Model Boat Mayhem

Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: AlanB on June 28, 2006, 10:19:18 AM

Title: Reducing from 12v to 6v
Post by: AlanB on June 28, 2006, 10:19:18 AM
Looking for a simple quick fix to my dilema, before I start ripping out motors etc.

I have a model which runs on 12v, all electrics including smoke generator, sounds etc require 12v.

Only problem is the motor, which was fitted by the previous owner, also runs on 12v supplied by the ESC and it is way too fast for the model.

I have reduced the prop size by 25mm, any smaller and it will look stupid.

So I need a quick fix that can be inserted between the ESC and the motor to reduce the voltage to the motor.

I have thought about two seperate batteries 1 x 12v and 1 x 6v but space is a problem.

Thanks
Alan
Title: Re: Reducing from 12v to 6v
Post by: Mankster on June 28, 2006, 10:25:16 AM
A variable resistor should do it. Turn the knob till you get the speed you want.
Title: Re: Reducing from 12v to 6v
Post by: DavieTait on June 28, 2006, 11:49:27 AM
Only problem with the resistor way is that they get Very Hot and need a heat sink.
Its been too long since I did my Electronics ( try 21 years ago !! ) so i'm way out of date on what available and suitable now.

Davie
Title: Re: Reducing from 12v to 6v
Post by: White Ensign on June 28, 2006, 11:57:45 AM
Alan, some electronic shops offer a unit which reduces the voltage. You should have a look, but I don`t have any idea about their names in the UK. But I know from a Electronic shop in Germany (Conrad) which has such a unit. The trouble with a resistor is, that he just produces heat but he will take nearly the same Amps on 6 Volts as the motor would take on 12 Volts.
Hope this was out of any help.

J?rg
Title: Re: Reducing from 12v to 6v
Post by: Malcolm Reade on June 28, 2006, 12:16:40 PM
Alan B

Why not phone Jim Casey?  He's an electronics expert and could help you I'm sure?

Regards

Malcolm Reade

Title: Re: Reducing from 12v to 6v
Post by: Doc on June 28, 2006, 12:23:06 PM
Alan,
One solution is to use two 6 volt batteries.  Just one for the motor and both for the electronics.  Not exactly the 'best' idea especially if there's no room.  Swapping the motor for another one would seem like the simplest solution, maybe not the cheapest though.  Reducing the pitch of the prop instead of the diameter is probably the easiest/cheapest route.  But, like the old coot who runs this place, "What do I know?"...
 - 'Doc
Title: Re: Reducing from 12v to 6v
Post by: AlanB on June 28, 2006, 12:40:29 PM
Two 6v batteries looks promising.

The current, excuse the pun, battery is a 12v 7Ah, so replacing it with 2 x 6v 4ah is feasible.

As for removing the motor ? It is well stuck in the hull. The hull would probably give way before the motor.

Thanks for the help so far
Title: Re: Reducing from 12v to 6v
Post by: malcolmfrary on June 28, 2006, 03:46:47 PM
The simple, cheap, down & dirty way is to measure the resistance of the motor, and the get an old electric fire element and measure off enough to give about 2/3rds the value of the motor.  Using the screw type connectors, (solder doesn't work on that kind of wire) insert it into one of the motor leads.  Make sur that there is enough air space around it because it will get hotter the more you turn the wick up.  This makes a good, high-power resistor of the appropriate vale for your application, and it need not be too large, but as you have a 12v 7AH battery installed, I suspect that space should not be a problem.  If the performance drops too much, shorten the bit of element, if not enough, either put a longer bit in, or dont turn the wick up that far.  If the element starts to get too hot, use two or more in parrallel.
I did a home-made resistor controller many years ago using an old element from a dead toaster, and it was completely trouble-free.  The resistor elements lived in a separate compartment from the rest of the "works", which occasionally collected water when enthusiasm for sailing exceeded good sense, and this would give a bit of steam from the hatch, but it still worked for many years.
Rather than two 6volt batteries, a six for the motor and a 12 for the rest might be better - two sixes implies that the motor would be powered from the "centre tap", which works, but the two batteries will not discharge at the same rate, and this can lead to problems when remembering which is which when re-charging.
Title: Re: Reducing from 12v to 6v
Post by: Malcolm Reade on June 28, 2006, 03:52:28 PM
Hi Malcolm

The downside is the amount of energy lost by the battery in heating up your "resistor".  Shorter running times etc?

Regards, Malcolm Reade

Title: Re: Reducing from 12v to 6v
Post by: johno 52-11 on June 28, 2006, 04:07:30 PM
Alan

Forget messing about with anything in the boat just restrict the movement of the lever on the transmitter. The advantage is that you will not be wasting any of the power in the battery and will have a longer running time.

John
Title: Re: Reducing from 12v to 6v
Post by: wombat on June 28, 2006, 06:30:14 PM
If you do have to do a step down from 12V to 6, I would suggest the way to go is by using a swotching regulator. National Semiconductors do quite a nice range (the SimpleSwitcher family). You may be able to find a cheapish module to do the job.

Any other form of dropper/regulator will reduce the efficiency of the motor drive to around 50%

Wombat
Title: Re: Reducing from 12v to 6v
Post by: AlanB on June 30, 2006, 10:05:07 AM
Johno,

I thought about restricting the movement on the TX lever, but the boat has a JJC sound unit fitted which runs on a Y lead from the RX and the TX lever also controls the sound.
So end result is less power to the motor and the sound does not sound good.

I have decided to buy an Electronize ESC and use the adjustable throttle speed on the ESC.

Problem solved ???

Title: Re: Reducing from 12v to 6v
Post by: Shipmate60 on July 01, 2006, 10:23:44 AM
Change the maim battery to a 6volt, this will be lighter than the 12 volt battery, and a bit smaller.
Use this extra space to fit a smalle 12 volt Nicad or MiNim battery to run Electronics.
It will mean wiring the Motor circuit and Electronics seperately, but this can have advantages of reducing interference.

Bobl
Title: Re: Reducing from 12v to 6v
Post by: Stavros on July 01, 2006, 05:36:51 PM
Come on lads the answer is so simple change the esc to an electrosise unit,for the simple reason they are ADJUSTABLE so turn the scew and hey presto you have the same motor same batts and a slower boat ;)Had to do eh same with a Riverman fitted with a MFA 6:1 motor,so forget all the other ideas and go for the obvious :D :D
Title: Re: Reducing from 12v to 6v
Post by: dougal99 on July 02, 2006, 07:17:03 AM
But what happens to the excess power? Heat I guess :(

Doug
Title: Re: Reducing from 12v to 6v
Post by: Stavros on July 02, 2006, 08:06:15 PM
Excess heat there isn't any because all you are doing with the elecronise speed controller is as I said reducing the percentage of power to teh motor
Title: Re: Reducing from 12v to 6v
Post by: dougal99 on July 03, 2006, 07:18:56 AM
Excess heat there isn't any because all you are doing with the elecronise speed controller is as I said reducing the percentage of power to teh motor

Don't take this wrong Stravros but I thought that's what speed controllers were designed to do anyway!!

Cheers Doug
Title: Re: Reducing from 12v to 6v
Post by: Stavros on July 03, 2006, 11:32:36 PM
Dougal99 no probs m8 I am running the riverman on reduced power by way of the esc and the esc does not get even warm maybee it is the quality of the product!!! pass on that one
Title: Re: Reducing from 12v to 6v
Post by: dougal99 on July 04, 2006, 08:00:05 AM
Hi Stavros

remembering my Physics lessons, the input power should be constant its the output that varies. Guess I'm out of my depth with ESCs. If it works for you then keep doing it - model boats as well. ;D

Cheers

Doug
Title: Re: Reducing from 12v to 6v
Post by: Made it to 80 (25p Richer now) on July 04, 2006, 08:37:25 AM
Oh I wish I was brainy like what you are Dougle  :o :o ???
Title: Re: Reducing from 12v to 6v
Post by: Malcolm Reade on July 04, 2006, 10:31:41 AM
Gentlemen

The basic laws of physics apply. - Ohm's Law

Take a look at http://www.grc.nasa.gov/WWW/K-12/Sample_Projects/Ohms_Law/ohmslaw.html

For a simple explaination and working examples.

Regards, Malcolm Reade

Title: Re: Reducing from 12v to 6v
Post by: HS93 (RIP) on July 04, 2006, 12:04:21 PM
ARE WE TRYING TO CONFUSE PEOPLE ???

Extract from electonize

As with all modern controllers, the motor speed is controlled by a P.W.M. (pulse width modulation) system. Power to the motor is pulsed on and off at high speed so that the speed varies with the on to off proportion.

so very little heat  ... as long as you are NOT trying to draw more curent than the controller rating..

Peter
Title: Re: Reducing from 12v to 6v
Post by: Made it to 80 (25p Richer now) on July 04, 2006, 12:30:26 PM
Hang on gents this is getting like 'A' level physics left all that behind when I left school ,we are talking model boats here isn't it simpler to say use an electronic speed controler when you want to go fast you push the stick all the way up when you want to slow things down move the stick nearer to the middle and when you want to stop you centre the stick
                 Cheers just of to do what life is all about sail model boats
                                                Cheers
                                                      Bob B
Title: Re: Reducing from 12v to 6v
Post by: malcolmfrary on July 04, 2006, 04:08:32 PM
Hi Malcolm

The downside is the amount of energy lost by the battery in heating up your "resistor".? Shorter running times etc?

Regards, Malcolm Reade



There will be heating of the resistor, but it should not be serious.  Obviously the run time will be reduced as agaist the optimum arrangement of the right motor and battery, but it should still be acceptable. The idea was to retain the full movement of the transmitter stick, not over-power the motor, and do it as cheaply as possible both in terms of cash and effort.  Of course, if a way could be found to use the energy dissipated by the resistor....
Title: Re: Reducing from 12v to 6v
Post by: Colin Bishop on July 04, 2006, 06:29:37 PM
Quote
Of course, if a way could be found to use the energy dissipated by the resistor..

Central heating for your boat....
Title: Re: Reducing from 12v to 6v
Post by: Peterm on July 12, 2006, 06:36:21 PM
Someone has already given the simple , reliable solution which I use regularly.   Use two 6v 4aH in series to get your 12v supply, tap off at the centre for your 6v supply.   Pete M
Title: Re: Reducing from 12v to 6v
Post by: sweeper on July 13, 2006, 12:16:21 PM
Well said Peterm,
I've been watching this thread as various suggestions have been put forward. I'll leave the subject of the various ESC's to those who have experience with them but I will add one comment. The suggestion of using simple Ohm's Law to evaluate the current taken by a motor is incorrect. If you consider the basic operation of a motor you will realise this.
Need proof? Apply an Ohmeter to the terminals of your motor (off supply) and note the reading. Connect your motor to it's normal battery and check the current taken while running on no-load. Now apply Ohm's Law. You will find that there is a difference (often considerable) between the calculated current and the current taken in operation.
I'll not go into the theory of the why and how it happens - after all as someone said "all you want to do is get the boat working on the water".
Hope this helps someone. ???
Title: Re: Reducing from 12v to 6v
Post by: Fast Electricals on July 17, 2006, 11:54:08 AM
Measuring the DC resistance using a multimeter will not give an accurate answer. When the motor is spinning, it produces a back EMF which opposes the current flow from the battery. The static resistance can be used to calculate stall current but little else.

Neil
Title: Re: Reducing from 12v to 6v
Post by: sweeper on July 17, 2006, 04:47:58 PM
I think you may have misread my suggestion. The idea of the reply was to show that you could not apply a simple Ohms Law calculation to a motor, as had been previously suggested.
Yes, we know all about Lenz's Law and back emf, my idea was to keep it simple without going into the realms of electrical theory.
Most people who aren't into motor theory cannot understand why a machine with a low ohmic value does not take a high current ie as it would appear to if they simply applied Ohms Law to the question.
Title: Re: Reducing from 12v to 6v
Post by: Made it to 80 (25p Richer now) on July 17, 2006, 05:48:00 PM
Blimey I thought this forum was all about model boats not advanced physics where did I lose my way ??? ??? ???
Title: Re: Reducing from 12v to 6v
Post by: dougal99 on July 17, 2006, 09:26:17 PM
Blimey I thought this forum was all about model boats not advanced physics where did I lose my way ??? ??? ???

Watford - 1968 ;D ;D ;D ;D
Title: Re: Reducing from 12v to 6v
Post by: sweeper on July 18, 2006, 04:30:15 PM
Advanced physics? nah, you need an ology to do that stuff (and I ain't got one of them).
This is just the stuff that young 17/18 year old boy electricians have to learn.