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Load from a single 2.5mm cable?


AbsolutelyN
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I'm looking to clean up the power to my scope and wondered if anyone can advise on the load I can put on a Nevada PSW-30 25-30A Switch-Mode Power Supply

The power supply will be positioned less than 10 meters from the scope and will be powering a CEM120, up to 2 ZWO cooled cameras and couple of dew heaters. On occasion I'd like it to also power a AZEQ6 at the same time. 

Will a single 2.5mm length of cable be sufficient to power all of these or would it be better to run two cables? I do have a spare Nevada PS-08 6-8A Regulated Linear Power Supply that could run a second line but ideally don't want to add that in unless needed to keep cables to a minimum. 

 Any advice much appreciated. 

Edited by AbsolutelyN
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Over 20 metres (there and back) the resistance will be 0.14 Ohm. Pulling say 7A will give a voltage drop of 1 volt, could be OK but when the current changes (ie controllers switch on or off) the voltage at the far end will 'jump'.

Mounts can be rather intolerant of volt changes!

4mm would be safer!

(I have the same supply.)

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I think 2.5mm twin and earth in a radial AC circuit (in a wall blah blah) will carry 27A. I'd say you'd be fine given you will most likely not run everything flat out at the same time and probably not for very long even if you did.

I'm not an electrician though.....

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I would run 2 cables from your 30amp supply, it is not the supply that is the issue but the voltage drop along the cable.

Use one cable ONLY for the mount.

The rest of the gear is fairly intolerant to minor voltage changes, the ZWO cameras regulate internally and the dew heaters are not at all troubled by any dip. They can all share the other cable.

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From the wording, I'm reading it that you intend to run the cable from the 12V output for 10m? You have two factors to consider, the safety rating of the cable and the voltage drop caused by it. In free air, 2.5 sq mm mains cable is rated at 27A, so you're probably just about OK from a cable safety perspective. In terms of voltage drop, you'd lose approx 0.15V per amp (you have 10m there on the "live" and 10m back on the "ground"). Add up the peak powers of your devices and see how that looks. Not speaking from personal experience but it may not be acceptable in terms of spikes and glitches it could cause.

It's never easy to transfer significant current at low voltage over distance. Can you not supply mains over the distance and limit low voltage to short leads? As long as you do that safely it's the better option. If not, I'd be tempted to use 4 sq mm at least. The other option would be some kind of smoothing/filtering at the remote end but that starts to get complex and possibly expensive.

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Thanks for the input here. Yes the cable would be running between the 12v power supply and the load. So it would only be carrying 12v. Basically I don't have an observatory, only a permanent spot to setup at so I don't know how else to keep power there permanetly as the supply has to be indoors. I've positioned the power supply as close as I can possibly get it, just measuring now I'd says it's between 6 to 7 meters. 

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I know you said the supply has to be indoors and in theory it does but given your kit will not be operating in the rain etc why not do what I do and put supply, laptop etc in a plastic storage case (with cable exit holes etc) at the side of the mount? That way one long mains extension run to the box will suffice.

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My 2p worth.

Is there anything preventing you from running an mains extension cable to the location and having the powersupply in a portable box of some description.  I've seen insulated plastic coolers being used before, the insulation preventing a lot of dew forming on the contents.

If this is not an option, use a multimeter and measure the actual current drawn directly from the supply for each item of equipment so you have a true understanding of the load.  I would expect the mount to draw around 1.8 - 2.3 amps when both axis are slewing, but have no idea about the cooling of the camera and dew heaters.  Then you have some real values to use in the calculations.  You may find 2.5m2 multi strand cable is better than solid core twin and earth, an electrician could advise you better on that.  

Power supplies are split into two types.  Constant current or constant voltage.  In order to provide a constant current the voltage is adjusted, with the opposite when a constant voltage is used.  Personally if the power supply needs to remain inside, then look for a constant voltage supply which can supply 7-20 amps.  There will be voltage drop of the distance which would be between 0.75 and 1v over that distance and depending on the resistance of the cable used and the true current draw.

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OK look at it from a materials and calculation viewpoint.
Copper wire restance is:

1.0mm sq, 17.2 ohms/KM.
1.5mm sq, 11.5 ohms/KM.
2.5mm sq, 6.9 ohms/KM.

It doesn't matter whether you have solid or stranded wire. It is the copper cross section that matters.
Though solid wire, like house ring main wire, is unwieldy.
For a 2.5mm sq cable, choose 50/0.25 stranded, or similar. That is 50 strands of 0.25mm diameter, which adds up to 2.5mm sq.

Assuming the power supply is 10M from the scope and you use a 2.5mm sq cable, resistance is (as stated in an earlier post) about 0.14 ohms.
You need to consider the out and back length, totalling 20M.

However, you are going to have a connector at the PSU. Multiple connectors at the scope where you share power between mount, dew heaters, camera, etc.
I reckon you must assume at least another 0.15 ohms for all of these connections, giving you 0.3 ohms total resistance.
If you have any dodgy cigar plugs, plugs with fuses, etc, this figure increases.

Taking 0.3 ohms total, what is the consequence?
It means you drop 0.3V for every 1 amp load at the scope.
So if you have a slewing mount, and a cooled camera maybe 3 amps? perhaps less.
But 3amps gives a 0.9V supply drop.
Mounts, cameras, etc can cope with this without problem.

Next come dew heaters. That have not been specified.
Some dew heaters have a resistance such that they provide enough heat to do the job, without a controller.
For example an eyepiece heater might take 0.2 amps, but a 12" mirror heater 2 amps.
How many heaters do you have and what size?
Refractor objective, guide scope objective, finder, etc.........

For this discussion, assume 3 amps of dew heaters, added to the 3 amps of slewing scope & camera gives 6 amps total.
Now you have a 1.8V drop between PSU and scope.
So a 13.8V PSU leaves you 12V at the scope. OK you can still work.

However, some dew heaters take more current and rely on a controller to give on/off switching to reduce heat to a desirable level.
Suppose you are using these heaters and they take 6 amps, but switch on/off at 50% duty cycle, giving 3 amps average.
This is a very different situation for voltage drop.
At worst case, all the heaters switch on/off together. In other words load is 0V to 6A switching once every few seconds.

In this situation the scope voltage is changing, in a step manner by 1.8V. This can upset (cost senstive designed) mount electronics and cameras.
Further, in this situation when the mount is slewing and the heaters are all on, the voltage only 11.1V if the PSU is delivering 13.8V.
Whether these step changes, and 11.1V cause a problem depends whether Skycelede or Primatron or whoever designed the mount electronics well, or cheaply.

If it was me doing the installation with the power budget described above, I would run one cable for mount/camera power, and another cable for dew heaters.
Actually no. I would as suggested in earlier posts put the PSU on a box off the floor keeping my 12V cablig short.

However, using a low cost multimeter it is easy enough to measure actual current consumption an establish voltage drop in the individual installation.

HTH,

David.

     
     
   
     
     
   

 

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As others have said, it's volt drop over distance that is your concern. 2.5mm for extra low volt IMO is not a starter. Put the power supply in a box and look to it being local to your stuff. I mounted the same supply in a box powered by a long mains lead, installed a set of connectors in the box, got rid of the cigarette lighter and ring lugs off the supplied cables and then just connected the supplied cables to the connectors. Job done.

box1.jpg

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Wow, thanks so much for all the info and ideas. Really, really appreciated. It sounds like the way to go is get the power much closer to the scope so I'll have a good look at running mains closer and into some kind of waterproof power box which can house the power supply. It should make for a much neater solution as I'd only need one power cable and an ethernet cable for the mini pc. 

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7 hours ago, M40 said:

As others have said, it's volt drop over distance that is your concern. 2.5mm for extra low volt IMO is not a starter. Put the power supply in a box and look to it being local to your stuff. I mounted the same supply in a box powered by a long mains lead, installed a set of connectors in the box, got rid of the cigarette lighter and ring lugs off the supplied cables and then just connected the supplied cables to the connectors. Job done.

box1.jpg

That looks brilliant, may I ask what the box is (and where to get one) please? Many thanks

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On 27/11/2021 at 12:56, M40 said:

As others have said, it's volt drop over distance that is your concern. 2.5mm for extra low volt IMO is not a starter. Put the power supply in a box and look to it being local to your stuff. I mounted the same supply in a box powered by a long mains lead, installed a set of connectors in the box, got rid of the cigarette lighter and ring lugs off the supplied cables and then just connected the supplied cables to the connectors. Job done.

box1.jpg

I would be more concerned if that's a metal box and you have a TN-C-S supply.
A broken neutral will would make that box live and more than likely give a fatal shock outside.
There doesn't even need to be a fault in the box to make that box live if the neutral on the supply side breaks.

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On 27/11/2021 at 20:24, AbsolutelyN said:

That looks brilliant, may I ask what the box is (and where to get one) please? Many thanks

Thanks, we have recently moved and everything is in boxes, garages and lockups but I will get a few pictures and a shopping list together soon as 👍

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12 hours ago, wxsatuser said:

I would be more concerned if that's a metal box and you have a TN-C-S supply.
A broken neutral will would make that box live and more than likely give a fatal shock outside.
There doesn't even need to be a fault in the box to make that box live if the neutral on the supply side breaks.

Good advice from wxsatuser here. Make sure that the 230V comes from a nice 30ma rcd or rcbo device. The boxes I have used are ip66 grp enclosures available from most electrical wholesalers or RS components. Use plastic 25 or 32mm compression glands drilled in the bottom only (yes I know I have one in the side 🤦‍♂️, I will plug it eventually). Oversize the glands, you are not looking for them to be weathertight but you will need sufficient space to get the cable plugs/connectors though. The box you can see houses all the bits and bobs with the second box housing the connectors and cables. Mistake I made was going too small with the second box, the cables just fit so I suggest getting the same size two boxes or at least one thats big enough to get the cables in easily. The boxes stand on unistrut brackets but any frame to space the boxes off the floor is the way forward as you need space between boxes, cable glands and a few spare inches for the cables. Enjoy :D

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8 minutes ago, M40 said:

Good advice from wxsatuser here. Make sure that the 230V comes from a nice 30ma rcd or rcbo device. The boxes I have used are ip66 grp enclosures available from most electrical wholesalers or RS components. Use plastic 25 or 32mm compression glands drilled in the bottom only (yes I know I have one in the side 🤦‍♂️, I will plug it eventually). Oversize the glands, you are not looking for them to be weathertight but you will need sufficient space to get the cable plugs/connectors though. The box you can see houses all the bits and bobs with the second box housing the connectors and cables. Mistake I made was going too small with the second box, the cables just fit so I suggest getting the same size two boxes or at least one thats big enough to get the cables in easily. The boxes stand on unistrut brackets but any frame to space the boxes off the floor is the way forward as you need space between boxes, cable glands and a few spare inches for the cables. Enjoy :D

Thanks for the advice, I was looking at RC over the weekend and they have a huge selection of wall boxes but would obviously select a plastic/abs one. It's going to take some time to put together but this thread has been very valuable in understanding how best to approach this. The power supply, currently located indoors, is already protected with an RCD located in my office so hopefully I got that part right already.    

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Just one other little tip.

If the supply side neutral breaks on a TN-C-S  supply the RCD will not trip as it will still carry equal current in each leg.
Current flows through live round the appliance via it's neutral and back to the appliance earth terminal making the metal appliance
live, if you touch it you complete the circuit to earth.
This is why powers that be don't recommend any metal appliances outside on a TN-C-S supply.

If the power goes off do not touch anything metal and check to see if the supply side neutral is still there.

 

 

Edited by wxsatuser
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 Ref @wxsatuserI would be more concerned if that's a metal box and you have a TN-C-S supply.

That looks like a polycarbonate or ABS box.

The result is that the 12V power supply inbuilt isolation will look after you in the event of supply issues.

Provided it is a 'proper' power supply with some approvals labels, it is fine.
Look for a known name sold by (for example) astro retailers.

Buying a ££ less power supply bearing a weird name from a dodgy online only seller could have serious consequences.

I have seen quite a bit of stuff from China with dangerous design/construction flaws, offered by 'marketplace sellers' using that south american sounding online retailer.
My reports have resulted in items being withdrawn from sale. Probably only to resurface elsewhere a week later.

HTH, David.

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I ran power to my pier from a leisure battery that was charged with solar panels in my shed.  The run was 7 or 8m and I went with 4mm armoured, buried under the lawn, I was getting a drop of 0.1 to 0.2v and never had a problem powering similar kit to yours.

Armoured cable into a small plastic box and that feeding an Anderson Powerpole distribution block.

IMG_20170820_125712.thumb.jpg.cd6f49e4e3c3197fb4b582c7b3b4457e.jpg.0aa3d080bfde21cbb92f933040031b6c.jpg

Over engineer it and whack a big cable in there 😁 

 

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8 minutes ago, Starflyer said:

I ran power to my pier from a leisure battery that was charged with solar panels in my shed.  The run was 7 or 8m and I went with 4mm armoured, buried under the lawn, I was getting a drop of 0.1 to 0.2v and never had a problem powering similar kit to yours.

Armoured cable into a small plastic box and that feeding an Anderson Powerpole distribution block.

IMG_20170820_125712.thumb.jpg.cd6f49e4e3c3197fb4b582c7b3b4457e.jpg.0aa3d080bfde21cbb92f933040031b6c.jpg

Over engineer it and whack a big cable in there 😁 

 

That's absolutely brilliant! Thanks for the inspiration.

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