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Old Jun 12, 2012, 08:44 PM
Registered User
Australia, QLD, Brisbane
Joined Apr 2012
8 Posts
Help!
Dropping voltage for motor+speed controller

Hey All

I'm having a bit of frustrating time working out how to drop the voltage down from a 12 volt battery down to 5 volts for a motor(it's geared and can only handle 5 volts) that sits behind a speed controller (http://www.mtroniks.net/details1.asp...r-marine10.htm)

I have a 5 volt regulator but I'm
A) Now sure how to to hook that up
B) Wondering if there is another way around this without the use of the voltage regulator

How should i do this?

-Trent
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Old Jun 12, 2012, 09:24 PM
Sea Dragon-Lover
Umi_Ryuzuki's Avatar
PDX, OR
Joined Dec 2002
10,202 Posts
What type of model boat is this installation for?

The simplest method would be to invest in a 6 volt battery.

Most regulators will not handle the amperage that a drive motor will demand.
Without knowing what type of regulator you are proposing to install, it is difficult
to say how your set up will fair.

If it is an appropriate regulator for your set up, then dropping the voltage prior to
going to the speed controller would be the best method.

Otherwise, the lower voltage battery is the simplest option.



.
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Old Jun 12, 2012, 09:43 PM
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Tollytime's Avatar
United States, MI, Macomb
Joined Nov 2006
1,251 Posts
Hmmm, I'm not an electric guy, but I do use resistors to lower the voltage for my LED lights in my slot cars.

Then there's the diode thingy.

http://www.ehow.com/how_5819671_redu...stem-volt.html
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Old Jun 12, 2012, 10:44 PM
Registered User
Australia, QLD, Brisbane
Joined Apr 2012
8 Posts
Quote:
Originally Posted by Umi_Ryuzuki View Post
What type of model boat is this installation for?

The simplest method would be to invest in a 6 volt battery.

Most regulators will not handle the amperage that a drive motor will demand.
Without knowing what type of regulator you are proposing to install, it is difficult
to say how your set up will fair.

If it is an appropriate regulator for your set up, then dropping the voltage prior to
going to the speed controller would be the best method.

Otherwise, the lower voltage battery is the simplest option.



.
I would agree to getting another battery though problem is space so having another battery is really out of the question.

the voltage regulator is L7805CV which is under the TO-220 series the PDF has more details (the 5 volt one obviously)
http://www.makershed.com/v/vspfiles/...ages/l7805.pdf
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Old Jun 13, 2012, 02:04 AM
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PDX, OR
Joined Dec 2002
10,202 Posts
I am not proposing to "add" a battery, I propose that you do not use the
12 volt battery at all, and use only one six volt battery for the boat.
Take out the 12 volt battery, and there should be room for two six volt batteries.


Linear current regulators burn off the extra voltage as heat.
They are inefficient, and will increase the drain on the battery.
However with a large enough heat sink, and perhaps water cooling or
good air flow, the heat can be dissipated.

With several of these in parallel you could overcome any issues with current, amp, draw.
Of course each regulator would need to be able to dissipate the heat it creates.
So what you propose, using a voltage regulator, could work, but you would have to design
the regulator circuit to meet the specifications, and needs of the motor under load.

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Old Jun 13, 2012, 02:15 AM
NeverAgainVolunteerYourse lf
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Australia, QLD, Regents Park
Joined Mar 2007
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I'm guessing its an auxiliary drive motor, not your main propulsion which is running off the 12 volt battery, what motor is it, what is it driving, do you know the amp draw?

For relatively low amp draw there is a thing called a UBEC normally used to supply a receiver and servos, there are some that can handle up to 10 amps.

Nick
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Old Jun 13, 2012, 04:57 AM
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Blackpool, Lancs
Joined Feb 2006
2,889 Posts
To give a proper answer, it would be helpful to know what the motor is actually doing, and how long it would be doing it for. If its just a deck accessory, with low current drain, a high wattage resistor will do the job, if the transmitter is programmable, just setting maximum speed to under half would do the job. If the motor is higher current and to be used continuously, a change of battery voltage is the best bet - a high power voltage convertor would probably take as much space as an extra battery.
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Old Jun 13, 2012, 05:23 AM
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Australia, QLD, Brisbane
Joined Apr 2012
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Ah it is just a deck accessory - a working anchor

the transmitter is a aurora 9 so i should think so

and unfortunately i haven't been able to find the amp draw
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Old Jun 13, 2012, 06:00 AM
NeverAgainVolunteerYourse lf
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Australia, QLD, Regents Park
Joined Mar 2007
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Well a highly geared small motor will probably run nicely on a 5 Amp UBEC like these

http://www.hobbyking.com/hobbyking/s...R-VR_UBEC.html

The 7805 VR like has been mentioned is limited in its current capabilities

Where are you in Brissy?

Nick
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Old Jun 13, 2012, 06:17 AM
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Australia, QLD, Brisbane
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I'm at st Lucia
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Old Jun 13, 2012, 04:13 PM
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Joined Feb 2006
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This is another one of those times when a cheap digital multimeter is your best friend. Saves a heck of a lot of guessing.
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Old Jun 14, 2012, 03:19 AM
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Wichita, KS
Joined Jul 2007
212 Posts
Don't go with the series resistor route. Half of your voltage will be dropped across the resistor, and you will need to determine what resistance value is necessary to drop 7 volt across the resistor. If your motor is pulling 0.5 amp, then the resistor would have to be at least 3.5 watt, that's a big expensive resistor. All the power disipated by the resistor is in heat, so the resistor would need to be heat sinked to pull the heat away. This method works fairly well for LED's as their current draw is in the 0.01 amp or less range and the wattage disapated by the resistor is less than 1/4 watt (low priced easy to obtain resistors that don't need heat sinks). Use the dropping resistor method so long as your supply voltage isn't greater than 18 volts. Remember, power (watts) is volts times amps. Leave some margin, don't make the device work at it's 100% capacity continously. That will shorten its life.

Going with a lower voltage battery is a possibility. It sounds like the capacity would not have to be great as it would be used intermittently.

Using the UBEC looks like a good cheap method. Understand the current draw of your motor and choose a UBEC with 10-20% more capacity then the max current draw of your device(s). Remember, starting current is greater than steady state current.

There is also a device called a DC-DC converter. It will convert one DC voltage either UP or DOWN in voltage to the specified value. I use these devices to hold voltage up on engine monitor displays during engine starts. As the voltage drops during a battery start of the turbine engine, the DC-DC converter holds a minimum voltage on the display so that it does not blank out due to low voltage. For example, the battery (24 volt sealed lead acid) will sag in voltage to below 18 volts (pulling 800-1200 amps) due to the starter power requirements. As the battery voltage drops below 22 volts, the DC-DC converter takes over and holds the output voltage to the engine monitor display at 20 volts, even while the battery voltage drops towards 12 volts to the starter. The entire start sequence takes about 35 seconds so the loss of critical engine displays durning starts is not desired.

Having said all that, the UBEC seems to be a similar type device and is a cheap (under $5.00US) method to achieve your 5 volt requirement. Hook it up to the 12 volt battery and you will have a 5 volt supply for your accessories. If you have LED lights, you could run them off the 5 volt supply too. You would need lower resistance 1/4 watt resistors in series ( you only have to drop about 3.5 volts across the resistor).

Have fun!
Derek
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Old Jun 15, 2012, 11:29 PM
Registered User
Waukesha, WI
Joined Dec 2006
257 Posts
Switching regulators

Linear voltage regulators are not very efficient
(meaning low amperage, heat generation and shorter battery life)
Switching regulators are the best way to drop voltage if you have to have
one battery.
Here is a great discussion page about merits of each:
http://www.dimensionengineering.com/info/bec

In US Castle Creations makes one:
http://www3.towerhobbies.com/cgi-bin...2&I=LXSWL3&P=K
Great Planes has another:
http://www3.towerhobbies.com/cgi-bin...?&P=9&I=LXWNS2
Dimension Engineering sells several smaller ones:
in left column of home page DC/DC
here is their biggest:
[http://www.dimensionengineering.com/products/swadjhv
These companies ship internationally.
Hope this helps if you don't go with just another battery-cheapest way out.

Jim Shander
KC9BOT
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Old Jun 16, 2012, 10:28 AM
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Blackpool, Lancs
Joined Feb 2006
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For the kind of usage, I would consider a box of 4 AAA rechargeables, 1000mAH are readily available, and use that as a base guideline for size, weight and price against the electronic alternatives.
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Old Jun 17, 2012, 05:46 PM
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CaptCB's Avatar
United States, CA, Orange County
Joined Aug 2011
1,994 Posts
Voltage Drop

Go with the Castle Creations unit. They are available up to 25 amps, I believe. They come preset at 4.8v, and with the computer link and software, the voltage can be adjusted. CaptCB
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