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L.V.B.P.R.C. (Low Voltage Battery Powered Radio Control)
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 Posted: Fri Dec 1st, 2017 05:57 am
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bobquincy
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These are just some ideas I am tossing out for comment or to see what others are thinking.


Low Voltage Battery Powered Radio Control or LVBPRC, a long enough acronym to please anyone.
By "low voltage" I mean one lithium cell (3.7 V nominal) or less although most of my models run on 2.5 V.

Why low voltage?
    There are some good reasons to design a system for low voltage:
        a single cell is usually the smallest battery we can get;
        a single cell has the best energy/volume package;
        no concerns about balancing cells;

       

Disadvantages:
    higher current draw (greater IR losses);
    still need a voltage converter (but only for some low current devices);
    requires careful selection of components to operate reliably.


Most of the BPRC models I build are stuck with 2xAA power (2.5 V of NiMHs) so I had to design around that.  Most radios require at least 3 V so a converter is required *but* it does not have to power the motor!  Powering the radio or controller with 3+ V is usually required but some motor drivers work down to almost 0 V so the high current part does not have to be boosted.

Why not use a voltage converter?
    Voltage converters work ok but initially lose 10-15% or more.  Voltage converters can shut down when a brief power requirement (like starting a motor) exceed their capacity.  Some voltage converters can be destroyed by a brief overcurrent surge.  Boosting 2.5 or 3.7 V to 12 V is not efficient.  Some inefficient (think cheap) low voltage motors take more current than commonly available converters can supply.



What's wrong with 12 V?
    There are no single cell 12 V batteries so we have to make battery packs from multiple cells: 3 LiPo or 10 NiMH.  This adds complexity and wastes space.  We may have to balance-charge the cells (some LiPo cells do not require balancing).




Why are our trains 12 V ?
    Legacy.  HO and other scales have been 12 VDC for a long time.  The 12 V has stayed for compatibility with existing systems but it may not be a good voltage for modern systems.  Computer chips started at 5 V, then 3.3 V, 1.8 V, and down to 1 V and less, all to reduce power consumption.  Our trains can do the same *but* we have to change the motor.

Are there any 2.5 - 3 V motors?
    Sure, Mabuchi makes millions of them.  The 130 size motors can be found for less than $1 but as expected they are not great motors.  Faulhaber, Maxon, and PortEscap make precision low voltage coreless motors that work great in trains.  New ones are $$$ but surplus or used ones can be had for less than $10.

What about LEDs?
    White LEDs generally take more than 3 V to reach full brightness.  Not that we need full brightness since it is usually too bright for our uses.  Still, 2.5 V will not light a white LED so we do need a small voltage converter.  Since many computer chips run at 3.3 V there are plenty of inexpensive converters for that voltage.
    3.7 V lithium systems would not need a converter.



Don't most RC receivers also take more than 2.5 V?
    Yes, for that we also use the 3.3 V converter.


But the ESC / motor takes a lot of power and won't work with the small voltage converter.
    Yes again, so we don't power the motor from the receiver's built-in ESC.  We use the outputs to control an external motor driver that will operate down to less than 2 V.  This powers the motor directly from the batteries, no need for a high power voltage converter!


Sounds like a lot of wiring.
    Maybe.  The external motor driver gets two wires from the ESC, two from the batteries, and two to the motor.  Six wires is not that bad.


So what are the savings?
    Single cell lithium or 2 NiMH, space and $ savings; at least 15% improvement in battery life by avoiding a high current voltage booster; balance charging is not needed;

Last edited on Fri Dec 1st, 2017 05:57 am by bobquincy



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 Posted: Fri Dec 1st, 2017 06:16 am
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Rod Hutchinson
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This motor crossbox combo I find extremely reliable with a 1S LiPo
low volt motor/gearbox/crossbox combo

Last edited on Fri Dec 1st, 2017 06:16 am by Rod Hutchinson



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 Posted: Fri Dec 1st, 2017 05:44 pm
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Bernd
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Bob,


I like what you are saying. I'm looking at converting several projects for 6 volt motors.


Keep the info coming.


Bernd



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 Posted: Fri Dec 1st, 2017 11:44 pm
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fallen
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Most of the 009 narrow gauge locos I have converted (all but one in fact) run on a single cell LiPo and no convertor. A single cell is easier to hide in a small loco body.

The single LiPo is ideal for Deltang Rx4 receivers of course but also this setup works well for modern n gauge commercial chassis like the Kato four wheel tram chassis and the current outside frame 08 chassis, both using the original motor. It also works well if you replace the normal motor in many other chassis with a small modern motor, not necessarily a specific low voltage one, just a modern efficient motor.

Keeping it simple makes for easier installation and charging and works really well for me.

Frank

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 Posted: Sat Dec 2nd, 2017 06:32 pm
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Bob R
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Bob,
Great discussion to start.  I have made several single cell Lipo powered engines using the Polulu step up regulators and often think about changing the motors to lower voltage.  Using 3 volt without a regulator or 6 volt with a 6 volt regulator would be more efficient.  I use Del Tang and the voltage requirement is only 3 volts.  
The legacy 12 volt systems are outdated and it would be great to see others approaches and results.
Your onto something here.....thanks.



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 Posted: Sat Dec 2nd, 2017 08:47 pm
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Helmut
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Well, well, those who honestly search will find.



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 Posted: Sun Dec 3rd, 2017 01:22 am
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bobquincy
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Another search, one of my 1S N scale BPRC models:
http://freerails.com/view_topic.php?id=7737&forum_id=45&highlight=monorail
similar to Helmut's model, even the same motor (except mine is 26:1 gearing).

I only run on batteries because I only have one rail and even that is non-conductive.  ;)

My main reason for starting this thread was to spur interest and discussion for less than 3 V power, where we have to use a step-up converter for the receiver but not for the main motor supply.

boB



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 Posted: Mon Dec 11th, 2017 03:40 am
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Simon M
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Just took a look for O gauge, ABC gears does consider 6v applications.

http://www.abcgears.co.uk/html/motor_information.html



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