Powering your setup

Astroshed in night illuminated by Moon

If you do astroimages these days you need some electricity. Long time ago it might not be a requirement, but today – yes it is 🙂 Usually amateur astronomical gears should be supplied with 12V DC voltage. 

Voltage

But since these devices do not work at edge conditions (like for example overclocked CPUs), so there is always some voltage range, that is allowed for given device. Like for example Sky Watcher HEQ5 and EQ6 mounts can be supplied with any voltage in the range 11-15V. Or Atik 383 camera can be supplied with voltage in the range 12-14.4V. Usually when device is equipped with cigarette lighter power adaptor it must be able to work at the voltage range 11-14.4V – that is the voltage range that can happen in the car electric installation. I encourage you much to read manual and specifications of your equipment before you start to work with them. It may work at the beginning at the conditions that exceed specifications. But once it will break at these conditions, manufacturer will refuse warranty repair. There are also usually some protections in the devices, like fuses or reverse polarity protection. But it is rarely mentioned in the manual – these are usually the last resort only. You should not test it deliberately. 

Setup ready for work

Current

There is another important parameter, that describes electrical device – that is current it draws. It is quite important information, and that is sometimes hard to understand the difference. The current is rated for both power supply and for power consumer devices. But. The current rated for power consumer (like for example 2.5A for QHY163 camera) is the current that this camera will draw or may draw. And the current rated for power supply (like for example 12V 7.5A power supply) is the current that this unit is able to deliver. Power supply will not deliver 7.5A all the time! It will deliver as much current as it is required by connected devices. So if you connect QHY163 camera to 12V 7.5A power supply the camera will draw no more than 2.5A at maximum cooling power. But you cannot connect QHY163 camera to 12V 1A power supply, because this supply will not be able to provide current required by camera and will break (or turn off if the supply is equipped with overcurrent protection). You can of course connect more than one receiver to one power supply. If the supply voltage match receivers voltage and the current that supply can provide is no less than sum of currents required by all receivers. And do not use low budget power supply units that usually have no protection at all. Electronic devices you use (mounts, cameras) are pretty expensive, and 10$ power supply will not fit 2000$ camera. This is so called “expense mismatch” that can lead to unexpected expenses 🙂 Cheap power supplies can have also worse parameters – they do not keep constant output voltage when load changes, the voltage ripple can be pretty high at output and basically they should not work at maximum rated current. 

Voltage drop

Once the supply voltage and current are briefly described, let’s focus on voltage drop. It comes from the simple equation called Ohm’s law (probably you know it):

V = I * R

It says, that voltage is equal to current multiplied by the resistance where this current flows. But is there any resistance in astroimaging power setup? Sure there is. Each cable you use has its own resistance. Cable resistance is higher for longer cable and also is higher for thinner cable. So for example 3m long copper cable with cross sectional area of 1 square mm has resistance of about 0.1 ohm. And 5m long copper cable with cross sectional area of 0.75 square mm has resistance of 0.24 ohm. It does not sound much, but it multiplies with the current. So if this second cable is used to power a setup that requires 5A total current, then you get 5*0.24 = 1.2V drop. And it is only for this cable alone. Each connector has also its own resistance. Common DC connectors can have resistance even as high as 0.1 ohm (when not cleaned for a long time). 

What does it mean? It means that if you have 12V power supply with such cable to your setup, and there are four connectors used to distribute power, then you got 1.6V drop, so at the camera or mount there will be only 10.4V supply voltage. Well, that is not good, and that is usually already below specification. So we should not use long and thin cables to power 12V devices, and not because such cable cannot hold the current (1 square mm cable can handle 10A current), but due to voltage drop. It is also worth to keep connectors clean. There are some chemicals available as small spray that can be used to clean connectors from time to time. Another way is to use power supply with higher supply voltage (for example 13.6V) – but first make sure all your equipment can work at this voltage. 

Power distribution box in roll off observatory

One or many?

So maybe it does make sense to use separate power supply for each astroimaging setup component? Well, using either one or many units have its own drawbacks and advantages. For one power unit you need to split 12V voltage, and you need to have good quality connectors and short cables to make voltage drop low. For many power units you need to split 230V voltage, and that requires much care, because this voltage is dangerous and can be lethal. One may say many power units are more reliable, because when one fails, all other component still work. But when your camera or mount fail, you need to stop work anyway. Plus when you have five suppliers the probability any of them will fail is five times larger than for one unit. It is also cheaper to have one good quality power supply with all protection circuits, than to have five of them of similar quality. There is no clear answer to that question, and sometimes it is just decided by manufacturer. For example many SBIG cameras needs to have its own fancy supplier that provides three different voltages. 

Ground loops

Ground loop is a scenario, when two points that should be at the same zero voltage level are connected with more than one cable. This scenario does not necessary cause problems, but it may – that’s why it is usually quite hard to recognise for example connection problems caused by ground loops. Let’s take a look at following scenario: you have one power supply for the setup that supplies mount, camera, filter wheel and industrial USB hub. Now, the ground is connected to camera both with main power cable between camera and power supply unit. But also ground is connected to camera in the path: power supply unit – usb HUB power cable – USB cable between USB hub and camera. And then depending on many factors (how camera and hub handles communication errors, is USB hub power galvanically isolated, quality of power and USB cables, etc.) this ground loop may cause connection problems or not. Connection problems may not happen at connection to camera, it may occur at connection to any device connected to USB hub. It can be random, or happen at some arbitrary moment (like turning off camera cooling), or both. That’s why they are so hard to investigate. If you encounter such connection problems in your setup, try to eliminate ground loops before replacing system components. You may for example try to use separate USB cable between camera and computer, or use separate power supply to power USB hub.

But in many cases despite of having ground loop in the system all works fine. Till the moment you connect another device to the hub, and then out of sudden problem pops up. Then you exchange cables and/or USB hub and all works fine again (because new hub handles this situation better), but root cause remains. Ground loops are nasty.

Summary

It is possible to write really much on power and electricity, but who would like to read it? Here are some key points to remember when planning or reviewing your astroimaging setup:

  • read your astro devices manuals and specifications, so you will know the conditions they can work at
  • optimise cabling length and cross sectional area to minimise voltage drops and/or use higher voltage power supply if setup components can handle higher voltage (like 13.6V)
  • rethink your setup supplying, maybe one good quality power supply unit is enough
  • avoid using power supplies of questionable quality – your expensive components deserve more
  • identify ground loops in your system, so in case of connection problems you can react
Industrial power supply unit mounted at the pier

Clear skies!

Leave a Reply

Your email address will not be published. Required fields are marked *