gs-280 RA drive to gel cell

[jinxsta]

Hi all,
does anyone know what the gs-280 RA drive motor takes in amps I know its 12 volt but can't find the amp amount anywhere .
I would like to use a 12 volt 7ah battery that can be recharged instead of the 8 C size batteries but would probally need something to keep it at 12volt and then something to keep the amp amount right so not to fry the motor.
any help would be great.

Jinxsta

[Kokatha man]

Hi Jinxsta - don't know the gs-280 but if it's 12 volts and you replace the 8 "C" cells (which as you know are 12 volts) with a 12 volt 7aH battery you aren't changing anything at all except the capacity to deliver current (amps) for a longer time - called the batteries "capacity" - which is the ability to deliver any specific current for an amount of time (much longer with the 7aH battery.)

The voltage stays the same (12 volts) and hence the current draw of the motor(s) is the same - having a greater battery "capacity" does not mean the bigger battery will push more current through the motor - there is absolutely no need to limit current (your "amp amount") - it will not "fry" the motor, current (amps) is a product of voltage applied to the load, generally called resistance, the "load" being the motor. Voltage divided by resistance (in ohms) = current (in amps.)

Your only concern would be having the motor running whilst you have this larger battery "on charge" from a battery charger - and even then it would only be a problem if the charger had a very unregulated and high output voltage. Take this as gospel from an ex tech/electrician.

[skwinty]

Hi Jinxstra
You can safely use a 12v 1000A/h battery.
The current drawn by the motor is determined by the internal resistance of the motor and the voltage of the battery. The Amp/hour rating of the battery will have no influence on the current consumed by the motor.
However, if there is a short circuit in the motor then expect a bigger brighter flash than you would if you were still using the c cells.
In either case the motor would be stuffed.
Regards
Steve;)

[jinxsta]

Thanks for the very informative post guys, Its what I thought but the salesmen at all the electronic stores told a different story and had me second guessing but you have confirmed and also taught me something new.

thanks again.
Jinxsta

[Karls48]

Not quite right. New, fully charged SLA 7AH battery will have 13.8V and therefore will push higher current to your equipment. In most of cases it will not do any harm, but you better check specifications for your telescope. If it says 12V +/- 10% it is usually OK to use SLA battery. Another thing to keep in mind is that it is very easy to reverse polarity and do serious damage. Wire power diode (1N4004 for one ampere or 1N5404 for 3 Amperes) in series in connection to your telescope. That will give you reverse polarity protection and the diode will drop up to 0.7V bringing voltage from the battery closer to 12V

[Kokatha man]

Karl, with all due respect that is getting a little pedantic: but if we are; a fully charged battery can have a terminal votage of anything up to 14.8V (sometimes even higher, depending on the charger) - but this is invariably an open circuit (no load) senario. I spent a great deal of my life doing load performance tests on lead acid batteries for telstra as well as living with wind and solar electricity generating systems: jinxsta will have absolutely no problems substituting his 12V 7aH battery for the 8 "C" cells; and ac/dc motors of the types used in drives have a relatively high overload tolerance. If you read my post you will note my precaution re using (very) badly regulated battery chargers and operating the drive from a battery hooked up to such a device - though your suggestion of diode protection to gaurantee against accidently wiring it up with reversed polarity is a good comment.

Cheers, Darryl.

[Karls48]

Hi Darryl, yes I’m little pedantic. But in this case it is warranted. I actually own this unit. When I purchased it about three years ago from Andrews, Lee warned me not to use it with rechargeable batteries, because in his words – they blow up and can not be repaired. RA drive kit consists of small unipolar stepper motor, large brass gear and hand controller. Naturally I was curious why would those controllers blow up when used with rechargeable batteries so I open the controller. From the memory (it must be about 3 years ago) it consists of PIC microcontroler, LM7805L, crystal, and ULN200x (again I don’t remember which one) Darlington Array driver. After analysing the circuit I come to conclusion that most likely reason for damage when used with rechargeable batteries is ULN driver. This device can sink/source about 600mA of current. There is little copper on the PCB to provide adequate heatsinking. With increased voltage provided by SLA battery and if the mount hits hard stop with corresponding increase of current this IC will most likely fail. Also there is no reverse polarity protection on this unit. Those two design shortcomings are understandable because unit was designed for use with non-rechargeable batteries so it is extremely unlikely that some one would insert all the batteries wrong way around and internal resistance of those batteries would limit the current.
Saying all that, I was using this RA drive with 7AH SLA battery until recently (I had converted mount to Autostar GO-TO) without any problems. But, I’m using power lead from the battery to the controller with small bridge rectifier wired in to it. It drops up to 2.5V and I can connect the power lead to the battery in darkness whichever way around and still have correct polarity going to RA drive. I’m also mindful not to let mount to hit had stop.

Jinxsta, wire or have someone to do it for you, W02 bridge rectifier to the power lead to your RA drive. As Darryl said, don’t use it while charging battery. And don’t let you telescope to hit something solid.
Karl

[Kokatha man]

Hi Karl - I accept the explained rationale for your comments (whilst still maintaining that Lee's comments were, knowing him, more apocryphal than "likely") - and I do like your use of the bridge rectifier viz incorrect hook-up.

Regards, Darryl.

[jinxsta]

Thanks guys very informative posts.
would like to have a go at soldering up the Bridge recifier myself but can't find a W02 but seen W04
Function/Type: Round 1.5A 400V Bridge
at dicksmiths would the W04 suit.
Thanks again.
Jinxsta

[Kokatha man]

Hi Jinxsta - WO2 and WO4 are both 1.5 amp bridge rectifiers, the WO4 rated for 400 volts and the WO2 for 200 volts: either would be suitable for your application.

Regards, Darryl.

[jinxsta]

ok got all the gear together and just want to check that I will be doing the wiring up right have attached a diagram (sorry for bad technical drawing) of how I think it should go.
thanks again for the help.


Jinxsta

[rmcpb]

Really informative post :)

[Karls48]

Yes that is correct. Battery outputs to ~ in any order, from bridge rectifier + to the center pin of plug.

[jinxsta]

all is well the lead for the RA Drive to the rechargeable battery works great and I'd like to thank all for the help once again the members of iceinspace has come up tops.:thumbsup:

Regards
Jinxsta

[graphworlok]

Hmm.. Thread Necromancy.. :)

I have this hand controller on my much-abused EQ3 - I modified it to have a pair of spade lugs for input as well as the normal 8x cell holder, well over a year ago, and it was doing fine - Until thursday night. I was testing a DIY dual axis controller modification, and somehow (I'm still not sure how!) i caused the factory controller to fail.. After verifying it was actually the factory controller (and not the motor itself) that had failed, i desoldered the ULN2k3 darlington array (the only part i had a replacement for, due to the DIY dual axis control system) and soldered in a socket.. New ULN2K3 in the socket, and the controller seems to be working as good as new - The case even fits back together with the added height of the socket!

Just thought you would all like to know.