RC model boat receiver power.

[davelectronic]

Recently picked up a model boat hull, it's a displacement type. Planning on using 850 size brushed DC motors, normally the RC receiver runs off 6 Volts 4 X AA batteries in a holder. I'm not sure if this will work, but for extra running time I was thinking of using the system voltage. Being 12 Volts I was going to drop the receiver voltage with a 78S06 linear voltage regulator for the receiver and speed controller, and servos, speed controller signal only. The motor power would come from a decent size SLA battery. So in summary, would the receiver run off of a power supply of 6 Volts DC from a linear voltage regulator ? Thanks for reading, any thoughts appreciated.

[Benta]

Most RC motor controllers have BEC (battery elimination circuit) which will supply your receiver and the servos with 5 or 6 V.

[jpanhalt]

Have you looked at the current requirement for your servos?  A linear 78xxx will not supply that current from 12V without getting quite hot.  Switch mode buck converters capable of doing that and more are available from hobby sources quite cheaply.

[themadhippy]

From experience what ever supply you end up with just make sure a flat battery dont end up with the throttle wide open  and no steering

[cstratton]

Have you looked at the current requirement for your servos?  A linear 78xxx will not supply that current from 12V without getting quite hot.

This is of course a concern, however a servo is not typically active with 100% duty cycle.  Also a big heatsink is quite possible in a boat.

Switch mode buck converters capable of doing that and more are available from hobby sources quite cheaply.

Indeed, however one should make sure that noise of a given switcher isn't reducing receiver performance.

As others have pointed out, most ready-to-go RC motor controllers (ESC's) already have a regulated output for the receiver and steering servo.

If going a fully DIY route, I'd personally look into using two distinct regulators - one for the radio chip and MCU comprising the DIY receiver, and a separate one for the servos.

One of the concerns is that a "stalled" servo (water plants or fishing line jamming the rudder, etc) could draw a very high amount of current on a continuous basis and potentially put a regulator into shutdown, killing the radio as well.

[james_s]

Things have changed quite a bit in the RC hobby. I haven't seen a SLA battery used in many years, everything is lithium ion now. Usually the speed controller has a regulator on it that powers the receiver but you can also get separate regulators.

[cstratton]

Things have changed quite a bit in the RC hobby. I haven't seen a SLA battery used in many years, everything is lithium ion now.

They still make a lot of sense in displacement hull boats if they can be positioned as part of the ballast that's going to be needed anyway, and they're cheap and robust.  Lithium packs mostly show up there borrowed from what's on the market to support settings where they actually make sense, like aircraft, cars, and planing hulls.

Also devices intended to work with 3S lithium will typically work with a 12v gel cell, provided voltage tolerances aren't too too tight and voltage monitoring lockouts not too aggressive.

[james_s]

They still make a lot of sense in displacement hull boats if they can positioned as part of the ballast that's going to be needed anyway, and they're cheap and robust.

Also devices intended to work with 3S lithium will typically work with a 12v gel cell, provided voltage tolerances aren't too too tight and voltage monitoring lockouts not too aggressive.

Yeah I suppose if you need the ballast it can make sense, and if you don't care about being able to rapidly recharge. Regardless of the power source the receiver will normally be powered by a regulator built into the ESC. Brushed ESCs are not as common as brushless but they are still widely available.

[cstratton]

Yeah I suppose if you need the ballast it can make sense, and if you don't care about being able to rapidly recharge.

The ballast requirements and modest operating speed of a displacement hull typically mean you can readily carry enough battery for an entire boating session.

Needing to swap out/recharge the pack is more a plane/car/racing boat thing.

[james_s]

Yeah I suppose if you need the ballast it can make sense, and if you don't care about being able to rapidly recharge.

The ballast requirements and modest operating speed of a displacement hull typically mean you can readily carry enough battery for an entire boating session.

Needing to swap out/recharge the pack is more a plane/car/racing boat thing.

That's what I've been used to, I had RC cars when I was a kid and I've been flying RC planes for quite a few years now as an adult. I've never had a displacement hull RC boat.

[Benta]

From experience what ever supply you end up with just make sure a flat battery dont end up with the throttle wide open  and no steering

How on earth would a flat battery result in a DC motor running "full throttle"?

Are we talking over-unity here?

[Benta]

@davelectronic:
That hull looks like a tug or a shrimp-fisher. The 850 motor will probably have too high RPM for it.
Apart from that, a SLA as ballast is quite a good idea.

[themadhippy]

How on earth would a flat battery result in a DC motor running "full throttle"?

in my case  the receiver and servos  were  fed from  one supply,and a separate pack for the motor.This was in the days when your speed control was a rheostat driven by a  servo ,no power and the servo stayed were it was.

[Benta]

How on earth would a flat battery result in a DC motor running "full throttle"?

in my case  the receiver and servos  were  fed from  one supply,and a separate pack for the motor.This was in the days when your speed control was a rheostat driven by a  servo ,no power and the servo stayed were it was.

Well, welcome to the '60s... I don't think I've seen a rheostat since then.

[davelectronic]

Thanks for your replies, yes the 850 size motors are for tugs and displacement type hull from 30 inches and above. My hull is just shy at 27.5 inches long. I've used my Son's weight lifting weights to see what the maximum weight load would be, it comes out at 6.25 kilo's to the natural water line. I hadn't thought in terms of lipo or NiMH battery, I only thought SLA for capacity and voltage, quite a few lipo / NiMH are 7.5 volts or near it. The hull was designed for 2 motors, and twin rudder. So I've got to keep all the weight with in 6.25 kilo's or less if possible. The speed controller I have in mind is rated at 30 Amps 12 Volts, as I know it the signal wire goes to the receiver as do the servos. And the main motor power has its own leads from the speed controller. It's the receiver servos and speed controller signal that need 6 Volts to power them. I know the limitations of a 6 Volt linear voltage regulator, but could Increase that with a bjt power transistor. But a buck converter does sound a good idea, I don't know how fussy the receiver would be on clean power. As usely the receiver is run from 4 X AA batteries for that 6 Volts. I just thought I'd get longer run time if I took 6 Volts from the main system battery. What I had in mind for an SLA is somewhere between 7.0 to 10.0 Ah battery. The higher is 3.8Kg in weight so might be pushing it, when building materials and motors electronics and running gear are added. But 6.25 kilo's is a reasonable weight to work with.

[Benta]

Again: almost all RC motor controllers will output 5 or 6 V for receiver and servos.
There's no need to build your own regulator.

[james_s]

Well, welcome to the '60s... I don't think I've seen a rheostat since then.

They were standard up through the 90s. Electronic speed controls existed but they were quite expensive. Seems like they cost around $200 in the early 90s which was quite a lot of money back then. A servo driven rheostat was a fraction of that.

[davelectronic]

I remember the rheostat servo speed controller from back then, a friend had a car with one in it. A 30 Amp 12 Volts speed controller for marine is £39.00 still expensive, but it's the option I'm going to take. Hopefully I can come up with the best power efficiency for the electronics. Thank you for all the help.

[james_s]

That £39.00 ESC sounds pretty reasonable to me, especially for a marine one. It will almost certainly have a regulator built in to supply power to your radio gear. Even a linear regulator will be fine, I use ESCs with linear regulators in most of my airplanes and those have 2-6 servos, they're not all active under full load at all times. In a boat the load will be even lower since it's typically just a single servo moving a rudder. 

[davelectronic]

Yes I'd imagined the current drawn by the servo would be reasonable, I'm only going with single rudder, so no prop wash against that. And it's just the single rudder servo and the speed controller. The biggest current drain will be the battery to motor leads.