13.8v PSU Cables

[matt_baker]

I'm basically a dummy when it comes to electronics and because I'm having communication issues due to a power brick, I'm wanting to get a 13.8v bench supply but I have no idea how to get from the black and red terminals into a DC cable for my power hub. Can anyone help with this?

 I was particularly looking at a mercury 13.8v 15A supply

Also the power hub I have is 12v 10A rated. Does it only pull however much it needs and 15A won't be too much for it?

Matt

[michael8554]

We need to know:

1) how long is the cable run from the bench supply to the power hub ?

2) How much current does all your equipment pull ? That's the important figure, hopefully not more than the hub and bench supply will take.

3) Do you know what kind of plug the power hub requires ?

Michael

[matt_baker]

1 minute ago, michael8554 said:

We need to know:

1) how long is the cable run from the bench supply to the power hub ?

2) How much current does all your equipment pull ? That's the important figure, hopefully not more than the hub and bench supply will take.

3) Do you know what kind of plug the power hub requires ?

Michael

The cable needs to be about 16-17m. I have the HEQ5 running at 12v 3A max, 1600mm at the same I think, raspberry pi 4 and a dew heater. It has the black and red terminals. I can power the dew heater separately if necessary

Matt

[wulfrun]

39 minutes ago, matt_baker said:

I'm basically a dummy when it comes to electronics and because I'm having communication issues due to a power brick, I'm wanting to get a 13.8v bench supply but I have no idea how to get from the black and red terminals into a DC cable for my power hub. Can anyone help with this?

 I was particularly looking at a mercury 13.8v 15A supply

Also the power hub I have is 12v 10A rated. Does it only pull however much it needs and 15A won't be too much for it?

Matt

The rating of a power supply is the maximum it can supply, not how much it "forces" into the load. If something requires (say) 5A and you have a 10A supply, that's fine. If the load demands (say) 15A the supply can't supply it. If it's a switch-mode supply, it'll likely simply shut down and refuse to work. If it's a linear supply it may supply the overload at reduced voltage but overheat in longer term. Basically, as long as the supply exceeds the demand, you're fine.

[wulfrun]

4 minutes ago, matt_baker said:

The cable needs to be about 16-17m. I have the HEQ5 running at 12v 3A max, 1600mm at the same I think, raspberry pi 4 and a dew heater. It has the black and red terminals. I can power the dew heater separately if necessary

Matt

If you want to supply low voltage over that distance, I'd re-think how you do it. There will be a large voltage-drop along 15m of wire unless it's unrealistically thick wire.

[michael8554]

Let's say 10 amps including the dew heaters, over 20 metres.

I found an online calculator that gave a figure of 12sq mm for a 0.5V voltage drop at 12V.

For example, probably the heaviest cable you find in a house for 240V is 6sq mm.

25M of 10sq mm black or red from Radio Spares costs about £60 each.

As wulfrum says, investigate a safe mains supply to your mount position, google "Temporary Garden Electricity"

Michael

[teoria_del_big_bang]

@wulfrun has given some good advice above.

Just as an example below are some links to calculators for voltage drops in DC circuits. Both give slightly different results but pretty close to each other and probably due to using different coefficients for the resistance of copper.

It is also very difficult to calculate exactly the current being drawn because the mount for example depends what it is doing. You state 3A that is probably the supply recommended. From what I remember a HEQ5 on average takes around 2A when slewing at full speed (even this can differ from mount to mount depending on how stiff the bearings are or how tight the gears mesh or if load is out of balance) but for 95% of the time it will just be tracking and draw 0.5 A or so. The camera cooler again needs a 3A supply but I think again draws around 2A when cooling down from ambient but when cooled may again only draw a small current and will continually be changing. The dew band should be fairly constant and I guess for a scope your size around 0.5 A.

So we may be drawing around 5A in theory but normally when all up and imaging it will be more like 2A. But we really have to take worst case and say it may be 5A.

Using the calculators for 13.8 V over 17 M using 1mm copper wire you would have a voltage drop of around 3V and only deliver 10.8V to the mount which is certainly not enough for the mount itself.

Voltage Drop Calculator 1

Voltage Drop Calculator 2

To have anything like a good 12V at the mount you would need to use 2.5 mm cable which would give a volts drop of around 1.2V when drawing the full 5A. Of course most of the time it will not draw this only around 1.5A and so the voltage using 2.5 mm cable will vary from about 12.6 to maybe 13.3 V.

When drawing 5A the cable may get a bit warm as it is displacing nearly 15W but suspect it maybe okay.

So maybe you will be okay with 2.5 mm2 cable.

 

Steve

[david_taurus83]

You'll only use about 4 amps max. That was my experience with AZEQ6, ASI1600, focuser, dew heaters, Stellarmate etc. A 1.5mm cable at that distance will drop 1 maybe 2 volts so you would need the ability to increase the voltage. I use this:

https://www.nevadaradio.co.uk/product/nevada-ps-30m/

I use an old yellow 110v building site cable, 1.5mm cores. I clipped the plugs off and put banana plugs on the PSU side and fitted a cigar socket adapter to the scope side, with an inline fuse for good measure. I'm hopeless with a soldering iron so most connections are crimped where needed. I used to use all 4 sockets and the USB supplied 5v to the Stellarmate but I only use the one socket now and have a small power distribution block for all 12v connections courtesy of Deep Sky Dad.

 

[teoria_del_big_bang]

7 minutes ago, michael8554 said:

Let's say 10 amps including the dew heaters, over 20 metres.

I found an online calculator that gave a figure of 12sq mm for a 0.5V voltage drop at 12V.

For example, probably the heaviest cable you find in a house for 240V is 6sq mm.

25M of 10sq mm black or red from Radio Spares costs about £60 each.

As wulfrum says, investigate a safe mains supply to your mount position, google "Temporary Garden Electricity"

Michael

At 10A that sounds about right but he will need less than 5A and to be fair if the supply is 13.8V he can stand a 1V drop and most of time he will niot be drawing 5A so I think 2.5 mm2 will suffice.

Steve

[matt_baker]

Thank you all for your input.

How unrecommended is it to put the psu in a box outside with the equipment connected to a mains extension lead and have a 2-3m wire to the DSD DCHub?

Matt

[wulfrun]

39 minutes ago, matt_baker said:

Thank you all for your input.

How unrecommended is it to put the psu in a box outside with the equipment connected to a mains extension lead and have a 2-3m wire to the DSD DCHub?

Matt

A mains extension lead to a covered connection nearer the scope is not unsafe - provided you use common sense. Clearly, any mains connections must be inaccessible to damp/rain etc. It would also be wise to switch off the supply indoors before connecting/disconnecting mains plugs etc outside. Does your mains supply have an RCD built in? Find out and be 100% sure. If it does, you're fairly well protected. If it doesn't, you can buy plug-in versions - just make sure it's the house-end of the extension lead that is plugged into the RCD adaptor, i.e. plug the adaptor in (in the house) and your extension lead into that. Don't have an RCD adaptor at the far end, outside.

EDIT: whatever type of RCD, plug-in or in-built to your consumer unit, test it using the test button - always!

Edited December 21, 2020 by wulfrun

[Tomatobro]

I made a little "dog kennel" that I used to slide the power supply in to keep it off the ground. Its own heat kept the unit dry. The kennel was open at both ends for ventilation. It was made from plywood.

I used it over a couple of winters with no problems but some may frown on the practice.

[teoria_del_big_bang]

35 minutes ago, matt_baker said:

Thank you all for your input.

How unrecommended is it to put the psu in a box outside with the equipment connected to a mains extension lead and have a 2-3m wire to the DSD DCHub?

Matt

You can put the power supply AND extension in a weather resistant box like this:

Outdoor IP55 Power Enclosure

Then connect the extension into a RCD adaptor:

RCD

Note that theIP55 box is not totally waterproof but more like splashproof and should not be left out in rain but then I assume you would not leave your scope out if rain was immanent either.

The RCD is essential for safety.

The RCD should protect you from shocks and will trip if a short occurs but I would also recommend if any damp about to plug everything in then switch on at the RCD and again switch off before unplugging again at end of session as stugg tends to get a bit damp if any moisture in the air. Maybe over cautions but better safe than sorry.

Much better is to get an electrician to fit a proper outdoor socket close to where you intend to image and that will go to an RCD in the consumer box. The power supply still really wants covering with an IP 55 type enclosure and make sure it is big enough to leave pleaty of air around the supply so it does not overheat.

Steve

 

Edited December 21, 2020 by teoria_del_big_bang

[matt_baker]

3 minutes ago, teoria_del_big_bang said:

You can put the power supply AND extension in a weather resistant box like this:

Outdoor IP55 Power Enclosure

Then connect the extension into a RCD adaptor:

RCD

Note that theIP55 box is not totally waterproof but more like splashproof and should not be left out in rain but then I assume you would not leave your scope out if rain was immanent either.

The RCD is essential for safety.

Much better is to get an electrician to fit a proper outdoor socket close to where you intend to image and that will go to an RCD in the consumer box. The power supply still really wants covering with an IP 55 type enclosure and make sure it is big enough to leave pleanty of air around the supply so it does not overheat.

Steve

 

My issue is that I have to image further up in my garden and then I'm not close to any sockets. My mains extension runs about 16m to the equipment as to why I need such a long cable but if I can have the PSU in a box outside, with the mains extension running to it so I can have a shorter cable and not have significant voltage drop, that would be great. Of course, the scope would never be outside when it's about to rain and it takes me around 15 mins to pack everything up. My main concern I guess would be moisture and how that could affect the PSU, as well as heating issues being inside a box

 

Matt

[teoria_del_big_bang]

51 minutes ago, matt_baker said:

My issue is that I have to image further up in my garden and then I'm not close to any sockets. My mains extension runs about 16m to the equipment as to why I need such a long cable but if I can have the PSU in a box outside, with the mains extension running to it so I can have a shorter cable and not have significant voltage drop, that would be great. Of course, the scope would never be outside when it's about to rain and it takes me around 15 mins to pack everything up. My main concern I guess would be moisture and how that could affect the PSU, as well as heating issues being inside a box

 

Matt

Then as I said put the extension inside a splashproof box that is plenty big enough to leave enough air round then to stop over heating, plug into a RCD. My suggestion of getting an outdoor socket near to where you image properly wired by an electrician to the consumer unit is just a better idea.

It is difficult for me to recommend this way of doing things as whilst I consider it safe if via an RCD many will shoot me down and say I should not be recommending this as electricity is dangerous - which it is if care is not taken.

I use a 240V ps in an home made box and will take a pic to show you. It is home made but I have never had any moisture get into it and is big enough to house a big 12V power supply and extension for any other smaller supplies (such as the RPI) to plug into. Only thing is I do have a proper outside socket next to the mount but this just means I do not have the extension issue and the RCD is in the consumer unit.

 

Oh and when using an extension always unwind all the lead, if a 25M extension and it needs to me 15 m do not leave 10M wound on extension. Even with small currents you would be amazed at how hot it can get.

Steve

Edited December 21, 2020 by teoria_del_big_bang

[matt_baker]

2 minutes ago, teoria_del_big_bang said:

Then as I said put the extension inside a splashproof box that is plenty big enough to leave enough air round then to stop over heating, plug into a RCD. My suggestion of getting an outdoor socket near to where you image properly wired by an electrician to the consumer unit is just a better idea.

It is difficult for me to recommend this way of doing things as whilst I consider it safe if via an RCD many will shoot me down and say I should not be recommending this as electricity is dangerous - which it is if care is not taken.

I use a 240V ps in an home made box and will take a pic to show you. It is home made but I have never had any moister get into it and is big enough to house a big 12V power supply and extension for any other smaller supplies (such as the RPI) to plug into. Only thing is I do have a proper outside socket next to the mount but this just means I do not have the extension issue and the RCD is in the consumer unit.

 

Oh and when using an extension always unwind all the lead, if a 25M extension and it needs to me 15 m do not leave 10M wound on extension. Even with small currents you would be amazed at how hot it can get.

Steve

That'd be great if I could take a look. And my extension is almost fully unwound, I think about a meter just maybe 2 is left over

[michael8554]

The calculator I found seems to have been pessimistic in advising 12sq mm.

But calculating for least load is not best either.

What happens when the mount slews and draws lots more current and the voltage drops? 

Mount runaway.

Michael

[teoria_del_big_bang]

4 hours ago, michael8554 said:

The calculator I found seems to have been pessimistic in advising 12sq mm.

But calculating for least load is not best either.

What happens when the mount slews and draws lots more current and the voltage drops? 

Mount runaway.

Michael

Michael,

No I agree we have to consider maximum load but if you add up all maximum draw of Matts setup it is really no more than 5A at 12V  (the least load is more like 1.5 to 2A) and I think 2.5mm2 would always keep the voltage over 12v with a 13.8 V supply.

Don't get me wrong I am not recommending this at all and do think 17M is far too far. Even at 2.5 mm2 it is borderline hence why I included links to the calculators, the cable would need to multistrand so actually this probably increases the required wire size so it is not what I suggest but with the lower load of 5A max I think it would be possible.

Steve

Edited December 21, 2020 by teoria_del_big_bang

[teoria_del_big_bang]

This is the box have made up and used for past 2 years +.

Steve

[matt_baker]

How does all of this sound?

https://www.rothervalleyoptics.co.uk/mercury-138v-15a-bench-top-power-supply-unit.html

https://www.firstlightoptics.com/astronomy-cables-leads-accessories/lynx-astro-silicone-power-cable-25mm-dc-jack.html (Ring tongue variant)

https://www.amazon.co.uk/Dri-Box-FL-1859-285-Weatherproof-Black-Medium/dp/B00D7DU6YW/ref=sr_1_5?crid=17J8AUOD94C5W&dchild=1&keywords=ip55%2Bweatherproof%2Bbox&qid=1608636293&sprefix=ip55%2Caps%2C160&sr=8-5&th=1

Mains extension plugged into an RCD plug reeled up the garden to the PSU in this IP55 box and outputted through the 2.5mm cable into the DCHub 2

Edited December 22, 2020 by matt_baker

[Tomatobro]

I guess that you spotted that the power supply does not have a cigar lighter socket but that is easy to sort.

Go for it.  I am going to get one of those dry boxes myself after Xmas

Hold on just checked the dimensions. PSU 22.6 x 19.2 x 10.8

Dry box 15 x 28 x 11 so the PSU will not fit in the box

Edited December 22, 2020 by Tomatobro
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