flyinrazrback

My Feedback: (69)

anyone using a 5735 digital from hitec? i want to use one for pull/pull on a rudder on a 30% edge. Does this require its own power source? I will be running 4 5645s, and the 1 5735, plan on using two switches with 2 1830nimh packs, thanks

Giant Scale

You will be fine with your described set up. The only time I can see using a seperate power source for these servo's would be if you have four of them ganged on the rudder in a 40-50% scale plane. I'd say 6-8 digital servos, a single receiver and 2 battery packs/switches is the average on most 35% planes.

mglavin

My Feedback: (31)

The 5735's are great servos... Speed, accuracy and power unequaled in a giant servo. I've been using them since the were released never had a problem with any of them... I don't use a separate power source, even with a ganged set-up on 40% machines. I don't think it's necessary, certainly not on a 33% model.

flyinrazrback

My Feedback: (69)

Thanks for the info, what I needed, good to head they are strong, would you suggest on the pull pull setup to run the cables to the servo, or use a tiller type setup? wingspan is only 90", probably 20-22lbs finished. Thanks and happy holidays

mglavin

My Feedback: (31)

I like the tiller-bar system's and pull-pull. Weather or not you can use push-pull or a pull-pull system is dependent on the requirements of your model for balance...

What are you planning on assembling?

Giant Scale

Well Mike here's a question for you..

You have 4 5735's on ailerons, 4-5735's on rudder,
4-5945's on elevators,1- throttle and 1 choke servo. Assume we use two receivers and want a redundant battery system. How many packs and what size do you use?

mglavin

My Feedback: (31)

What are you building? That's some line-up of servo power...

I'd have to re-think the power requirement's for this bird... That said, if possible I would locate the power source as reasonably close to the servo's as possible.

What battery technology or type are you contemplating?

Rudder servos in the rear or in belly?

At least three HD switches with multiple contacts, 2700mAh, 5-cell packs for the radio system. Probably be inclined to go with 4000mAh capacity packs. One for each side of the model and the third for the rudder servos.

Drive the rudder servos from one RX to prevent the possibility of multiple commands causing the giants to work against themselves.

How redundant do you want to go? I like the idea cross feeding the RX power systems. Multiple output's on the switches and batteries is a viable concept. Custom heavy gauge wiring should be fabricated IMO. Eliminate as many connectors, wyes and excessive wire lengths as possible.

A recent test confirmed the 5945 easily consumes 2.1A on a 6V system. I'll have to test a 5735.

flyinrazrback

My Feedback: (69)

My setup is going to be 1x 5645 per aileron, 1 5625 per elevator half, 1x5735 giant digital pull pull on the rudder, standard on throttle, 2x1830nimh batteries, 2 switches, going on a 21lb lanier 30% edge 540

Giant Scale

Mike
All these servo's are going in a 50% CAP 232. The rudder servos will be mounted in the belly using a pull-pull system. I may use the 5735's or 805's with a matchbox depending if I need more holding power. I have been using NiMH packs from
superbatterypacks.com but have not decided which packs to use yet. They have some hi capacity Li ion packs out that I may try. I may use 4 packs and 4 switches for the receivers and servos(ailerons,elevator,throttle and choke) for redundancy, and a seperate pack for the rudder servo's. Cross feeding the receiver with a jumper is an option but I don't know much about doing this. Obviously with a plane this large, safety is a major concern. All those batteries are heavy so this will have to be taken into consideration. Let me know if you have any more ideas on this install.

mglavin

My Feedback: (31)

How heavy is the Cap expected to be?

Four 5735's on rudder is most likely overkill IMO. I have several 40% Aerobats [typically 38lb's] with two of these servos with nary a problem for rudder power. My flying partner has a 42% Edge again same servos but three of them, more is better I guess, LOL... Are you aware of the holding power of these servos on 6V? The 805's are very good servos as well, slightly more power, not as much holding power and slower with less accuracy.

I think three in your case would be more than adequate, but what the heck it is a big model. The secret here is going to be providing adequate energy to these servos. The standard configuration with 22 gauge stuff passing through one switch and source pigtail doesn't make it IMO. A bank of multiple contact switches and multiple pigtails from each battery pack/source would be preferable. You could easily fabricate a power buss with heavier wire all the way to the battery for the 5735's on rudder servos. The 805's could benefit from a Matchbox type device, whereas the 5735's don't need it...

Heavy duty multiple contact switches can be mounted inside the fuse side by side with a piano wire interconnecting them with a single wire/pushrod operating them form the exterior of the fuse. Three banks of switches would do the job. Left, right and rudder.

You can provide redundant sources of power for the RX's by simply having multiple pigtails from each battery feed one switch for each side of the model. I wouldn't worry about diodes for isolation. The likely hood of a short in this specific area is very, very low and you'd only be addressing this one area when several areas would be unprotected.

I to am using Super Battery Packs equipment, presently using and compiling data with their 4000mAh Lithium Ion cells. These are 2000mAh cells wired in series-parallel. Approximately 8 amps is available continuously. Due the nature of your unusually HIGH current demand, I think I would stick with NiCd or NiMH technologies. These cells are capable of providing much more current on demand... Or you could use multiple 4000mAh Li-Ion's for each source requirement, each battery pack would require multiple switches and switching regulator's capable of passing high current draws, in this scenario more components are required elevating the expense and increasing the likelihood of failure points, not to mention power system resistance would go up. That said, this would be a state of the art system.

Giant Scale

Mike,
The CAP-232 should weigh 60-65lbs ready to fly, powered with 3w-240. Blaine Austin has one and I think he is using 4-futaba 5050's, he also has a 50% edge that is using 4 multiplex jumbo's on pull-pull for the rudder. Which switches would you use? Do you have a particular make and model in mind? Using a conventional redundant system 5 packs would be used, 4 for the receivers/servos and a seperate pack for the rudder servo's. In your opinion would this provide enough power?

jrjohn

My Feedback: (24)

I'm confused. I was told that digitals use more battery. What am I not understanding???

I was on the servo city site looking at the 5945mg digital Hitecs. they show at 6 volts 3MA/idle and 230MA no load. with 180 oz of torque

then I look at the 945MG (none digital brother) and it shows the following specs. at 6 volts 6.6 MA/ idle and 500MA no load with 152oz of torque.

the 5735 say 3 MA/idle and 840MA no load!

Am I reading something wrong? it sure looks like the digital has more torque and uses less battey.
However the 5735 looks like a pig

Information obtained on the Servo City website.

John

mglavin

My Feedback: (31)

C&K makes a quality switch which is rated at (RMS) 12A @ 28VDC, PN (part number) S602031SS03Q. This same switch is available from Digi Key (http://www.digikey.com) under PN CKC5013-ND, around $3.00 if I recall. Remember this is a DPDT switch. It essentially is two switches in one housing, built in switch redundancy. The secret is to wire accordingly and have multiple pigtails from each battery.

I would definitely wire the switches and batteries for redundant power supply to the RX it serves and also to the 2nd RX, it just makes sense to me.

60-65lbs. is a lot of model aircraft... I can't see the additional weight of a redundant power system being very significant... If your anally weight conscious use three batteries wired for redundancy through the switches to both RX's. However two battery packs per RX sounds reasonable to me, why not two for the rudder system as well? Conventional NiCd in the AA size can easily handle 40A plus, NiMH 30A plus. Either of these should be fine for your application. With consideration for redundant battery packs I'd look at NiMH 5-cell 2700mAh battery packs or better. I have seen information that was compiled from a credible source that suggests that actual power seen by or at the servos with a 6V system was 4.2V under load....... This is a significant loss...

This animal your contemplating would most likely work fine with less servo [s], providing the power distribution scenario was optimized and capable of providing the required energy to the servos, rather than add more servos and the weight and failure points associated with them... I guess where I'm going here is it rarely done correctly, typically we just add more servos to make up for the lack of servo power.

mglavin

My Feedback: (31)

John

Hopefully the answer to your questions is noted below. This information was authored by a known RC Techno Guru, I was interested in an in depth analysis and laymen's understanding of our servos. Below is what was provided to me.

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Let’s talk about permanent magnet dc motors like we use in servos. The torque is proportional to the current. More current more torque.

At rest, motor not rotating, the current is equal to the voltage at the motor terminals Vt divided by the resistance R of the winding. As the motor starts to rotate it becomes a generator and produces a back emf (voltage) that has a polarity opposite the terminal voltage. The current then becomes Vt minus Vemf divided by R. The faster it rotates the greater the back emf and the lower the current. Therefore a motor at no load draws relatively low current.

As we load the motor it slows down, the back emf drops, the current goes up to create more torque to overcome the load. If we stall the servo, the back emf goes to zero, the current goes back to Vt divided by R and if you’re lucky you won’t burn up the servo. In the old days the servos were designed to withstand being stalled but I’m not so sure about the modern day servos.

So the answer to your first question is no, digitals do not pull max current regardless of the load. More load more current.

Let’s talk about model airplane servos. The output circuit of the servo amp is essentially four switches in an H configuration. Two switches are closed to drive the motor one way and the other two to reverse it. This is the same for so called analog and digital servos. At large error signals the switches are closed continuously until the servo gets to the commanded position and then all switches turn off. Large error signal means a large difference between the commanded position and the actual position like you would get if you slammed the stick over. You can now see that if analog and digital servo amps are driving the same motor they will draw the same current.

The difference comes at small error signals. Both types pulse the switches at small error signals in order to slow the servo down so it stops without overshooting. The so-called digitals pulse more often with shorter pulses but the net result is they are closed for a greater proportion of the time and therefore the average current is higher and there is more torque at small error signals. This is why they feel stiffer when you try to drive them off neutral.

The limit on how much current you can put into the motor is a function of the windings and we are getting close to the point where they will burn up if you put too much voltage on them which would in turn put too much current through them either at stall or during oscillations where it is continuously reversing itself and never builds up a back emf. So the plateau is really a cliff where it burns up if you increase the current.

Your final question. In an ideal system design your servo would drive to the commanded position and it would null out and draw no current. Then it wouldn’t matter if you had 4.8 or 6 volts except it would get there faster with 6. In the real world the servo might not get to the commanded position because as it gets close it starts pulsing, the current goes down and the torque goes down so it can’t overcome the load. If you have more voltage, 6 instead of 4.8 the current is higher, the torque is higher and you will get closer to the commanded position. Again digitals will do better because they have a higher average current under these conditions than the analogs. So the answer is 6 volts will do better under heavy loads but you could have a design where 4.8 is more than enough.

The problem I see now a days is we don’t really know what the voltage is at the servos due to all the drops due to high current through inadequate connectors. I’m betting there are a lot of people getting less than 4 volts with a 5 cell pack under extreme conditions like starting six servos in a snap roll.

dirtybird

My Feedback: (5)

You might consider putting a battery pack at each servo position and control your servos from the receiver with a fiber optic connection. There is a fellow in Canada that sells fiber optic servo connections I think that could be modified to do this. It sure would simplify your set up.