New Observatory preps and build

[Gina]

Oooh I know

[fwm891]

Sunday: Been working on a minor problem with the south wall top section and I'm having to adjust its 'fit'. Seems I made it too good a fit to start with making it difficult to slot back into place when closing. That or the sliding side walls are springing in slightly when the front panel is lowered...

Either way I'm going to try putting some guides on the south wall top section to position the side walls when the flap is raised into its closed position.

The R0R runs well enough along its tracks so the problem doesn't appear to be there. But I'm reluctant to take material off the front drop-down panel for fear of making it too sloppy.

All closed up now waiting for the predicted rain, not sure just when weather will clear this week so I can start on the guides - cloud already building in the west....

TBC

[fwm891]

Had a few problems with the weight of the front flap and hauling it up and down. I tried the rope trick mentioned earlier but it just proved too awkward to operate smoothly. I've now cut the flap in two horizontally and made in it a bi-fold flap. This is hinged at the bottom to the top of the south wall so it folds outwards. Then hinged again across the middle so the top half folds back over the lower half. I have put a central bracket outside which supports the flap in a horizontal position. It can now be opened and closed quite easily against the closed sliding roof's front edges.I have also been building a power distribution box to mount below the telescope on the pier. My aim is to take 240VAC into the obsy but then only have 12 VDC at the scope. For this I'm using a Maplins 7 amp regulated power supply and splitting this inside the box to power the LED dimmers/dew heaters, mount, 383L+ and powered usb hub which will all be in continuous use while thescope is powered. There will be an additional suppy to my 450D when that is also in use.The 240 VAC will also be used to power a monitor and PC when setting up.

1 in photo shows the original dropdown mode, 2 - shows the bi-fold flap in its open position

[Gina]

That's interesting Francis As for the PSU - I have the same model of PSU but sitting on the floor of my warm room with low voltage cable through the ducting to the pier top but I'm finding I'm getting voltage drop since adding the extra load of dew heaters. I'm now considering putting the PSU on the pier just below the top. Either that or a PC PSU to supply the dew heaters separately - I'll have to work out my maximum load current. There's something to be said for separating the loads.

[Tinker1947]

That's interesting Francis As for the PSU - I have the same model of PSU but sitting on the floor of my warm room with low voltage cable through the ducting to the pier top but I'm finding I'm getting voltage drop since adding the extra load of dew heaters. I'm now considering putting the PSU on the pier just below the top. Either that or a PC PSU to supply the dew heaters separately - I'll have to work out my maximum load current. There's something to be said for separating the loads.

I have the Maplin 7AMP PSU and driving the Mount, CCD, its been fine, added a Dew heater for the ST-80 and i think out of the corner of my eye i noticed the Mount LED flicker, i will check it next time its set-up, as for the Dew Heater awesome, the ST-80 stay crystal clear while the Tube got covered in frost.....On the Maplin Output the spec shows 5 amp continuous and 7 amps 50% Cycle

[opticalpath]

Gina, are you getting the voltage drop because of the total load on the PSU, or because of resistance in the low voltage cable run? I'm guessing it's the cable length since you're considering moving the PSU closer. How about just using some heavier cable, or doubling up, to reduce the resistance?

I've been wondering how I might simplify my power supply requirements. At the moment I run a mains extension cable to the shed (no separate warm room) and then have a motley collection of plug-in adapters to run the various components: mount, camera, dew strap, focuser, EL panel, USB hub ... the list goes on! I still want mains power for laptop and dehumidifier, but I wondered if I couldn't simplify all the other stuff that's nearly all 12V. I've always been unsure about interference if, for example, I ran the camera from the same 12V source as other equipment. Or is that already as much of an issue as it's likely to become since I already run the separate adapters from the same mains line? I just don't know.

Adrian

[r3i]

Gina, are you getting the voltage drop because of the total load on the PSU, or because of resistance in the low voltage cable run? I'm guessing it's the cable length since you're considering moving the PSU closer. How about just using some heavier cable, or doubling up, to reduce the resistance?

I've been wondering how I might simplify my power supply requirements. At the moment I run a mains extension cable to the shed (no separate warm room) and then have a motley collection of plug-in adapters to run the various components: mount, camera, dew strap, focuser, EL panel, USB hub ... the list goes on! I still want mains power for laptop and dehumidifier, but I wondered if I couldn't simplify all the other stuff that's nearly all 12V. I've always been unsure about interference if, for example, I ran the camera from the same 12V source as other equipment. Or is that already as much of an issue as it's likely to become since I already run the separate adapters from the same mains line? I just don't know.

Adrian

Gina: your voltage drop issue has got alarm bells ringing for me. I am planning to have my 12V PSU in the warm room and running a cable to a 12V distribution point on the pier and was hoping that the 2m or so length wouldn't be affected by voltage drop. For 12V distribution I currently have a home built box that splits the power out to 5 cigarette lighter sockets - these are not the most reliable of connections so for my permanent setup I'm thinking about using Anderson Powerpole connectors and investing in a distribution box like one of these: http://www.powerwerx...r-distribution/

[yesyes]

You will always get voltage drop on any cable, but with very short cables and/or low current it might not make a difference as the drop is very low.

A certain length of cable of a certain type has a certain resistance R. The voltage drop U at a certain current I can be calculated with Ohm's law

R = U / I -> U = R * I

That shows that a higher current (more load) will result in a higher voltage drop. A higher resistance (longer and/or thinner cable) will also result in a higher voltage drop.

So to reduce voltage drop you can either make the cable shorter and/or thicker or reduce load.

Dew heaters are quite a high load and should probably be run off a different power supply than your sensitive equipment if the cable run is quite long. Assuming a 1 Ohm resistance for your cable run (a 2 metre run actually has 4 metres worth of resistance as the current needs to go there and back) and a current of 3Amp for the dew heater, the voltage drop would be:

U = R * I

U = 1Ohm * 3Amp

U = 3Volt

That would leave you with only 9 Volts. That's probably fine for the dew heater itself but not enough to power your mount and other things...

[yesyes]

(hit send a bit too early)

So at 12V and 3 Amp the example dew heater would have a power of 36 Watt (P = U * I). If it was mains voltage (240V) it would only draw 0.150Amp (assuming a 100% efficient 12V power supply for the previous example, which doesn't exists). Over the same 1 Ohm cable run you would only lose:

U = 1Ohm * 0.150Amp

U = 0.150Volt.

That is why it is much better to run mains as far as you can and only run low voltage cables as short as possible, unless the current draw on the low voltage side is so low that you can ignore the voltage drop.

[r3i]

Thanks Chris for the detailed explanation

I might be hijacking the thread here (apologies): My PSU (one of these: http://www2.ripmax.net/Item.aspx?ItemID=O-IP2002) has a dual output - so would I be right in thinking I should run one cable from the PSU for dew heaters then another cable for other stuff such as the mount and USB hub? The other piece of kit I have is a QHY8 which sucks up to 4A - which of the 2 cables should this be run off?

[opticalpath]

Agreed. You need to choose a cable with a low enough resistance per metre to carry the maximum forseeable load over the distance required, that produces a votage drop small enough to be acceptable.

There is a useful calculator here for low voltage lighting cables that shows the voltage drop for a given length and load, for various actual cable gauges, so you can select the appropriate one:

http://www.tlc-direct.co.uk/Technical/Lighting/VoltageDrop.html

So for example, a 3m length of 2.5 sq.mm conductor twin-and-earth cable, in a conduit, with a load of 84 Watts (12V x 7A), would result in a voltage drop of 0.4V.

Adrian

[Islander]

You could also interpose a distribution box with multiple output sockets and run individual power leads to devices. That way you don't need to oversize the cables and voltage drop would per cable rather than on a single connection. I've been thinking about this problem for my own obsy when I get around to building it as I'm intending to run it off grid with solar panels and batteries instead of running an armoured cable in.

[fwm891]

Interesting - thanks.

I'll be running my 12 VDC cable some 1.5 M to the distribution box then short individual cables from there to mount, 383L etc...

I had been thinking about getting a second 12 VDC regulated supply to separately power my 383L to avoid any noise from heaters switching etc...

[opticalpath]

I wonder if there's a way of ensuring that items connected to a shared 12V DC supply (or maybe even separate PSUs from a shared mains line?) don't interfere with one another. The camera in particular I'd want to keep away from any electrical noise sources.

Any electronics expert able to advise?

Adrian

[fwm891]

Hi,

I've been working on a new power/data box for the obsy and below is a basic schematic to show in's and out's.

I purchased a second Maplin 7 amp (5 amp cont) regulated supply and intend to run only the mount and 383L+ from this via respective fuses. The original 7 amp supply is powering the dew heater (there are 3 dimmable connections but only intending to use 1 at present), the USB Hub, Canon 450D and stepper/focuser board via voltage regulators. There are 2 small fans, one blowing air into the box the other sucking air out to keep the stepper board, voltage regulators (on heatsinks) and arduino cool.

The Atik 383L+ USB connection will link direct to the PC, the USB hub via a second cable.

(Hope this makes sense )

[opticalpath]

A lot neater than my arrangement, Francis .

Is the 383L happy to run on the slightly higher than 12V nominal voltage? I assume it has its own on-board regulator anyway that would cope OK.

Adrian

[opticalpath]

A lot neater than my arrangement, Francis .

Is the 383L happy to run on the slightly higher than 12V nominal voltage? I assume it has its own on-board regulator anyway that would cope OK.

Adrian

Francis, out of curiosity I looked up what the manual says about supply voltage for my (QSI) camera:

" ......... are designed to operate on stable, regulated 12V DC power and consumes less than 2 amps at full power with 100% cooling, fans running at full speed and the filter wheel moving. DC power input above or below 12V will decrease the maximum cooling capability of the camera by increasing power dissipation or lowering cooling efficiency. If the input voltage is below 11V or above 14V the camera will report an error........"

Good point about how even OVER voltage would reduce cooling capacity. Never thought of that!

Adrian

[fwm891]

A lot neater than my arrangement, Francis .

Is the 383L happy to run on the slightly higher than 12V nominal voltage? I assume it has its own on-board regulator anyway that would cope OK.

Adrian

Hi Adrian, The Atik documentation is lamentfull with regard to details: http://www.atik-came...anual.pdf just says 12 VDC 2.5A supply - no tolerances.

I was running the Atik from the same supply in the old obsy before we moved for some time so therefore thought no more of it

With the exception of the mount I've had everything connected up for testing in the house today. Only problem I've found is with the LED dimmer and the long heat strap for the MN190 - its getting very hot even on its lowest setting. Probably need to add a big ish resister in the chain, that and check my calc's again!

[fwm891]

OK Pilot error found out where I went wrong. I made my heat strap by laying two strands of nichrome wire down onto gaffa tape. Each strip was turned back on itself giving me four almost parellel lines. I intended joining the two centre lines to make a continuous length but somehow got things wrong... When I checked the resistance at 4.2 ohms instead of the 17 ohms I expected I new I had a problem. So de-solder all the connections and try again.

Now works fine.

[opticalpath]

Mmm, toasty!!

Adrian

[Gina]

I need to sort out my electrics. My PSU keeps cutting out even though the ammeter is showing less than 5A so need to look into that.

[Tinker1947]

I need to sort out my electrics. My PSU keeps cutting out even though the ammeter is showing less than 5A so need to look into that.

I brought a 2nd Maplin 7amp, i suppose in the long run should have gone for the 30amp and be done with it...

[Gina]

I'm going to run my dew heaters and other sundries off an old PC PSU - that's rated at 15A. Guess it would make sense to run the 5v things off the 5v it provides too.

[fwm891]

Well got back into the obsy today and fitted the floor bearers. 18mm ply for the floor arrives Monday (bound to be wet!) and I plan on fitting it in sections screwed down to the floor bearers. TBC...

[Gina]

Good