DIY Power/USB Hub concerns

[mrflib]

Hey all

After trawling through lots of power-box threads I thought I would ask I final couple of questions before I go ahead and blow myself up.

I'm looking to make a smart all-in-one power and USB hub - I have most of the parts now and it will be running on a leisure battery. My major concern is with the laptop power - I have Windows running on a Mac and I think it's going to suck a lot of juice.

Does anyone have any advice on safely and efficiently powering a laptop in the field?

Additionally is there anything I should know about water/dew proofing or necessary insulation for both the battery and USB hub (this one from Startech: http://www.misco.co.uk/Product/162640/StarTech-7-Port-Rugged-Industrial-USB-Hub)?

Sorry for the general nature of my questions - I just don't want to make it poorly!

[Oddsocks]

This 12V input car/air universal laptop PSU from MAPLIN will provide 120W (~ 6 amps at typical laptop 19.5V DC input).

http://www.maplin.co...g-socket-614037

Check your existing laptop PSU, the power/current rating will be printed on the case, as long as it is equal to or less than 6A or 120W output then this device will work and is quite safe for use in the field.

The USB hub (and any other electronic device) will possibly overheat if you attempt to dew-proof or waterproof by enclosing the device totally but it will be quite ok working inside a closed and ventilated box.

Any heat generated by the device(s) will act to counter dew formation.

During battery charging make sure the box is extra-well ventilated so that pockets of hydrogen gas or acidic vapour do not form as this could lead to an explosion or corrosion of the electrical devices.

The battery can withstand temperatures down to minus twenty or so without freezing, you don't need extra insulation inside the box.

If possible install your supply 12V output connectors inside the lid of the box and leave a side opening to run the cables from the mount, heaters, laptop etc inside the box to connect up, I've seen a few boxes where the builders had put all the connectors on the outside of the box where they promptly get dewed up or frozen, although not a hazard in itself the connectors become corroded quite quickly unless you run to expensive gold plated versions.

Avoid at all costs the temptation to fit a 12V DC to 230V AC sine wave inverter, they are not safe for field use as in certain circumstances, and while under normal operating conditions, up to half the high-voltage AC output (115V AC) can exist between the earth/neutral connection of the power output sockets and the battery negative terminal. This can be lethal for you as well as anything sharing the high and low voltage power source, such as a laptop powered off the AC supply and connected to a 12V DC supplied camera or mount via USB or serial line.

Good luck with the build.

William.

[mrflib]

William - thank you. It was a post I saw by you that prompted me to post here (camping accident).

As it is a Macbook would you suggest this is acceptable: http://www.amazon.co.uk/In-Car-Charger-Apple-MacBook-Unibody/dp/B006LMEMQ4 ?

If possible install your supply 12V output connectors inside the lid of the box and leave a side opening to run the cables from the mount, heaters, laptop etc inside the box to connect up, I've seen a few boxes where the builders had put all the connectors on the outside of the box where they promptly get dewed up or frozen, although not a hazard in itself the connectors become corroded quite quickly unless you run to expensive gold plated versions.

Would it be possible to explain this a little more? Are you suggesting not to drill holes for the cigarette lighters in the side of the box, instead mounting them on a panel inside?

Do you have any links to high-quality builds you have seen - I have googled as much as I can but it seems there are so many terms to describe the build it's not too easy to find them all.

Pete

[Oddsocks]

Hello Pete,

That Lavolta car adaptor on Amazon for the Macbook looks ok, should have the correct "L" magnetic plug and is the correct rating for the 15" and 17" Macbook Pro (but not the latest models with the "Retina" display, which have a different plug and higher power requirement I think)

The Lavolta products do have a few quality control issues but as long as you buy it from Amazon UK you will be able to return it if it arrives faulty or fails during the warranty period.

The only issue you may have with it is that it is not aircraft rated, only in-car, so it will probably be quite electrically noisy and may cause interference noise patterns with any camera you connect to the leisure battery.

If you do get noise problems then this is not difficult or expensive to deal with though, a couple of ferrite beads, capacitors and a line-choke in a small plastic box fitted between the Lavolta adaptor and the battery will filter out most of the noise and not cost more than ten pounds for the components.

If you are ok with soldering and wiring then consider using a group of XLR sockets to distribute power to the various devices instead of the cigar lighter sockets, I never could understand why this method of power connection has been so widely used for astronomy products given how unreliable they can be.

The XLR plugs and sockets are ok for up to 15 amps and very low contact resistance so they don't suffer voltage drop and overheating as much as the cigar lighter plugs do.

Here are the links to the XLR plugs and sockets:

http://www.maplin.co...sis-socket-1026

http://www.maplin.co...n-xlr-plug-1025

If you did use these it would mean removing the cigar-lighter plugs and fitting the XLR plugs to each of the devices you want to power from the 12v supply.

Yes, when I said mount the sockets internally I meant to mount them on a panel inside the box so that you have to open the lid to plug the devices in, you need to ventilate the box anyway so that when the lid is closed a gap of say a 100mm long and 20mm deep between the battery box wall and the lid remains (bit like a recessed handle in a chest of draws) so that once you have run and laid the device power and usb wires you can simply close the lid to keep everything nice and dry.

There's quite a few threads here on SGL of various battery-box builds, here are a few to begin with but if you search using the SGL search box at the top of the page using the term "powertank", "power tank" or "battery box" you'll find several more:

http://stargazerslou...ated-power-box/

http://stargazerslou...-neither-did-i/

http://stargazerslou...hl__battery box

You will find that some builders prefer to keep things simple and use the cigar-lighter sockets while others upgrade to XLR plugs and sockets (or similar).

I'm afraid, being an retired engineer and with lifetime leanings towards carpentry and cabinet making that if I built one it would be made of Mahogany and Teak, totally over-engineered and cost more than my scope!

It isn't that difficult a build really, I think the only thing I would do that I've not seen anywhere else is to put a couple of large fat wheels and a handle on it.

William.

[Herrman]

Hi,

I must agree with William about the cigar-lighter plugs & sockets; they can be most temperamental, and I do wonder why they're still fitted as standard to so much astro-kit. I've been replacing them with XLRs which I've purchased through Amazon.

Regards, Herrman