50m 12v DC power extension

[rambaut]

I thought this might be of interest to others who want to run a low voltage DC power cable into their garden. I wanted an alternative to long AC extension cords and issues of weather proofing. The issue with DC cables are that unless you go for seriously large diameter cables you get significant voltage drop due to resistance in the cable (a 50m cable has a 100m round journey). The other issue is that you can't compensate just by having a higher voltage at the start because the voltage drop is dependent on the power being used which will vary. So the solution is to start with a higher voltage and then use a power regulator at the telescope end to get exactly 12V. I packaged this in a waterproof box with a volt/amp meter to keep tabs on things and a cigarette lighter socket to connect the power to my mount. This allows me to have the cable out in my garden all the time, tucked away in the hedge without worries about it being cut and I just pull it out when I need to use it. Total cost was about £100 with stuff from Amazon, but you could probably do it cheaper. 

Details are posted here:

https://github.com/astrogear/12vdc-extension

I would be very happy to have comments on how to make this better/cheaper (you probably don't need the in-line multimeter - but it is fun). You might be able to get away with cheaper cable (smaller diameter, more voltage drop) but I was erring on the side of caution (I used automotive cable with a 1mm diameter). I am not sure about the relative efficiency of running at 24vs vs closer to 12v given loss in the cable against the efficiency of the DC converter.

Has been working very nicely and saves me time setting up.

Edited January 31 by rambaut

[Alien 13]

The ideal way of supplying 12V over long distances is by using the 4 wire method, two current carrying wires and two thin sense wires which all go to the mount, unfortunately few commercial power supplies have this. The method you are using does the same thing but with 4 wires the PSU senses the load so adjusts the output voltage accordingly showing a constant 12 or 13V on the display even though the actual output wires might be a lot higher.

Alan

Edited January 31 by Alien 13

[rambaut]

Hi Alan,

Thanks for this. I think buck-boost converters have improved a lot and are very cheap and efficient so I would be interested in how this compares in terms of efficiency. Running a higher voltage over the long cable has the advantage of significantly reducing power loss due to resistance (which would then be traded off by the efficiency of the converter - if you used an old regulator you would probably loose even more). This solution was certainly easy to throw together over a weekend using bits off Amazon. 

Andrew

Edited January 31 by rambaut

[AstroKeith]

The new converters are really quite efficient. 90+%.  But efficiency probably isn’t a worry?

stability of the 12v is more important and your arrangement will be near optimum. 
24v is probably a good choice as there is a good range of components available.

[Carbon Brush]

An interesting idea.

When constructing my observatory I considered running 24V to 48V from the house and using local 12V regulators for equipment.
In the end I stuck with mains voltage as a length of armoured cable became available at a good price.

Back to topic....

I would add some very large good quality electrolytic capacitors near to the switcher supply input.
They take current in pulses and can get upset about the cable resistance in the line.

A word of warning.
If the switcher supply pass transistor fails short circuit, all of your expensive astro kit can be subjected to 24V.

Fit a BIG 15V zener diode on the output of the switcher supply.
If the switcher supply decides to provide a high voltage, the zener starts to conduct, protecting your camera, etc.
If the zener overheats, they usally fail short circuit, protecting your camera.

Now what happens on a short circuit zener?
It all depends on the 24V power supply response to overload, and any in line fuse.

HTH, David.

[Mandy D]

16 hours ago, Carbon Brush said:

If the switcher supply pass transistor fails short circuit, all of your expensive astro kit can be subjected to 24V.

Fit a BIG 15V zener diode on the output of the switcher supply.
If the switcher supply decides to provide a high voltage, the zener starts to conduct, protecting your camera, etc.
If the zener overheats, they usally fail short circuit, protecting your camera.

Now what happens on a short circuit zener?
It all depends on the 24V power supply response to overload, and any in line fuse.

HTH, David.

Brutal, but effective! I've always used a zener diode to control the gate of a large SCR to crowbar an errant PSU.

[drjolo]

Elegant solution for all the fat cables and voltage drops.

16 hours ago, Carbon Brush said:

Word of warning.
If the switcher supply pass transistor fails short circuit, all of your expensive astro kit can be subjected to 24V.

Fit a BIG 15V zener diode on the output of the switcher supply.
If the switcher supply decides to provide a high voltage, the zener starts to conduct, protecting your camera, etc.
If the zener overheats, they usally fail short circuit, protecting your camera.

A TVS diode would be also a solution.

[Carbon Brush]

26 minutes ago, Mandy D said:

Brutal, but effective! I've always used a zener diode to control the gate of a large SCR to crowbar an errant PSU.

Yes a solution I also use. This allows an accurate low power zener, or voltage reference to trigger the thyristor at a precise voltage.
In this case, most of the readers are not electronics technicians or engineers so discussion of SCR choice and triggering adds confusion😁.
 

24 minutes ago, drjolo said:

A TVS diode would be also a solution.

TVS diodes have very high energy withstand, often more than may be generated in this instance. They are also fast acting.
The downside is they have a 'soft' turn on characteristic and wide voltage tolerance. Rather like MOV devices.
I use them in high power less precise applications. For example limiting transients to 30V or so in a 12V vehicle supply.

Thank you both for your comments,

David.

[rambaut]

Good idea about the zener diode - an easy addition. 

In my case the power is going into a ZWO ASIAIR Plus which then does the power distribution to the mount, camera etc. ZWO website suggests it has over-voltage protection (won't turn on above 15v). Probably best not to test this and a zener would make the whole thing a bit more general purpose.