Electrical Engineers Help Needed

[Singlin]

This is my intended setup:

Servo Cat:

The current requirements are (from your 12V source): Tracking: typical 0.25 – 0.4A Slewing (one axis): 0.5-1.5A Slewing (both axes full speed): 2-4A [momentary peak during acceleration only] Average user: 0.4A [400mA] average or 0.4Ah The unit will operate for 6 – 12+ hours on a single 7AH battery assuming most of the time is spent tracking and not slewing (or doing a GOTO). A larger 12Ah battery is however suggested in order not to have to charge the battery as frequently .

 

Sky Commander:

11-13.4 Volts-210 mA

 

Pc Fans:

2 x fans at 12v and 0.1 amp

 

Secondary Dew Heater:

Current(A@12V) Power 3W max

I am thinking of using the following battery:

https://www.firstlightoptics.com/batteries-powerpacks/tracer-12v-24ah-lifepo4-battery-pack.html

Would this battery be able to provide power for a nights viewing?

What connectors and fuses would I need?

 

 

 

[MarkyD]

Taking the worst scenario you will be pulling 10Amps so that battery will last for 2.4 hours at that rate.  If an average of 1.1 Amps (Say 1.5amps) is pulled from your specs then the 24Ah will last for approx 16hrs so I would say from fully charged that battery will be OK.  The nice thing about LiFe is that it will stay at 12V until the last moment then just drop off unlike SLA which drop the voltage as it is used.  I use these as connectors as they are secure

http://www.maplin.co.uk/p/maplin-2-way-audio-locking-connector-line-socket-fk22y

Use 20mm fuses rated to what the maximum the devices will draw

[Singlin]

Thanks Mark,

What would you use as a hub to connect everything to?

[MarkyD]

I've used these before as they easy to mount.  One for + and another for -  A tad overkill on current capacity but they do the job

http://www.hobbytronics.co.uk/power-distribution-bus?keyword=Powerbus

[RayD]

Well with the information you've supplied that lot is about 13w (ish) and given an amount of slewing etc. according to the specs of that battery you are going to get comfortably in the region of 12-14 hours, which I would say is a good night's viewing.  I like the assessment of 6-12 hours, made me chuckle.  Would be like telling my Mrs she looks between 50 and 100 

As far as connectors go it's a minefield with so many differing opinions it's just a case of picking one you like the look of, although most tend to steer away from the cigar type sockets as the plugs easily disconnect.  There are lots of different preferences here on connectors, and every one of them is right.  I use XLR's but you may want to stick with 5.5/2.1 as that is what the output of the Tracer has, or chose from the googles of them that do a great job.

Fusing is a whole subject on its own, so as it has been raised a few times recently perhaps I can give my view on it in more depth here.  

Fusing depends on what the nominal load and maximum load on that circuit will be (along with type of load and cable size).  Each of those mentioned individually in your case is small, with only a 150mA to 1A fuses needed, although the Servo Cat will probably need a 3A time delayed fuse as the declared momentary peak is actually quite high for the noted nominal during tracking.  You would normally have a "main fuse" on the output of a supply to prevent damage to it if the cables shorted, but it may be the Tracer has a built in fuse, I've not seen one up close so don't know.

As a rule of thumb with fusing you would look to have 120% (this should be at least 110% but I find 120% to offer more consistent reliability) of the maximum load being drawn on that circuit, but within the maximum current carrying capacity of any cable or connector feeding from it.  To clarify this, you don't want a small spike on the equipment blowing a fuse, hence the 120%, but you also don't want any cables melting if you get a direct short because they are rated at 6A and you have a 10A fuse fitted.  In your instance let's take your 250mA resistive load for example (dew heater), add 20% and the nearest standard fuse size to that would be 300mA.  If you had big enough cables even fitting a 10A fuse will work with a direct short, but we tend not to go that route as we really want fuses to blow with low resistance faults, and not just short circuits.

Don't forget technically you could argue that the primary purpose of the fuse isn't really protecting the equipment at all as such as it won't blow unless there is already a problem, so it's actually protecting the circuit, and as a consequence preventing further damage to the equipment.  For example if you have an electric fire in your house and it goes short circuit, well the fuse will blow, but it hasn't protected anything, the fire is already broken.  However, the fuse disconnecting does now stop all the cabling and the incoming mains supply going up in smoke.  It also prevents you getting killed if the reason it went short circuit is because you touched it with the metal spurs on your cowboy boots. Note: RCD's are there for this and this is an example.

One final example is a ring main in a house.  It is normally rated at 32A and wired using 2.5mm2 cable.  Now I can plug many loads with no problems in to this ring until I exceed 32A then the trip will break.  No problems with the loads, they're all fine, but the trip has gone because it is protecting the circuit, which in this case is the cable.  Now If I left the trip at 32A but changed the cable for 0.75mm2 and did the same thing, I could expect to have a very warm evening by the fire with my burning cables.  Eventually of course we are likely to see a short and the trip will open, but the point I am making is that the fuse is there to protect quite a lot of things, including the cable, and not jut equipment at the end of it.

The reason I give these examples is because fusing isn't really an exact science as such, but it is the result of a few variables such as nominal circuit current, maximum circuit current, type of load and cable size.  Once you have this information you chose a fuse which is best suited from a range, and usually that is the next readily available size above 120% of the full load current.  If the load is an inductive load such as motors etc. this fuse may need to be time delayed.

Hope this all makes sense and at least helps you select the right fuses, if not the plugs .

[Singlin]

Thanks Ray that is a great help.

So a 3A fuse for the servocat, 250mA fuse for the sky commander,150 mA fuse for the fans and 300mA for the dew heater.

Is the above correct?

What size wire should i use to go into the 5.5/2.1 plugs?

Thanks again

Simon

[RayD]

1 minute ago, Singlin said:

Thanks Ray that is a great help.

So a 3A fuse for the servocat, 250mA fuse for the sky commander,150 mA fuse for the fans and 300mA for the dew heater.

Is the above correct?

What size wire should i use to go into the 5.5/2.1 plugs?

Thanks again

Simon

No problem Simon.  Yes assuming the figures you have are accurate.  I don't know what a sky commander is?

Standard cable for this would be speaker type (figure 8) which I think is 0.5mm2, and is generally what is supplied with mounts like Skywatcher and Celestron, so this would be fine at these loads, but slightly bigger (0.75mm2) is not going to hurt at all.

Don't forget at 3A the mount fuse will probably need to be time delayed as for some reason it has quite high starting currents on the motors, so I can only assume it accelerates quite quickly.

[Singlin]

THe sky commander is a digital setting circle computer attached to my two encoders.

On the link below click on the specifications window.

http://www.skyeng.com/

Where and how do you put the fuses in?

Sorry for my ignorance but I am taking advantage of your expertise.

[RayD]

1 minute ago, Singlin said:

THe sky commander is a digital setting circle computer attached to my two encoders.

http://www.skyeng.com/

Where and how do you put the fuses in?

Sorry for my ignorance but I am taking advantage of your expertise.

It's ok.  Easiest way if you're cabling yourself is to use standard 20mm fuses as noted by MarkyD which can use a variety of holders such as these but any kind of inline one would do. 

[Singlin]

Which wire does the fuse go on, the negative or positive? 

[RayD]

3 minutes ago, Singlin said:

Which wire does the fuse go on, the negative or positive? 

I'd suggest the positive 

[Singlin]

Thanks Ray.

Astrosystems make a power unit for the Dobsonian groundboard which enables power from one lead to the ground board to then reach the mirror box.

This unit is shipped with a connector box as seen in the below pdf.

https://www.astrosystems.biz/ASPFTinst.pdf

[Singlin]

WILL THE FOLLOWING POWER SUPPLY BE ENOUGH FOR THE WHOLE SYSTEM ?

 

[Gina]

Depends what you want to run off it but probably not.

[Singlin]

This is my intended setup:

Servo Cat:

The current requirements are (from your 12V source): Tracking: typical 0.25 – 0.4A Slewing (one axis): 0.5-1.5A Slewing (both axes full speed): 2-4A [momentary peak during acceleration only] Average user: 0.4A [400mA] average or 0.4Ah The unit will operate for 6 – 12+ hours on a single 7AH battery assuming most of the time is spent tracking and not slewing (or doing a GOTO). A larger 12Ah battery is however suggested in order not to have to charge the battery as frequently .

 

Sky Commander:

11-13.4 Volts-210 mA

 

Pc Fans:

2 x fans at 12v and 0.1 amp

 

Secondary Dew Heater:

Current(A@12V) Power 3W max

[MarkyD]

That PS is rated at 5Amp 12Volt.  Do your calculations and that will tell you your current demand.  Although those PS are rated at 5Amps I wouldn't want to run them at max capacity for very long

[Singlin]

Seeing that I could peak 4 amps when starting the motors when slewing I will probably update to a 10 amp powersuply.

I had my cooling fans on when I started a slew with the servocat motors and the servocat went into safety mode.

I turned the motors off and the servocat on again and all was well.