RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

According to Futaba Rep, the general rule is 1 servo per channel. 9 channel reciever = 9 servos(including high torque servos) I asked this question at the Toledo show a couple years ago.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Well it's def. an interesting debate to say the least. I guess there are valid points on both sides.

I can think of 3 40% Cardens that are run through extreme 3D constantly without so much as a matchbox which is why I don't see the need for a power expander. I will say they do have 2 receivers though.

Thanks for sharing some of the more detailed knowledge

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Mike described it better than I could, the best analogy I can come up with is a conventional light bulb. The bulb filament will conduct a large amount of current but the voltage drop across it results in energy being dissipated in the filament. In the light bulb this is the desired result, the heat generated produces light via the white hot filament.

In a car,boat, or airplane the wiring and termination are just to route the electricity to the device that is going to make use of the energy. In a model aircraft those devices are servos.

You or at least I do not want to waste the energy in heat generated in the termination or due to conductor loss. Ohms law says current (amps) X current x resistance = watts dissipated

The total resistance in a circuit includes internal battery resistance, termination to the battery, wire from the battery to the terminal, terminal to wire to switch, the switch, the wire to the receiver, the terminal to the receiver, internal in the receiver, the termination from the receiver to the servo wire, wire to the servo, and any terminations between, and do not forget the path all of the way back to the battery.

Every wire, every termination every junction has a definable resistance and current carrying rating.

Every place in the circuit where a voltage drop occurs, and it does occur even during a momentary current spike, the Ohms law equation does not have a time element, robs the servo from energy to perform its job.

Terminal and wire ratings are !QUOT!never exceed!QUOT! values for 100% duty cycles at a given ambient temperatures. Above this you risk thermal runaway and light bulb filament creation.

Now as Mike said the peak current values are momentary and are in fractions of seconds,
thusly no observable physical results of circuit overloading are apparent.

To those of you that are flying your aircraft on a single receiver, essentially pulling 6, 8, or 10
servos through a single 22 gage wire and 3 amp pin .... I hope you continue to have good luck, I simply cannot afford that kind of a gamble

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

There must be a lot of good luck going around!

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Yes there is a lot of luck, however I keep reading about unexplained servo failures that work perfectly after the accident, intermittent reciever failures, just luck I guess.

Luck, the bring together of preparation (or lack there of) and opportunity.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Actually, having disassembled a PowerBox SC-12 and traced things a bit, I can tell you that it is laced with potential single points of failure. There is no redundancy at all in any of the signal paths between the receiver and the servos outputs. Also, in the one spot where there is a measure of redundancy, i.e. a pair of linear regulators, one per battery input, they appear to have compromised that by introducing yet another single point of failure in the form a 47CTQ020S by International Rectifier. That part is a single D2PAK (single epoxy case) housing a pair of Schottky diodes that are internally connected as a common cathode pair. Each diode leg is rated at 20A max forward current.

If for some reason, one of the two diodes in that single package fails, then the full load being drawn will have to travel through the remaining leg. Because of that, it is possible that a failure of that type would result in higher heating of the junction that is still functioning. That extra heating may also contribute to a failure of the only remaining path for power to the servos. It is not a true redundant design. Perhaps there are some PowerBox models that are designed differently that don't have these shortcomings? I've only had the chance to peak at the SC-12 so far.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Yes, there are different models and manufacturers of servo interfaces, i.e., ModelBau PowerBox, Emcotec DPSI and Smartfly each has its own merits and limitations IMO...

And yes while the power routing from a single power/battery is not truly redundant with the SC-12, if one route fails the model fly’s home with halve the battery power. Even at 20 Amps the system is more than capable as a typical 40% model with fourteen 5945/8411’s consumes an average current of 3.6A performing aerobatic maneuvers. Emcotec’s approach differs considerably, specifically with power routing and the dual diode housing as noted above and then some.

I don’t think any of the manufacturers suggest they have redundant signal paths to the servos, I'll have to dig in to that one...

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Mike, are you suggesting two receivers, or an Emcotec/smartfly power expander in the 35-40% stuff. I can not find anywhere in the in the U.S. can you even buy an Emcotec DSPI. One of the Duralite Powerboxes has similar capability to the Emcotec but double the cost. Two PCM receivers are about the same cost as an Emcotec/Smartfly.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

All of this brings me back to a question I asked on page one...

If you used 2 receivers and a properly designed NiMH setup with high-end cells that had a nice low ohm rating would you not be able to build a system that was less money and was more redundant than a power expander setup? IE: 2 switches, 2 recievers, 2 NiMH packs.

I direct you to a battery setup like.. http://www.hangtimes.com/weightcompare.html

I mean you keep seeing people who say the battery channel on a reciever can only handle 3 amps. Yet you are constantly told to buy NiMH packs with the lowest resistance so they can put out the power when needed. You are also told to use the best quality wire with atleast 22awg or better and ultimately if we are well aware that a digital servo can draw 3-5 amps does it seem remotely logical that everyone designs receivers that only handle 3 amps per channel?

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Mike, I think you may have missed the point. The Powerbox SC-12 has a design that uses a common component to route the output of both regulators into one common point. If that one component fails in a way that takes out both paths, then the game is over. It's not a redundant design.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

The US Distributor for Emcotec is Desert Aircraft...

I use a single RX in my models with the Emcotec Mini/Duralite-PowerBox 40/16 for 35% models and the Emcotec DPSI-RV LDO/Duralite-PowerBox 40/16 or 40/24 for the 40% models. I have also used the dual RX set-ups too; and there is the DPSI Twin interface as well specifically designed for two RX’s one is used as a primary and the second comes online as a back-up RX when the primary fails, is locked out or activated from the TX.

When you add the total cost up for a second PCM RX and two fail-safe ON switched regulators the cost for a servo interface is not that hard to pass up considering you have eliminated several ancillary devices together with there associated connectors, electrical losses and shortcomings comparatively and NO you don’t really need these gadgets several goodies will fill the void without the added benefits of servo interfaces thereof.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

I understand that there is a remote possibility of some unknown failure mode of the dual diodes housed in single enclosure utilized by ModelBau could possibly render the system inoperative BUT, this is a distinct and remote consideration although it’s plausible nerveless. Power is supplied to the entire flight system in the event of a single power route failure, thus the REDUNDANT claim I believe. Remember the power routes are short circuit protected and each diode operates independently of the latter common cathode or not. FWIW, not all interfaces use the dual diode housing…

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

That's what I was interested in, which are models that are designed without that limitation. BTW, for a truly robust and redundant regulated power system , you need to use the right linear regulator chip. There are some really superior regulator chips available that eliminate the need for added parts such as isolation diodes on the output. Frankly, I was really surprised to see that part in there. Made me do a double take. Any time you have to add a part to overcome the shortcomings in a design, you're getting less than you bargained for.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

All Emcotec Servo interfaces use two single housed Schotkky Rectifiers for each power route coupling the power prior to the linear regulators...

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

And that makes perfect sense to do. I know exactly why they would do that.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Well my question keeps getting slightly bypassed.. so I am going to have to conclude that a dual reciever setup with 2 mpi switches, and 2 NiMH battery packs is a better solution and costs less. Batteries cost less, no regulators to worry about, and not a bunch of extra gadgets to deal with. The weight penalty will only be 2-4 oz as well.

From what I am reading here if you wanted to get crazy and use 4 switches you could possibly be more redundant than a power expander system... personally that's a little overkill if you ask me. hehe

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

This is the set up that makes more sense to me. KISS is always better....

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Less money yes, more redundant no If a switch or power route fails you loose halve the model with your set-up. If you loose the switch with a servo interface or one power route fails the entire model is powered up with a single power route. The likelihood of the both power routes failing with a servo interface is remote at best IMO.

While its true the OEM connectors are rated for 2-3 amps continuously are needs for high current draw are momentary and with the advent of multiple power pigtails to unused RX channels its all good. Low impedance cells provide high current demands while maintaining voltage even though our needs are momentary the loaded requirements are around 24 amps model/servo dependent while performing aerobatic maneuvers. Current is consumed in spikes and our servos will work with what’s available to them a robust power system will net notable differences in performance comparatively. In the past many modelers simply through more servos in the mix rather than address the need for a robust power distribution system.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

Mike you keep insisting the average current is low, which I can agree on BUT:
The IR drop across the system resistance determines the voltage and current available to the servo(s) at peak load, precisely when it is most needed. The IR drop obviously affects servo response times and ability to actually deflect the control surface as the supplied voltage and current are less than optimum.

Your 200 In-Oz servos are what 150 In-Oz?

I was always taught to design for peak values, and in given conditions factor in saftey margins.
I can understand the concept of underdesign, if everything is structurally lighter then the saftey factor comes back to an acceptable level. After all we are not building a tank.

In my experence electrical designs do not follow this thought process, under design results in less final performance (and introduces unnecessary failure modes) than a design to accomodate known circuit requirements.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

A power jumper takes care of that problem nicely. Still, with two receivers, each with a battery and switch, there is no single point failure in the radio system. You might lose some functions but you'll have plenty to land the model with. Kind of hard to land the model if your single receiver or single power box fails.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

You're on the right track here. This is where a well designed regulator system combined with batteries that have low internal resistance and good cell recovery from instantaneous loads can be helpful. When you add a good regulator system in, something happens which seems counterintuitive to many. The bypass capacitor(s) provide alot of the instantaneous response to the rising edge of the current spike, i.e., they begin to discharge at a rapid rate. As they discharge, the voltage droops and the regulator(s) respond through their feedback network, which effectively shores up the voltage droop that the capacitor can't keep up with. So the current spike effect on the line voltage is tamed a bit. This is a good thing, partly because it does help a little bit with the effort to get the servo motor going (or stopping), but also in good part because it can significantly reduce the induced noise that may sometimes affect the other electronics in the model.

The other thing that the regulator system does it often not considered at all. It has the effect of reducing the effect of all of the resistance behind it. That includes connectors, wires and the internal resistance of the battery. Provided that the battery voltage doesn't dip below the dropout point on the regulator under load, the regulator can make those resistance effects behind it virtually disappear during peak demands. Unfortunately there's nothing that it's going to do about the IR drop along all of the wiring ahead of it that runs out to the servos. That requires a different approach. It's do-able, but probably not of interest for the question at hand.

Anyway, you can still construct a pretty good redundant battery setup with just a pair of batteries and switches. Mike is correct in that it's not a total solution, but if you happen to be using two receivers with a battery/switch on each, you can add a jumper between the receivers and get more protection. The main objection that exists then is "what happens if a battery shorts". Well, that's a pile of trouble no matter how you look at it, but you might survive the event by adding a high amperage, slo-blo fuse in the jumper between the two receivers. One side might keep working. But here again is where using a properly designed redundant pair of regulators can help. Not every regulator has this feature, but some will prevent any reverse flow of current back into a failed battery. So if you arrange for a pair of batteries to feed a pair of regulators into one receiver or a pair of "jumpered" receivers, you have the capability to keep everything working until you bring old Betsy home. See a Tech-Tip at http://www.tech-aero.net to help with some ideas along these lines.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

No argument from me on your assertion. This is exactly why a robust power distribution system is paramount to performance levels. Quality HD gold plated connectors, extensions, switches and components are required to minimize system resistance along with minimum connector counts and low IR power sources. I preach this info on a regular basis; back in the day when everyone started using digital servos we had huge problems with all of the above. The old tried and true stuff simply fell short of the needs for high power servos then and does to this day IMO.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

As noted above the achilles heal with this approach is the common jumper... Yes a fuse may solve a shorted power mode, BUT what happens if a servo or RX realizes a failure mode and bounces undesirable stuff around and through the jumper (unwanted signal contamination) or how about a smoked/shorted servo? Stuff happens anything that increases RF operational headroom IN THE EVENT that it might come in handy is a good thing! Enter servo interfaces…As I mentioned previously and I can’t speak for all OEM’s simply because I don’t have the data, one manufacturer has yet to have a documented failure of said device.

Seems no matter what approach one takes there are considerations...[:@] In the end its all good IMO.

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

This looks interesting for 35% and up. Price seems high at 1st but once you see what your getting its not to bad if your a person using a power box/power expander type device.
http://dreamworksrc.com/catalog/prod...roducts_id=979

RE: Can Futaba R149DP handle 5 5955 servo or do i need a Powerbox?
 

I think you guys are taking it pretty far out on the what if situations.... Lest we forget there is only one batt, one fuse, one diode, gobs of programming, and one pilot on the other end of that plane.

:)

Seems that it all comes down to a point of perception. But I think !QUOT!Thumbs!QUOT! are the worst culprits that need to be worked on....