Bartelised scopes

[Rod]

I am having problems with the laptop driving my bartelised dob. I wanted some feedback on the solutions I have in mind.

I use small laptops to save weight and bulk. Currently I use a Toshiba Libretto 110ct. It is very compact - about the size of a Video cassette. I have turned off all power management in the BIOS but it will not run the motors smoothly unless I also remove the battery, it then runs the scope perfectly in the workshop. However, when I observed in the cold last night I had the following behaviour:

Tracking was fine. Manual slewing with the handpad was fine. Gotos would start OK but just before completion, the laptop would shut down. So the motors would run together OK at the start but I think the computer crash is occurring when the slew is switching from two motors back to one.

My power supply is 4 x 12 volt, 7AH SLA batteries wired in parallel. This supplies all my power (for laptop, motors serial bluetooth adapter and red LED light). I use a small inverter for the laptop.

I am thinking about doing one of the following:

1. adding an independent power supply for the laptop - I could add another battery but wondered about just using one of the four I currently have and leaving the other 3 for the motors.
2. A friend suggested adding a large capacitor in series with the power lead to the battery. He suggested this because I once had a similar problem with another laptop. However, in this case the laptop battery could be used. When the battery was in place the computer did not crash. My friend felt that battery was supplying a little extra current as back up. Any suggestions on the size capacitor required?
3. This has all left me wondering if my power supply is inadequate?

I should also add that this issue with the laptop seems connected to my use of higher voltage for faster slews. When I run at 12 volts the laptop behaves itself.

Sorry for the long winded description. I would really appreciate any advice.

Thanks,

Rod.

[bojan]

Yep, Rod, your power supply is inadequate.. and capacitor will not help. Only bigger battery...

[Rod]

Originally Posted by bojan

Yep, Rod, your power supply is inadequate.. and capacitor will not help. Only bigger battery...

Thanks Bojan, what size do you recommend? Would adding another SLA in parallel be enough?

[bojan]

Not sure.. but - bigger the better.
Use the biggest you still can consider reasonably practical .

[alistairsam]

Originally Posted by Rod

I am having problems with the laptop driving my bartelised dob. I wanted some feedback on the solutions I have in mind.

I use small laptops to save weight and bulk. Currently I use a Toshiba Libretto 110ct.

My power supply is 4 x 12 volt, 7AH SLA batteries wired in parallel. This supplies all my power (for laptop, motors serial bluetooth adapter and red LED light). I use a small inverter for the laptop.


Rod.

Hi Rod,

the Libretto specs show that the adaptor required is 15V, 3A, which is 45W. http://www.batterysupport.com/toshib...r-p-38868.html . What is the wattage of the inverter you're using.

you could use two 18AH SLA batteries, but to identify the cause of shutdown, you'd have to measure the voltage at the different points, the battery voltage under full load (DC, should stay close to 12v), voltage at output of the inverter at full load (AC), voltage at output of laptop adaptor (DC, should stay close to 15v).
reason is that if current draw is high from a battery, voltage tends to drop, and if that happens, your inverter won't provide the right voltage for the laptop adaptor to provide 15v at the output even if it is regulated as regulators have input thresholds.

the output of the inverter with your laptop connected should stay at 230v ac or whatever the rating is with load. if that drops, then the output of your laptop adaptor will also drop to below 15v which might be causing your laptop to shut down.

by adding more batteries in parallel, you'd be increasing the current delivering capacity or AH capacity, but the voltage would still remain the same thats 12v.
so you need to measure voltages and you'd know what's causing the problem.

you mentioned a capacitor in series to the battery, not sure if you meant filter caps, the capacitor won't work this way, it has to be across the DC power rail, that way it will provide power during sudden demand, but only momentarily.
the other effect it has is that it will raise the voltage slightly, as rms value of the rectified output increases.

eg, if a 2000mfd cap is connected in parallel to a 12v dc supply at the output, it will raise voltage slightly by 1 or 2 volts.

one option would be to try powering the laptop from the battery directly with sufficient caps in parallel, ensuring the voltage is close to 15v. this would eliminate the inverter and reduce losses as inverters have losses. just an idea.

[Rod]

Originally Posted by alistairsam

eg, if a 2000mfd cap is connected in parallel to a 12v dc supply at the output, it will raise voltage slightly by 1 or 2 volts.

one option would be to try powering the laptop from the battery directly with sufficient caps in parallel, ensuring the voltage is close to 15v. this would eliminate the inverter and reduce losses as inverters have losses. just an idea.

Thanks for the suggestion and the detailed response. The inverter is a
Powertech can sized inverter (MI-5121):

• Output Power (Continuous): 150W
• Surge Output Power (Surge): 450W

I used to use a simple dc voltage converter designed to run laptops off a 12 volt battery. This worked for years when the laptop battery still had some life in it in the previous laptop. But the shutdowns began when the battery died. That's when I switched to the Libretto and the inverter. This seemed to resolve things in the workshop - but not when I was out in the cold observing.

I like your suggestion re the capacitors in parallel. It would simplify my equipment. Would this give a small reserve of power too? Can you give me specifications on the capacitors I need to try it - I'm guessing 2 to 3 2000mfd caps? I can make things up OK but I don't understand the theory very well.

Rod.

[alistairsam]

hi,

if you have access to another battery, do try adding that in parallel first and checking if the same behavior is noted.
else, usually one car battery should provide ample power and their chargers are fairly inexpensive.

the capacitors won't give sufficient reserve as they only provide power momentarily depending on the load, so this could be from a few milliseconds to one or two seconds and largely depends on load draw. they're mainly designed to filter rectified dc and ripples in the waveform when used as filter caps.
but like I mentioned, they do increase voltage slightly even with load.
in Audio amps, usually 2000 to 10000mfd capacitors are used in parallel and they provide power during bass notes which require higher power to move the driver.

my suggestion is purely theoretical as I haven't tested this. I would try a load other than the laptop first that's rated at 15v.
measure voltage at battery before applying load, after applying load, and after adding 2 x 2000mfd caps in parallel, but the capacitors need to be rated at least for 50v. I don't remember the formula to calculate voltage rating.
one aspect to be cautious is discharging the capacitors. the higher you go, higher the charge retained. so a simple load like a 15v dc bulb with a 1w resistor and an spst momentary switch would help discharge them.

also try your original configuration with the inverter, but separating the batteries for the laptop/inverter and the motors. that way, if you slew at higher voltages, that won't affect the laptop, and keep observing which battery set drains quicker, and you'll get an idea of what AH rating you'll need and how many batteries in parallel for which.

[Rod]

Originally Posted by alistairsam

hi,

if you have access to another battery, do try adding that in parallel first and checking if the same behavior is noted.
else, usually one car battery should provide ample power and their chargers are fairly inexpensive.

I don't have another battery but I have a 12 volt 5 amp dc power supply. Could I run that in parallel with the batteries? If it resolves the problem, I could then buy one or two more 7AH batteries.

Thanks,

Rod.

[alistairsam]

Originally Posted by Rod

I don't have another battery but I have a 12 volt 5 amp dc power supply. Could I run that in parallel with the batteries? If it resolves the problem, I could then buy one or two more 7AH batteries.

Thanks,

Rod.

not sure what you meant by running in parallel with the batteries. is this an ac to dc adapter?

[Rod]

Originally Posted by alistairsam

not sure what you meant by running in parallel with the batteries. is this an ac to dc adapter?

It's a power supply I bought from Jaycar. You plug it into a wall socket and it supplies 12 volts DC. It is rated at up to 5 amps. I was thinking of adding it to the bank of batteries just like it was another battery - adding its positive lead to the battery positives and the negative lead to the battery negatives.

Hope that makes sense.

Rod.

[bojan]

Originally Posted by Rod

It's a power supply I bought from Jaycar. You plug it into a wall socket and it supplies 12 volts DC. It is rated at up to 5 amps. I was thinking of adding it to the bank of batteries just like it was another battery - adding its positive lead to the battery positives and the negative lead to the battery negatives.

Hope that makes sense.

Rod.

Wow Rod, that is dangerous.. Power supply you have is not a charger and it should not be connected to battery just like that.

Previously I mis-understood you and/or didn't read your post carefully enough.

If you have a problem with computer when slewing, I suggest you sort this out where the problem is: get a new computer battery or different computer.

[Rod]

Originally Posted by bojan

Wow Rod, that is dangerous.. Power supply you have is not a charger and it should not be connected to battery just like that.

Previously I mis-understood you and/or didn't read your post carefully enough.

If you have a problem with computer when slewing, I suggest you sort this out where the problem is: get a new computer battery.

Thanks for the warning Bojan. As the laptop's own battery causes it's own problems I might try powering the laptop separately with one or two of my 7AH batteries. What do you think?

Rod.

[bojan]

Yes, this would be much better approach.