan is 12v so it runs off any powertank available. I didnt want the hassle of a different voltage out in the field.

I did consider mounting the control pot on the rear of the scope but as the pot gets hot (well warm anyway) I decided to keep any heat sources away from the mirror. The pot is a very low profile one (I cant recall its exact size) but it was a spare left over from my polarscope dimmer project. The fan controller is just a pot plus an on/off rocker switch with an LED to let me know its on (cos the fan is very quiet).

In normal use its plugged into the scope and then run over one of the tube rings lock nuts so the controller is draped over the rear of the scope so its to hand without too much faffing about (thats another reason I didndt go for the pot on the rear of the scope - on its dinky handcontroller I can have its on/off and rate switch in my hands while I observe if need be). The other end of the lead has a standard 12v ciggy plug for connection to my powertanks.

The handcontroller is about the size of a matchbox and it was originally going to have a 2-3 position switch with a resistor between each pole so the fan would run on only 3 speeds but I couldnt find a switch that met the spec that was small enough with the resistors soldered onto it hence the variable pot approach.

I also considered a temp sensor on the mirror with an automatic control mode but gave up on it as it was over complex and a bit of a techy overkill approach. It bloated the size of the handbox and also the circuitry to run the controller would have been an extra drain on power and in the end it just seemd too complicated a system and one more thing to go wrong in the field.

The fan blow air into the tube as the main idea was to get air onto the mirror back surface to cool the mirror down. At full power the fan is easily powerful enough to disrupt surface air on the mirror face. I didndt want the fan sucking air down the tube towards the mirror as that seemd a good way to get generel drek pulled into the tube and onto the mirror face - blowing it in from the back means drek will blow past the mirror and out the tube.

Edited by Astro_Baby
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Larger Fans are better for cooling PC's and can be run quieter, I think these are also valid points for a Mirror Fan, Vairable Speed is an ideal mod at the same time, as most computers do that also via Bios and heat sensors.

Edited by Earl
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The pot I used actually has its 'hot' bits inside the outer pot (in the plastic knob bit) and your right the metal plate would act as a heatsink.

I just thought as the whole idea was to dump heat away from the mirror it was kind of logical to not add anything that could add heat. I know its only a tiny pot but I am fussly like that :blob10:

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Wayne - I agree its not that hard BUT its mkore stuff to go wrong. A temp sensor, more chips, more wires etc. As someone who has had a few nights viewing wrecked by technical failures of one kind or another and as someone with limited viewing time I wanted simple, robust, to reduce technical failures.

This was born out fter SSP this year where on the only clear night my HEQ5 mount packed up with a problem. Lucky I had the TAL on a simple manual EQ mount in reserve but it was still a real pain not being able to use either the 200 to test out some new eyepieces OR the SKyMax 180 on a night of great seeing.

Heres a few pics of the finished article as it looks these days.

post-14805-133877480766_thumb.jpg

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A_B, I'm with you, keep it simple. My final plan has come down to a 12V gel battery, a basic maplin speed controller and a nice quiet 90mm 12V fan (withg mesh guard). The battery will live in the base of my dob mount complete with charging socket and may also end up powering a red LED to illuminate the AZ setting circle.

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Mel, do you remember what size pot you used for that, and was it a linear? Oh and what range does it give? ie if you did it again would you use same sized pot? Does it go nice and slow at bottom end? oeerr missus!

Edited by TopHouse
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Um - I think - and unfortunately the pot carries no markings - its a 470k ohm pot and its linear. If I can find the spare one I had I could test it out. AT the slowest the fan actually stops altogether. If yo wind the pot up a bit the fan will kick off and the pot can then be trimmed back to the point where the fan is only getting enough juice to spin in a close to freewheel mode.

I think if I were doing it again I might use a small pot size of perhaps 100k ohm. I;d probably buy a few different ones in a mixed bag and see what works best to be honest. One thing is I would probably use a smallt (or at least slimmer fan) and have it inside the metal plate so the scope can be stood on its end if need be.

When I started on this I had all sorts of super elaborate designs in mind but kind of opted for the KISS approach in the end.

By the way my scope is an older 200P with the metal plate at the back from the factory and Phillips collimatiuon adjusters rather than the knurled type. While I was doing the fan job I also drilled out the back plate so I had access to the collimation adjusters.

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I have a feeling that there may be some confsuion about the speed control of fans using a pot. If you use a simple pot between your motor and battery it will have to be a very low resistance pot, maybe a few 10s of ohms max. The maplin controller I am intending using is a linear electronic regulator where a high resistance pot controls the regulator circuit to adjust either the current or output voltage to the fan. In the first case, a 100K pot will work for a tiny fraction of its rotation and will get very hot. In the second case the regulator circuit (usually a package designed to dissipate heat) will, well, dissipate the heat. Ohms law will give you the info you need: if the fan pulls (say) 100mA of current at 6V and your battery is 12V then the pot will need to be (12-6)/0.1) ohms = 60ohms and it will dissipate 6 * 0.1 = 0.6 watts of heat. This is a lot for a tiny fraction of the track of a 100K pot (60 ohms remember). However, if you got a high power 100R pot (you can get them) you would be OK. Either way, these approaches throw away the 'spare' energy which is wasteful.

The efficient way is the suggestion of using a thing called a PWM controller and as someone said, there are loads of circuits out there that will do the job. You don't actually need a microcontroller, you can do it very simply. The advantage is that you dont burn off the spare power as heat, you only use the power you need at the chosen speed so your battery will last longer. If anyone is interested, I'm now pondering a basic PWM circuit design for my own installation.

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I dont disagree - and I am not sure what pot size is in the controller. It must on hondsigight be smaller than 470k cos most of its track is in use. Unfortunately cos its not marked theres no way of knowing for sure cos there is a resistor inline to limit the LED from memory. Wish now I had taken some pics before I glued the back onto the hand controller.

I agree about power saving but you have to remember the fan isn't running continuosly - its only used at full power for 10-15 minutes or so at the start of set-up to help get the scope down to ambient and then the occasional slow speed buzz to break up surface air on the mirror when doing planetary so the power loss is pretty negligable considering for most of its use the fan is being run at full power (so no power savings) or just the occasional stab to break up air currents.

I did look at a more techy approach including one with an automatic sensor on the primary mirror but as I say it all started to get complicated more and more wires, more and more connectors and more and more chance of stuff going wrong when it 3am and the world is covered in wet dew (which seems to get in everything). It also increased the package sizes and with weight and size being at a premium it kind of militated towards basic.

I'd be interested to see what you come up witn though cos it might be good on a future project for something else.

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The timless (pun intended) 555 can be configured to act as a PWM controller too.

I recomended a microcontroller as they really are all the rage at the moment and in the case of the PICaxe is a nice easy to use device which can be programmed in a BASIC like language or graphically and the little PICaxe8 is just a couple quid.

I'll have a look in Maplins later and see if they got any 4 wire fans with the PWM input.

Not totally sure but I thought most PC style fans were brushless now anyway and therefore basically little 3 phase motors?

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Wayne, you are probably right about the PC fans and yes a brushless fan is a bit like a stepper motor. It would make great use of a PICaxe or similar device. Personally I hate 555s with a passion (28 years of professional electronics design and I have never used one) but again, they can be made to do this kind of thing. I'm with Astro_Baby and will be opting for a straightforward brushed motor fan and the simplest controller I can make work, I do complicated electronics for a living and don't want to do it at home as well:).

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