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gajjer

Power supply : Work in progress

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Well the weather has been rubbish so I thought I'd get round to doing a bit of construction.

First of, Amazon were doing a toolbox with wheels for £25 so that was the starting point. I don't go to remote sites but I like to put electrical stuff in the garage after a session.

Next a moderate car battery fixed to a plywood base which is in turn fixed to the toolbox base. I don't want that sucker roaming about.

The tool box has a drop in tray which I cut all the features out of to carry a plywood panel. A lot of messing about making that fit nicely I can tell you.

At first I ordered 5 off XLR sockets from Amazon but then decided I should allow for expansion and possibly functional segregation.

Two banks of 5 will be separately fused. One for essentials ( camera, mount etc ), the other for heaters etc.  I might even break the second bank up and drive individual sockets with a PWM control.

Mains sockets are for the laptop, USB hub supply, hair dryer etc

There is also a battery charger giving 4A output. The battery is the supply, the charger will just top it up.

There will be an in-line fuse right at the battery just in case.

The panel has 3 fuses - one for each bank of the XLRs and one for the charger.

There is a battery voltmeter with blue backlight ( would have preferred red but hey )

There is provision for two switches - one to isolate the charger and one to isolate the meter while not in use.

I've also removed a load of sectioning in the lid so I can leave mains plugs in and put the lid on.

Hoping for an opportunity to use it one day. Ha

cheers

Gaj

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Hi Gaj,

A very neat unit with room for future expansion and improvements etc.

I would suggest that you fit individual fuses for each output, rated for the attached load... having a single fuse per bank means using a much larger fuse rating which would not fully protect an individual attached device.

One word of caution though... DO NOT charge that battery whilst enclosed in the unit.

Lead acid car batteries give off Hydrogen gas + Oxygen whilst under charge and the small volume of the box will soon get filled with these... especially with the top board and lid fitted.

Hydrogen gas is explosive at 4% by volume in air and the additional vented Oxygen will add further to this explosive condition... all it would need would be a small spark, say from plugging something in to one of the sockets, to cause it to explode.

Remove the battery for charging in a well ventilated space.

They also give off sulphuric acid fumes when charging, and even when not connected, which will play havoc with your electronics and electrical connections.

A better battery choice would have been a sealed gell type leisure battery, albeit they are a somewhat more expensive.

I trust you are connecting the mains via an RCD device.

Stay Safe.

Sandy. :grin:

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Hi Sandy

yes, good advice.

As I am sure you are aware, fuses protect the wiring not the attached device and I intend to use wiring that will more than adequately carry the 5A or so I expect to draw from each bank.

I'm glad you raised the point about charging the battery. I intended to use a brushless motor fan to provide cooling for the charger and ventilation. Though I was still pondering the best way to

ventilate through the top without providing a route for condensation. Hole can easily be put in the base without problems. It's no big deal to charge with the lid off after a session.

One of my first jobs was at British Gas research and I helped a colleague do some explosion tests on a small furnace. The last test we did was with a gas air mix at 25% of stoichiometric.

The pressure rise was only around 1psi but it was enough to blow the, chained down, explosion relief doors open ( steel doors which were about 4ft by 2ft and I could barely open! ) and send insulation material floating into the air.

I have a healthy respect for combustible gases.

Thank you for your concern.

cheers

gaj

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Quote  -  "As I am sure you are aware, fuses protect the wiring not the attached device and I intend to use wiring that will more than adequately carry the 5A or so I expect to draw from each bank."


Well yes, and no.


In a domestic setup, the big 30Amp etc fuses in the fuse box / Consumer Unit protect the wiring.

Downstream of that you have smaller fuses in the 3-pin plugs that should match the current draw of the device, so that if a fault occurs the big fuse doesn't blow and switch off half the house, and the small fuse should give some damage limitation.


Given the arduous conditions of astro setups it's well worth fusing every device - scope motor boards aren't cheap to replace.


Michael

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Michael

the reason you have fuses on each of the appliances in your house is to prevent one device fusing causing the drop out of the entire ring. ie you wouldn't want a clock to take out the telly, cooker, etc.  It will not provide protection to the device in question.

For my set-up, if either the camera, the mount or the filterwheel blows the fuse, then that is the end of my night. The second fuse circuit is for items I can afford to live without; like heaters.  

I repeat. The fuse is not to protect the electronics - it will only limit damage.

cheers

gaj

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Michael

the reason you have fuses on each of the appliances in your house is to prevent one device fusing causing the drop out of the entire ring. ie you wouldn't want a clock to take out the telly, cooker, etc.  It will not provide protection to the device in question.

For my set-up, if either the camera, the mount or the filterwheel blows the fuse, then that is the end of my night. The second fuse circuit is for items I can afford to live without; like heaters.  

I repeat. The fuse is not to protect the electronics - it will only limit damage.

cheers

gaj

I don't beleive you are right and I'm prepared to back down given adequate proof but one thing I will say is that there are 0.5 amps for a reason and I struggle to believe any wiring would be unable to carry that given the size of the fuse wire. My astrotrac runs at 0.25A and is protected by a 0.5A fuse. given that the cable supplying power to to device is 1mm and running at 12v and therefore can safely carry 10A all day long, what good reason other than protecting the electronics is there to run a 0.5A fuse when a 5A one will protect the wiring?Fuses protect both the applience and the wiring 

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Quote  -  "As I am sure you are aware, fuses protect the wiring not the attached device and I intend to use wiring that will more than adequately carry the 5A or so I expect to draw from each bank."
Well yes, and no.
In a domestic setup, the big 30Amp etc fuses in the fuse box / Consumer Unit protect the wiring.
Downstream of that you have smaller fuses in the 3-pin plugs that should match the current draw of the device, so that if a fault occurs the big fuse doesn't blow and switch off half the house, and the small fuse should give some damage limitation.
Given the arduous conditions of astro setups it's well worth fusing every device - scope motor boards aren't cheap to replace.
Michael

Well stated Michael.

Gaj,

I would point out that wiring also includes the printed copper tracks on your mount circuit boards... some of which are quite small in cross section... whilst these will be of adequate size for normal running currents it is quite possible (and often the case) that they get burnt off when something causes the normal working current to rise rapidly, such as both motors stalling during a slew due to a trapped cable.

In such a case the large fuse protecting the whole group of sockets is less likely to blow before the tracks, or other circuit components, are damage beyond repair, wherase a small individual fuse rated say 1amp higher than the normal maximum current would prevent any such damage... it would also help to prevent unwanted/uncontrolled shut down of other devices connected in the chain of sockets which could, in some cases, also cause unnecessary damage.

As Michael has stated... mount motherboards don't come cheap... and are not readily available in a lot of cases.

A small fuse and holder costs only pence.

Your understanding of flammable gas/air mixes is good to hear... sadly so many don't realise the potential danger.

The test you described may have only increased the pressure by 1psi but that would have placed an instantaneous load of over 1/2 ton on each door... so I am not surprised they blew open quite rapidly.

As for the forced fan venting... I am not a believer in using any type of electric motor in a potentially explosive atmosphere, however, providing the brushless motor is of the GAS SAFE sealed type and the control electronics are outside the enclosure then it should be ok.

Perhaps it should be set up to draw fresh air in via a shielded and filtered side entry (to prevent condensation ingress) and with an exit in the base (this should be well off the ground in your case due to the wheels etc).

Alternatively, if you can get access to the vents on the battery, it may be possible to fit vent tubes directly to these and pass them through the sides, or bottom, of the case via sealed glands.

Stay safe and hope you get clear skies.

Best regards.

Sandy. :grin:

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gaj is absolutley corrcet, the purpose of a fuse in a domestic 13 amp plug is to protect the flex and not the appliance - the appliance internal fuse, if any, properly protects the appliance.  It's a common misconception that the plug fuse is there to protect whatever appliance is connected, it is certainly not.  Moreover, the fuse in the plug will only give full protection when an effective earth is also connected.  In a typical scenario a fault in the appliance causes the appliance to draw an overload current. The fault could easily result in the appliance continuing to draw the high current continually. Such a current can easily damage the flex (it may exceed the rated capacity of the flex) at best degrading the insulator capacity of the flex and at worse resulting in the flex overheating and catching fire.  In a situation where the fault causes a live connection to the exterior of the appliance (non double insulated), anyone touching the appliance would be electrocuted without the protection offered by the fuse and the earth connection.    With an earth connected, the overload current with have a suitable path to earth, this results in the current rising further in turn blowing the fuse and disconnecting the power. In a sense the fuse does offers a degree of protection to the appliance in that it will ultimately cut the power but it's primary purpose is to protect the flex. In a fault condition the rated current carrying capacity of the flex may be greatly exceeded, and would continue as such until power is cut. The insulation covering our cables is highly toxic when it burns - electrical cables burning are nasty things.

As for battery venting while charging, a reasonable degree of venting is sufficient  with a lead acid battery.  Remember that Hydrogen is lighter than air and will readily escape to atmosphere.  Unless the battery is charged in a gas tight over container there is little practicable risk, especially given the likelihood of the low charging rate to be used.  One only has to consider the nature of a car battery in use in a car - it is continually under recharge by the alternator, and while small quantities of hydrogen will off gas the volumes concerned are readily dispersed.  Sensible venting of the gaj's battery box would be more than adequate and certainly recharging with the lid open would be completely acceptable. 

Gaj, that is a cool setup; how do you intend to connect the 13A sockets, are you looking to use an invertor or connect from the mains?  I must admit I do like its portability and the nicely varnished wood base gives it a real professional look.

Jim

Edited by saac
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Hi Jim

thanks for the comments.

The mains will be supplied from an outside RCD socket at the back of the garage, which is about 10ft from the pier. I only do imaging in the back garden so there is no

need for real portability.

Still have some work to do mounting the charger and cooling fan.

When the weather is rubbish I find it quite enjoyable making stuff.

Next project is putting a focus control on the Equinox 80 pro. I've built the drive and written the software. I just need to fix the motor to the lens.

Over forty years since I graduated as an electronics engineer and I love it as much today as I did when I was 10.

cheers

gaj

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Quote" the appliance internal fuse, if any, properly protects the appliance."

And there in lies the issue... Whilst I totaly agree that with a domestic AC mains appliance the plug fuse is there to protect the wiring not the appliance, however, we are talking about a DC installation, probably unearthed on the DC side and where the only return path is via the Negative line, and an expensive mount which does not contain any internal protection for overcurrent or for reverse polarity.

It would seem prudent to me (also a fully qualified electronics engineer) to fit a suitably rated fuse at the input of the mount to provide some such protection in the event of an overcurrent situation rather than rely on a much heavier fuse protecting the wiring to a whole bank of sockets... the likelyhood of this blowing before the tracks on the mount motherboard vapourising would seem to me to be tempting fate.

The only mount protection provided by Skywatcher is a fuse fitted inside the Cigarette plug on the power cable supplied with the mount and since this is not being used in this case it would seem resonable to replicate it by fitting an individual fuse at the appropriate socket.

Good luck with your new supply and I hope you will get your electronic focuser up and running soon... I did mine a year or so ago and it made a huge difference.

Best regards and clear skies.

Sandy. :grin:

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I too am a qualified electronics and electrical engineer and fully endorse what Sandy says.  Skywatcher decided that their mounts needed fusing and hence provided a fuse in the supplied power cable plug (cheapest way of doing it) so if you use another way of powering the mount you should provide a similar fuse.  For the sake of a little more effort and small cost, it's a "no brainer".

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Regardless of what anyone says, any lead acid battery in an enclosed space whilst being charged can be dangerous. The 4% by volume air to hydrogen mix needs only be concentrated in a small volume at the top of the box, exactly where the switches are. Any switch on, or off can cause a spark and ignite the air fuel mix. The propagation of the explosion wave front will very likely cause the flame front to enter the battery gas vent and the battery will explode if hydrogen is present. The X:LR  connectors are not sealed and the hydrogen can escape from them amongst other things, e.g. the 240 sockets.

If you intend to use a lead acid battery the safest ones are the sealed gel types, otherwise please do not charge the battery whilst in use.

Please read "Power Supply in Observatory" #23 and  "A reminder about charging your PowerTanks" in Discusssions- Mouints, post #63 for a bit about Lead Acid batteries.

The battery should always be vented to the outside of the box and a flame filter fitted, these are easily available from boat chandlers.

Personally I do not use ordinary lead acid batteries in these situations.

Hope this is of some help.

Derek

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Hi Jim

thanks for the comments.

The mains will be supplied from an outside RCD socket at the back of the garage, which is about 10ft from the pier. I only do imaging in the back garden so there is no

need for real portability.

Still have some work to do mounting the charger and cooling fan.

When the weather is rubbish I find it quite enjoyable making stuff.

Next project is putting a focus control on the Equinox 80 pro. I've built the drive and written the software. I just need to fix the motor to the lens.

Over forty years since I graduated as an electronics engineer and I love it as much today as I did when I was 10.

cheers

gaj

Gaj, I look forward to seeing your focus control, something I'll eventually need to get around to.  I'm already planing to copy your power box so you may want to take out some patents :smiley: I'm generally happy with power electrics but electronics and especially programming is the stuff of magic for me - my background is mechanical engineering (aerospace) - spent a few years in many an explosive environment in the RAF as an aircraft engineer.  I know exactly what you mean about using your professional skills and knowledge to build astro kit.  I think that's the beauty of this hobby.  We may not get as much time at the eyepiece due to poor weather but the opportunity for designing and building, and keeping hard won skills honed is great;  there is such a sense of achievement to be had  when a project comes out well.  Good luck with finishing off the power box, please post updates, I'm keen to see what you add. Oh and be careful, lead acid batteries are heavy, just as well you have wheels on that box  :smiley: .

Jim

Edited by saac

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Well there has been a little more progress but I have some measurements to do first.
Firstly here are some pictures of holes I have drilled for ventilation.

The attachments show where the fan is mounted. It is extracting air from the box.

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There are also holes along the bottom to allow air in.

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There are also holes at the highest points to allow any gas to escape.

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I also have a vent tube fitted to the battery that vents to outside the box. So in theory there shouldn't be any gas to ventilate - better safe than damaged.

I haven't changed the fusing arrangement to the sockets.

I have done some measurements on the mount but I only had a 5A moving coil meter and a clamp meter that is really intended for higher currents.

Anyhow. Under normal conditions it draws about .7A. When slewing on 2 motors it draws a bit over 1A. When stalled it initially kicks higher but very quickly ( ~ 1 sec )

drops to just above 0.5A. I would stress that these are not accurate and I will try to borrow a current probe for my scope to get better data.

The point is that the stepper motors don't stall in the same way that a brushed motor does. They make a hell of a row when they are stalled but it sounds worse than it is.

Still more to do.

cheers for now

gaj

Edited by gajjer

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Hi,

This is all a little OTT as you would need to charge the battery without the fan running then start the fan for a buildup of gas to happen but.......With the fan in the current orientation if there was an explosive mix it would be extracting it through the fan so if the fan sparked, not that it should because its a brush less motor, it might be a source of ignition. Have you thought about reversing the direction of the fan, so it blows fresh air into the box. 

Cheers

James

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Hi James

I take your point. I am currently doing some tests on the battery to see how much gas is generated at different charge currents and voltages.

I think you are right that forcing it in will be a better option than sucking it out and possibly stirring it up.

Despite the tube directing the gas outside the box, there is always the possibility of a leak. It's an easy mod to reverse the fan and blow the gas out the vent holes - especially the top ones where gas is likely to accumulate.

Thanks

gaj

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Well I will watch this thread with interest as my battery sits in a battery box I bought in a caravan shop and then in a big black bin bag in case it rains.

Your use of XLR connectors is interesting as they are probably considerably cheaper than the 30A gold plated shrouded connectors that I currently use.

In all these projects we have to start somewhere . My Celestron ASGT and a dew band on my C8 probably only required about 2 amps.

But my EQ6 with my SBIG camera and dew heaters in my OO AG12 + fans + focusser and mount hub Pro consume nearly 8 amps when moving in both axis.

I could not see what type of fusing you intended using but I will be adapting my current box to use car type fuses (I suspect they are called blade fuses) as I find the "car cigarette connectors" convenient but unreliable. I will use both a main supply fuse and individual feed fuses to provide maximum protection over the next tweny years.

I do wonder what a fully qualified electronics engineer is perhaps someone will enlighten me.

Andy C.Eng MIET

Edited by AndyKeogh

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Hi Andy and Gaj

 Have a look at Speekon. I have just started using them they can easily handle 20 Amps. Some types are rated at higher current. The music industry uses them and they are rugged. I have used XLR on my control boxes but doubled up pins on some to take the current.  Speekon are too big for those connections as they would foul none moving parts. I have used from 3 pin XLR  up to 7 pin XLRs, so it is impossible to connect the wrong connector to the wrong socket in the dark. The Speekon connections are used for the power supply to the rig. They take an 8 metre length  of  6mm2 wiring from the batteries.

As for fuses I use 20mm standard panel mounts but with a water seal (Item No FF01336 available from CPC), RS do similar but more costly.

Derek

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Hi Andy

I'm not sure what a 'fully qualified engineer ' is either. For me, I know what I know and I find out the about bits I don't.

The fuse to the mount is 5A but also feeds an Atik camera and filter wheel. I don't intend to run the heaters off that circuit.

I don't like the cigarette lighter connectors either. I've also changed the connector on the mount for one that screws in. I had a few drop=outs because of the one fitted.

I have been running the set-up with another battery box I built and had no problems - even with the mount stalled a couple of times. Whoops!!

Of course that doesn't mean it's correct. It's like a parachute - you only find how good it is when you need it.

Derek. I don't know of Speakon. I think what I have is adequate for my present needs. However, that could all change. You can only go with what you know at the time.

Your point about not connecting the wrong connectors is a good one. I may colour code them or similar. The mount is easy because it is the only one of that type.

Something else I would point out is that this is to meet MY needs as they are at present. Others may require something different.

I will post my findings on the battery and the current consumption of the mount shortly.

Of course I would rather be taking pictures, but fat chance of that.

cheers

gaj

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Hi Gaj, I wasn't suggesting you change the connectors, but just passing on info that I have come by. I have changed all sorts of things as time has past over the last 9 years or so. As I become aware of a better way (for me anyway) it just seems logical to stop the same problem re-occurring. I hate loosing imaging time as I get so little of it.

Best of luck with your build.

Derek

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Hi Derek

I quite agree. I am really hoping that one day I'm going to completely wear those suckers out!!

They do look like nice connectors. As you probably know there seem to be millions on types of connector and it's good to know what other people are happy with.

I didn't mean to sound dismissive. I have a reasonable amount of free time and may well get to starting again after 12 months of use.

This is my 'B' model. My first attempt had a camera timer controlled by a PIC which also monitored the battery. After a period of actual use I now have different needs

and this one better fits what I need to do.

I like to make things and adapt them as necessary. So your recommendation will no doubt be of use.

I had a few instances with the power connector on the mount. The cable got pulled - not disconnected - and the blooming thing reset. Oh boy was I pleased! ( NOT )

Hence that got changed to a screw in one. I'm much happier with the new one.

I'm always open to suggestions - but the management reserves the right to ignore it!!

cheers

gaj

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Just one point that may be of interest concerning the Speekon plugs and sockets. Speekon sockets seem to come in two varieties. One of them is a direct replacement (size) fitting for the XLR sockets you and I  have used. It is the reason partially, why I changed to some speekon types. i.e. no alterations needed to the boxes the XLR were fitted into, just a quick swap out. You are completely correct, it is a bit of a minefield the number of sockets and plugs available, I have made plenty of costly mistakes over time.

Derek

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Me again.

Just a few results to report.

Firstly a picture of the battery venting

post-15982-0-04814000-1450617966_thumb.j

This should show how the vent hole in the battery is taken via a red elbow joint and plastic tube to an exit point.

Next I did some measurements on the quantity of gas given off under different charging conditions.

A arranged it so that the gas from the outlet tube would be captured in a measuring cylinder filled with water.

Starting voltage was about 12.5V and charging current from the charger circuit was about 5A

After 5hrs the voltage had risen to 12.9V and the current was down to 1.4A. The amount of gas captured was a small bubble about 8mm dia.

I began to wonder if there was leaking so I checked with soapy water looking for bubbles. Nothing.

The charger circuit was struggling to get the voltage much higher than 13V so I modified it to give 13.8V out.

Another 5hr charge period started a 13V at 5A and finished with 13.6V at 1A - still virtually no gas.

The charger was turned off and the set up left over night. In the morning a small amount of gas had been collected - I'd say about 20cc.

Swapped the charger for a power supply with current limit to get a higher voltage.

I was now able to run above 14V.

Left the set up to run over night and measured 14hrs later.

Start voltage was 13.8V at 2A and finish voltage was 14V at 0.15A.

Gas collected was approx 100cc.

Next I switched my attention to the mount. I was unable to borrow a current probe, so resorted to a resistor in series with the supply. ( 0.22R 5% )

Just after power-up without selecting alignment the current was about 0.4A

After executing alignment and slew on one axis the current was about 0.65A. Slewing on 2 axes the current was 0.9A.

Selecting a rate of 9 and slewing on 2 axes gave 1.5A.

When stalled at this setting the current was still only 1.5A.

So. There you have it.

cheers

gaj

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Hi Gaj,

Can I suggest one thing for your battery vent. Normally in an automobile  the wind passing the vehicle will remove any Hydrogen gas away safely. In a static environment that is not the case. What I have done in the past is to buy battery vents from a boat chandlery. They are cheap and made in plastic material. They contain a mesh to act as a flame stop.  I have also made my own from stainless steel mesh. I used two or three layers of mesh.  What they do is to reduce the heat of the flame front by conduction so that the flame is extinguished and cannot pass through the mesh. (think of a Bunsen burner in a lab, they use the same principal to stop the flame impinging on glassware by having a steel mesh between the flame and the glass). Thus the flame cannot continue on into the battery compartments. You can ask Skipper Billy I would think as he probably sells them for use on boat battery compartment venting.

As I said in another post here some time ago I had a battery explode in front of me. I was very lucky only to loose the battery and not my sight. It was pretty scary.

Derek

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