Model Boat Mayhem - Forum

Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: ianb on December 16, 2007, 02:09:28 AM

Title: !2V to 6-7.2V
Post by: ianb on December 16, 2007, 02:09:28 AM
I'm building an Aziz using 2 Graupner 720 Torque motors at 12V.

The bow thruster (Graupner 400) is rated at 3,6 - 7.2V.

Batteries are 2- 12V 6.5Ah Sealed Lead Acid type, connected in parallel. For reasons too difficult to explain here I can't obtain 6V SLA's, so 12V batteries they must be.

Electronic speed controllers are Electronize, 2- 15A for the motors and 1- 10A for the thruster.

So far I am OK with the connections for the drive motors, but I don't know the best way to obtain about 6V for the thruster from this set up. Is there an easy and efficient way of obtaining the required voltage drop?

Thank you for reading this.


Title: Re: !2V to 6-7.2V
Post by: HS93 (RIP) on December 16, 2007, 02:19:56 AM
Hi not a fan of connecting two 12 volt in parallell , but if you need them for ballast and are not going to sail for hours and hours at one go think about swapping to the second battery half way through the day, as weight does not seem to be a problem simplest way is to use a 6 volt nicked pack as it only gets used in bursts and stops the Neda for resistors that get hot or complicated circutes.

Title: Re: !2V to 6-7.2V
Post by: Circlip on December 16, 2007, 07:31:39 AM
FLJ, Would A BEC regulator work? Is It the 317T? Just to feed the thruster?  NOT trying to deloop you HS93, sorry.
Title: Re: 12V to 6-7.2V
Post by: Faraday's Cage on December 16, 2007, 10:34:30 AM
FLJ, Would A BEC regulator work? Is It the 317T? Just to feed the thruster?

Think I'm correct is saying that the 317T will only carry 1.5 amps (with a good heatsink) max.
From memory, I'm sure that my thruster draws around 2 to 3 amps of current, therefore another solution would be required.
The LM338 reulator might work as it will carry 5 amps.

My solution though, would be a separate pack for the bow thruster.
Title: Re: !2V to 6-7.2V
Post by: ianb on December 16, 2007, 11:08:55 AM
Thank you for your informative replies.

I will go with a separate 6V pack for the bow thruster as you suggest.

Peter, I would really like to know why you don't recommend connecting the two 12V batteries in parallel. What problems might occur? My Toyota has a 3,0l diesel engine and has two 12V batteries connected in parallel for electric power. Am I comparing apples to oranges? As you can tell, I don't know very much about electrical things.

Thank you

Title: Re: !2V to 6-7.2V
Post by: malcolmfrary on December 16, 2007, 12:47:47 PM
Nothing wrong with parallel batteries as long as they are as near identical as possible.  You do have to keep checking them to make sure that a failing battery is not taking its mate down with it.  If your Toyota handbook doesn't mention this, they haven't done you any favours.
Probably the simlest down and dirty way would be to run it off 12 volts but have a resistor of about the same resistance value as the motor, in series with the motor, of a high enough wattage.  Some power is wasted, but a thruster is only used intermittently, so it should not be a problem.
Title: Re: !2V to 6-7.2V
Post by: OMK on December 16, 2007, 01:42:37 PM
The LM388 regulator would do the trick. An even cheaper solution might be to bolt a bog-standard 2N3055 NPN transistor to a sufficient heatsink, with a 5.6 volt zener diode across the Base/Emitter, and approriate dropper resistor from the Base to Pos' supply rail. Since the tranny drops approx' 0.6V, means the output is nicely regulated at five volts.

Size might be a problem. The '3055 is a bit of a clunker. 12V to 5V means 7 volts has to be lost somewhere, which manifests itself in the form of heat. So a pretty hefty heatsink is needed.
Apart from that, three components costing around thirty shillings could be a cheaper alternative to stuffing a 6-volt SLA inside the model.
Title: Re: !2V to 6-7.2V
Post by: J.beazley on December 16, 2007, 02:16:43 PM
Just a thought but could you not run the Bow thruster off another 4.8volt Reciever pack??? that way you have both 12volt SLA's wired upto the main drive and seeing as the bow thruster only gets used to push the front round performance isnt essential.

If you have both motors wired up seperate she should spin in her own lenght anyway.

Title: Re: !2V to 6-7.2V
Post by: OMK on December 16, 2007, 02:20:08 PM
The circuit...

Nice answer. Why didn't I think of that?