High Voltage servos on Futaba receivers - options ?

[gmorales83]

Quick question:

Has anyone tried using the new higher power servos Hitec 7950 or Jr 8711 HV with unregulated LiPos on current Futaba 2.4 receivers?

I know one option is to use the SmartFly Comp 12 and just plug the battery straight in with no reg. Can I use a standard receiver instead? and no, i'm not switching to Spektrum just so I can use their 9100...although that is a VERY nice option to have.

Futaba...are you listening ?

GM

[Pilotpete]

Futaba FASST receivers are good for at least the full charge level of 5 cell Nixx and 2S A123, but as to whether they can handle the full 8.4V level of 2S lipo, good question. I agree that Futaba and others should address this issue.
One option is the relatively simple and inexpensive power expander from Servo City, it could be configured to run the full lipo voltage to the servos and use a regulator to power the receiver.
http://www.servocity.com/html/servo_power_boards.html
The other option would be to go with 2S 123 cells for your power source, but then you will not get the same torque from those HV servos.
Best of luck,
Pete

[gachezem]

sub

[Julez]

I accidently ran an 606fs on 3s, and there was no problem.

When going from 5s Nixx (6v) to 2s (7.4V) Lixx, you have 46% more heat that the regulator has to dissipate. Considering that Futaba some time ago reduced the current demands of the Fasst recievers to 50%, today's recievers on 2s Lixx run even cooler than early recievers on 5s Nixx.

[Mithrandir]

	Quote: Originally Posted by Julez
	I accidently ran an 606fs on 3s, and there was no problem. When going from 5s Nixx (6v) to 2s (7.4V) Lixx, you have 46% more heat that the regulator has to dissipate. Considering that Futaba some time ago reduced the current demands of the Fasst recievers to 50%, today's recievers on 2s Lixx run even cooler than early recievers on 5s Nixx.

Umm... if the Rx is drawing the same power, then shouldn't the higher available voltage reduce the current??? And the heat comes from the current ... No???
Or would the Rx draw the same curent even if the voltage is higher???

Enquiring minds want to know!! lol


I ran a 14 ch FASST Rx on two cell LiIon for over a dozen flights.... (accidentally forgot to replace reg) and I didn't notice any change in performance.....

Prolly you can get away with doing a 2 cell Li
Prolly "FUTABA" is going to tell you not to.

[olddog1956]

I was told not to by futaba.

[Paul5992]

By a 12x or a 9303.

[Wojcigitty]

	Quote: Originally Posted by Mithrandir
	Umm... if the Rx is drawing the same power, then shouldn't the higher available voltage reduce the current??? And the heat comes from the current ... No??? Or would the Rx draw the same curent even if the voltage is higher???

My understanding is that regulators within the receiver that supply it current are linear regulators and not switching regulators...that means that excess voltage is "burned up" as heat. The current draw does not change.

[MODE 1]

sounds like a recipe for disaster

[Julez]

No, it does not.

I once confirmed that the internal voltage is stabilized to 3.00V by a low drop linear regulator.
This means, that, as said above correctly, the excess voltage is "burned".

The excess voltage is 3V with 5s Nixx, and 4.4V with 2s Lixx.

The recent Fasst RXes with dots draw around 40mA, the first generation used 70mA.

So a first-gen RX will produce 0.21W (5s Nixx) vs. 0.308W (2s Lixx).

A dotted RX will produce 0.12W (5s Nixx) vs. 0.176W (2s Lixx).

It is easy to see, that a current reciever with Lixx will run even cooler than an old one with Nixx.

[Wojcigitty]

Thanks for posting real data, Jules. Good to know.

[gareth.ky]

	Quote: Originally Posted by Mithrandir
	Umm... if the Rx is drawing the same power, then shouldn't the higher available voltage reduce the current??? And the heat comes from the current ... No??? Or would the Rx draw the same current even if the voltage is higher??? Enquiring minds want to know!! lol I ran a 14 ch FASST Rx on two cell LiIon for over a dozen flights.... (accidentally forgot to replace reg) and I didn't notice any change in performance..... Prolly you can get away with doing a 2 cell Li Prolly "FUTABA" is going to tell you not to.

I saw this and wrote a very details response but I erased it. Its too much geeky detail.

The short answer is that you should follow the manufacturers specs. Use some kind of power distribution (like the Smartfly) if your receiver is not rated for the voltage of your supply batteries.

The long answer is that Ohms law states Current = Voltage/Resistance. The Rx has a fixed resistance and adding more volts will increase the current drawn from the battery.

The chips in the Rx operate at 3.3v. There is a small Linear Regulator in there that drops the input voltage to 3.3v. the extra current drawn from the battery is converted into heat. Heat kills electronics.

Too much voltage and the linear regulator will succumb to thermal runaway and fry like the BEC circuit in a speed controller. BEC circuits have to deal with the variable resistance and current draw of servos. The regulator in the Rx only has to deal with a constant current draw of the chips and the radio receiver. The components behind the regulator in the circuit have a fixed resistance and current draw. Furthermore it's a small current draw. That's why you can get away with a somewhat higher voltage. This will shorten the lifespan of the regulator and someday cause it to fail.

We can get into details about how those linear regulators work, transistor trace separation, dielectric constants, leakage etc. Really its not necessary. Electrically there is no difference between a receiver and a fuse. You pump enough volts in and it will blow.

Abuse the specs with caution.

[Julez]

Gareth,

I'm sorry, but you are mistaken on some levels.

First, you contradict yourself here:

	Quote:
	The long answer is that Ohms law states Current = Voltage/Resistance. The Rx has a fixed resistance and adding more volts will increase the current drawn from the battery.

	Quote:
	The regulator in the Rx only has to deal with a constant current draw of the chips and the radio receiver

What is ist now? Increased current, or constant current?

You can believe me: No way will the current consumption of a Fasst RX depend on the supply voltage.

	Quote:
	Furthermore it's a small current draw. That's why you can get away with a somewhat higher voltage. This will shorten the lifespan of the regulator and someday cause it to fail.

This is plain wrong. I could look up the specs for the regulator in use, but most of them are rated for at least 80°C.
In our recievers, they are so underpowerd, that they become luke warm at best. If one uses 2s Lixx now, and they become a little warmer than luke warm, this will cause it to fail - after 1000 years or so. Seriously, when operated within specs (and I speak about the specs of the regulator manufacturer, not Futaba), lifetime of the components is really not an issue.

	Quote:
	Too much voltage and the linear regulator will succumb to thermal runaway

There is no thermal runaway concerning linear regulators. Every regulator I have overheated so far, decreasd it's output voltage. I do it routinely with ESCs to know their limits. And I have never had any permanent damage.

But what is more important: Before our regulator reaches a critical temperature, the ML2724 transciever will quit. I found this out during my tests about which component caused the heat-related problems some time ago.

So, in a nutshell:

- A current reciever with Lixx will run even cooler than an old one with Nixx.
- The regulator laughs at the load of either battery option.
- The regulator is not the critical component when it comes to heat-related problems.
- From the fact, that there are no problems reported with the old RXes and 5s Nixx, we can conclude that there will be no problems with new RXes and 2s Lixx, as the latter constellation runs cooler.


Cheers,

Julez

[gareth.ky]

I think we are actually both saying basically the same things.

I think you are still a little confused about the regulators power usage vs the receiver components themselves. Its a series circuit.

The ML2724 spec sheet gives us enough information to do the math. Max operating voltage is 5.5V and the nominal voltage is 3.3V, minimum is 2.7V. Receive mode current draw is 55mA.

If you were to measure the voltage and current used by that component in the circuit it would be constant. The regulator regulates the voltage to that component. It's the regulator itself that burns up the extra power and turns it into heat.

It would be nice to know what the regulator actually is so we could just do the math. Now that im thinking about it I want to go do this for the new Hitec stuff.

[Julez]

What you write is also correct.

But I still fail to see the claimed problems when operating a Fasst RX with 2s LiPos...