Servos HV Servos - How do they get used?

[Derek]

I've noticed, on a lot of newer and large helicopters, HV servos are being used? Many of them up to 8.4 volts. Obviously, if I were to use my LiFe Rx pack (6.6v), I wouldn't be able to get full use of the HV servos. So, I guess, here is a short list of my questions:

1. How many volts can a Spektrum rx handle? Is there a limitation?
2. Instead of the 6.6v LiFe rx pack, what would I use to power the Rx and servos?
3. Are HV servos required for larger helicopters or is it more for the higher torque and speed that they can produce?

I know that Question Number 1 doesn't relate to this forum section but it's a part of my "overall" question.

 

[HeliDinoRC]

Hey, Derek! I can answer #2....I am using a 2S LiPo in my Goblin 630 with the iKON FBL and 2 DSMX Sats. I made power cable with two servo leads that plug into the LiPo with an EC3 connector. I removed the signal lead and plug both of the servo connectors into the FBL. The servos are then powered through the iKON FBL.

Also, a Spektrum AR7200BX, I believe can handle 8.4V......can someone confirm this?

 

[pvolcko]

Yes. 7200bx is 8.4v max.

And as you guessed, they are faster and have higher torque. They also chew up more power, get hotter, more or less require metal cases, and in high load applications can over stress bec based power systems. Also cost more. So it's a balancing problem like all the other stuff we buy for this hobby.

 

[Derek]

What servos are you using in your 630? and....could you possibly upload a picture of short video of the wires that you made running from the lipo to the Ikon? That would be appreciated. I have the Ikon on my 600e Pro and I'm using a 6.6v LiFe battery. I also have the CC Pro BEC. The BEC has two leads that go to my Ikon, so I imagine that our setups aren't too dissimilar....as far as the wires.

anyway...if you could show me a picture or video of the wires you made, that would be cool!

 

[HeliDinoRC]

Very similar, the difference is that the two leads running to your iKON from the BEC will run from the Rx battery.


In the picture you can see the 2S Rx Battery layed out as if to plug into the cable I made. The two servo leads are soldered into the EC3 connector (both red to positive side and both black to negative side of the battery/EC3) and then run up to my iKON and plug into the AUX ports....DO NOT PLUG INTO THE BIND PORT....bad things will happen! The 12 gauge ground wire running to the black ESC wire is there as a reference ground....to be honest, this is there because Paul told me to! LOL! At first thought you would think that two separate circuits are being utilized: one is the 12S batteries to power the ESC and motor; two is the 2S Rx battery to power the FBL and servos.....this is not necessarily the case as you then plug the throttle lead from the ESC into the FBL, which then combines the circuits, so to speak.

I had this discussion with Paul on here. Here is the link

Paul, can you expand on this?

 

[Derek]

that's awesome! Thanks for the picture and the explaintion!

 

[HeliDinoRC]

My pleasure man! I'm glad I could help....

Forgot to answer one of your questions: My servos are RJX FS0521 HV Cyclic and Tail. Both are rated at 6-8.4 volts. The specs are killer! They are actually slightly faster than MKS and Torq!

There are many ways that work. I think I am going to like this setup for this bird. It's nice and simple. Plus I can bench test with out having to disengage the motor. Yeah, I got another battery to keep track of but I have two and from what people say the Rx doesn't drain the batteries that quickly. I just need to remember to keep the second one charged and with me when I am flying the Gobby. I am thinking of hooking up a voltage sensor so I can keep tabs on the battery through telemetry in my Tx.

One thing I forgot to mention is that I ran two power leads from the Rx battery into the iKON because I thought that two wires carrying the amp load would be better than one as they are rated at 3 amps (I think) and it offers a little protection if one should uplug........then again if one uplugs then they both would probably.......I'm thinking to much again!

 

[pvolcko]

On the wire connecting the negatives of the two batteries. It is not strictly necessary when dealing with an opto-isolated throttle input in the ESC and a digital PWM type signal which is used for throttle signalling. But it is a good idea to have it none the less.

In general there are two issues when dealing with independent batteries and circuits which have the potential for connection:

1) Due to the negatives/commons not being at the same voltage level, when the two circuits are connected in a way that they are not isolated it can result in unexpected current draws. For isntance, relative to a reference earth ground the voltage of battery 1 negative may be .6v and the negative of battery 2 at .3v. This will result in a .3V differency between the two "commons" when the circuits get connected. If the circuit doesn't isolate the two power sources via transformers, opto-isolators, etc. then there can be unexpected current draws through it which can burn out circuit components or cause electrical noise (60hz hum in AC circuits are an example of this), etc.

2) Even if the circuit can handle a current generated by the mismatched supply voltages in the two connected circuits, depending on the type of signal involved it may lead to one circuit misjudging what the real value of the signal is. For instance, if the signal is a simple DC voltage and it is 1.5V relative to the negative of battery 1, when the circuit 2 sees that 1.5V level, but it has a reference voltage .3V lower than circuit/battery 1, the voltage will appear to be 1.8V to circuit 2. For digital signals this isn't so much of a problem since the signal value is carried by a series of pulses between full and 0V. It is only a problem if the difference in reference is so large as to make the detecting circuit miss the threshold for considering a pulse a good pulse. PWM and other digtialish analog signals are largely immune too, but depend on the detecting circuit to buffer the input to a re-referenced pulse so the D-to-A conversion process, if there is one, is not influenced by the mismatched reference voltages. Again isolation techniques can eliminate this problem but the simple solution to both is...

Connect the negatives of the two power sources.

This will pull them to a common voltage level and give both circuits a common reference to work from.

 

[Derek]

Oh wow! Well, I'm going to have to come back and re-read that post, Paul, when I'm not "just home from work." LOL...I'm not very knowledgeable when it comes to electronics, so I think I understood about half of what you said. I prefer the "plug and play" setups, lol. However, my 600e Pro DFC has larger servos than what I'm use to, a CC 20amp BEC, HV ESC, and a 12s battery setup. That being said...it's info, like what you posted above, that I want to learn. This thread, about HV servos, is about a "possible" project, later this year.

Thanks for taking the time to perplex the "perplexed" some more, lol.

 

[Derek]

I just noticed something....a while ago, I posted a video (I think it was a video) about a stray black wire in my wiring on my 600 Pro. It was a wire that was soldered in and I had no idea "why" it was there. I was sure there was a purpose (because the helicopter had been flown before) but I didn't fully understand why it was there. After looking closely, again, at the picture that Dino posted above....I can relate his setup to mine and I can see why that wire is in my setup. Granted, our setups are a slightly different but I can see the purpose for that stray black wire.

Awesome!!!