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Wiring advice for my KK2 & Fatshark

#1
I posted my question on RCGroups, but didn't get much response. Need some advice on my tricopter build. I really want to use the low voltage alarm built into the KK2, but it looks like I need a direct source to the battery. I also want to power my Fatshark TX and tricopter all from a single battery. Currently using a 4S for the tricopter, and my Predator V2 won’t work with 4S. It only takes 2S or 3S.

Here are a few thoughts so far (for you guys to beat up):

I bought the power harness for a quad (4 outputs from 1 battery), so I currently have 1 +/- lead that isn’t being used. The other 3 +/- leads are going to the 3 ECSs (tricopter). Thinking about using the spare +/- lead to go to the KK2 board for the low voltage alarm. Will that work?

I’m not sure what the best approach is to powering the Fatshark. I could build a harness from the balance lead on the battery to power the Fatshark. I could only grab 2 cells from the 4S battery to power my fatshark TX. This would also be nice because then I can choose when to power my Fatshark TX instead of it being on all the time. I also think this would be best because it would reduce static in the video since it will be tied directly to the battery. The downside is that it will unbalance my battery during flight, and I will have to keep an eye on my flight times.

I know I cannot be the only one out there with this dilemma. What are you guys currently doing?

Joshua
 

kah00na

Senior Member
#3
There's something called a step down regulator, you can use it to convert 4S to 12v for the FPV gear.
Can't you use a BEC that outputs 12 volts to power the transmitter? Since the 4S battery would always be above 12 volts, the power coming out of the BEC could be a constant voltage, right?
 

Cyberdactyl

Misfit Multirotor Monkey
#4
Hi Joshua. I would say you could wire into two of the cells in your balance lead, but you really do need to filter the current since it's somewhat dirty powering your harness and getting all sorts of waveforms from the ESCs. THIS is basically what you get with the Immersion Tx, but with the added 4 cell balance lead plug.
 
#5
The KK2 needs a mod to read voltage. See here: http://www.youtube.com/watch?v=_jmjiIpQWTA

There's something called a step down regulator, you can use it to convert 4S to 12v for the FPV gear. Look in your closest hobby shop for one or try this one:

http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=18787&aff=524347
Thanks for the youtube link. I was already planning to do this mod to my KK2 board and was going to plug these wires into the 4th +/- lead from my ESC harness that isn't being used. That way the KK2 is getting a direct current from the battery with no ESC in-between.

I actually already have that step down regulator but I am worried about noise interfering my FPV because it is a switch mode instead of a linear step down.

Hi Joshua. I would say you could wire into two of the cells in your balance lead, but you really do need to filter the current since it's somewhat dirty powering your harness and getting all sorts of waveforms from the ESCs. THIS is basically what you get with the Immersion Tx, but with the added 4 cell balance lead plug.
My Fatshark Predator 2 kit came with this filter, so I already have this. I was just assuming I would have to grab 2 of the 4 cells from my balance lead before plugging into this filter. I am OK with doing this, but I am not excited about unbalancing my battery in flight. I just thought I would check to make sure there isn't another option.

Someone on RCGroups recommended David's linear step-down regulator as shown here: http://rcexplorer.se/projects/2012/07/the-tricopter-v2-6hv/ but I don't have the necessarily skills to make this.
 
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Cyberdactyl

Misfit Multirotor Monkey
#7
I am OK with doing this, but I am not excited about unbalancing my battery in flight. I just thought I would check to make sure there isn't another option.
While it isn't the best option, the imbalance would be minimal. The Tx and camera draw only a fraction of current the motors draw. Probably little more than the normal divergence the ESCs and motors subject to the battery 'normally'. And if you balance charge your batteries, I would think the harm would be insignificant.
 
#8
So what about using a 12 volt BEC like this 12V 2.5A UBEC 2-5S Lipoly (12-23v) from Hobby King? Would it work? How is it different than a step-down regulator?
This would work great for powering my FS TX, however this would create a lot of interference. This is a switch mode. David's design is a linear mode which uses heat rather than a capacitor.

While it isn't the best option, the imbalance would be minimal. The Tx and camera draw only a fraction of current the motors draw. Probably little more than the normal divergence the ESCs and motors subject to the battery 'normally'. And if you balance charge your batteries, I would think the harm would be insignificant.
This is good feedback. I was worried that my homemade harness to only grab 2 of the 4 cells would create a huge imbalance.
 

kah00na

Senior Member
#9
This would work great for powering my FS TX, however this would create a lot of interference. This is a switch mode. David's design is a linear mode which uses heat rather than a capacitor.
So the BEC is a bad idea? I was planning on using it to power my TX and camera. Can you explain the difference between switch mode and linear mode?
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#10
Linear mode acts like a variable resistor, wasting energy as heat to get you a fixed voltage for a WIDE range of current. They're simple, cheap, and clean, but very wasteful in return.

A switch mode effectively lets you draw power off a large capacitor, which is rapidly connected/disconnected to a higher voltage by a transistor switch to keep it topped off at the fixed voltage (much more complicated, but that's the basic approach). When well designed, they're extreamly efficient, but they're compicated for the designer to get right, more expensive and can have some nasty ripple on top of the DC at inconvienient frequencies.

I'll half agree with Josua -- a switched BEC *CAN* be a source of noise on a video system, and a crippling one at that. It all depends on the quality of the BEC and the filtering installed in and downstream of it. The quality can be linked somewhat to price, since low-end OEMs can cut some corners in the filtering to lower costs. In short, it's hard to say if this one will hurt the signal, but the chances are good. Filtering can be placed downstream -- either home built or purchased -- but it might take a lot of playing with it before you're happy.

If you've never played with power regulators, and you have better options (seperate battery or 12v linear regulator), I'd research those first.
 

kah00na

Senior Member
#11
Linear mode acts like a variable resistor, wasting energy as heat to get you a fixed voltage for a WIDE range of current. They're simple, cheap, and clean, but very wasteful in return.

A switch mode effectively lets you draw power off a large capacitor, which is rapidly connected/disconnected to a higher voltage by a transistor switch to keep it topped off at the fixed voltage (much more complicated, but that's the basic approach). When well designed, they're extreamly efficient, but they're compicated for the designer to get right, more expensive and can have some nasty ripple on top of the DC at inconvienient frequencies.

I'll half agree with Josua -- a switched BEC *CAN* be a source of noise on a video system, and a crippling one at that. It all depends on the quality of the BEC and the filtering installed in and downstream of it. The quality can be linked somewhat to price, since low-end OEMs can cut some corners in the filtering to lower costs. In short, it's hard to say if this one will hurt the signal, but the chances are good. Filtering can be placed downstream -- either home built or purchased -- but it might take a lot of playing with it before you're happy.

If you've never played with power regulators, and you have better options (seperate battery or 12v linear regulator), I'd research those first.
Thanks for the explanation. I have that 12 volt output BEC coming but I also have a 2S battery that is the size of a stick of gum that I have been using. Maybe I'll stick with that.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#12
If you get good video times on the small 2s, it's a simple setup -- give it a try.

If not, you can build a simple 12v linear regulator with a 7812 chip and a pair of capcitors -- all reddily available at at radioshack (or similar electronics parts store), and could be done with only 2 solder joints. Roybro posted an example using a 7805 (for a 5v linear reg), but can't seem to find it. If you can find it (or someone else post the link) it'll work identically for 12v by substuting the 7805 for a 7812.
 
#13
Sorry to bump such an old thread. Just got my parts from hobbyking and want to have someone double check my wiring before I turn on the solder iron :)

Once again I am trying to step down my 4S battery to 2S via the balance lead on my 4S battery. My plan is to:

-connect red wire to red wire
-connect blue wire to blue wire
-connect black wire to black wire
-leave orange and yellow wire not connected to anything
pics073_zps9b0e84c6.jpg

Will this work?
 
#17
Right. You only want 2 adjacent cells for the fatshark to think it's hooked to a 2S battery.
Ok I did a quick search on RCgroups and found someone elses recommendation. They said to only grab 2 wires from my 4S balance lead:
balance.jpg

Is this correct? If so would my wire method previously mentioned above be incorrect?
 

xuzme720

Dedicated foam bender
Mentor
#18
If you are using only the red and blue, then yes that will work also.
Do you understand how the balance leads are wired to the battery?
Using the colors from the diagram:

(- cell 4) black wire

cell 4

(- cell 3), (+ cell 4) yellow wire

cell 3

(- cell 2), + (cell 3) blue wire

cell 2

(- cell 1), + (cell 2) green wire

cell 1

(+ cell 1) red wire

Using the red and blue wires (in this diagram) would connect to the + of cell 1 and - of cell 2 giving you voltage equivalent to a 2S (2 cell). Using your picture, from the first post, of your balance lead, you still need to hook red to red, black to yellow...


This is the way, by combining any 2 adjacent wires, you can isolate that cell and the charger can test cell voltage individually, or in your case, you can power from the balance lead. So, by hooking the red and blue from the 4S balance lead to the red and black of the fatshark lead, you are in effect hooking cell 1 and cell 2 together and your Tx will see 2S. Keep in mind that wire color is to keep track of different wires, it's the polarity that matters, and most harnesses will have at least a red and a black. I have balance harnesses with only 1 red and the rest are black. It's always better to test, but usually the red shows + and the black is -, the colors in between are interchangeable depending on what cell you are connected to... I'm only bringing color up because you have different colors on your balance lead picture from the first post and the diagram above.
 
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#19
If you are using only the red and blue, then yes that will work also.
Do you understand how the balance leads are wired to the battery?
Using the colors from the diagram:

(- cell 4) black wire

cell 4

(- cell 3), (+ cell 4) yellow wire

cell 3

(- cell 2), + (cell 3) blue wire

cell 2

(- cell 1), + (cell 2) green wire

cell 1

(+ cell 1) red wire

Using the red and blue wires (in this diagram) would connect to the + of cell 1 and - of cell 2 giving you voltage equivalent to a 2S (2 cell). Using your picture, from the first post, of your balance lead, you still need to hook red to red, black to yellow...


This is the way, by combining any 2 adjacent wires, you can isolate that cell and the charger can test cell voltage individually, or in your case, you can power from the balance lead. So, by hooking the red and blue from the 4S balance lead to the red and black of the fatshark lead, you are in effect hooking cell 1 and cell 2 together and your Tx will see 2S. Keep in mind that wire color is to keep track of different wires, it's the polarity that matters, and most harnesses will have at least a red and a black. I have balance harnesses with only 1 red and the rest are black. It's always better to test, but usually the red shows + and the black is -, the colors in between are interchangeable depending on what cell you are connected to... I'm only bringing color up because you have different colors on your balance lead picture from the first post and the diagram above.
Thanks for the detailed explanation xuzme720. This has been very helpful. If its not obvious I have no clue how the balance lead is connected to the 4S battery. I have made my own 2S bats so I understand the basic concept... but 4S seems a little more complicated.

Maybe I will stick with my first wiring diagram:
-connect red wire to red wire
-connect blue wire to blue wire
-connect black wire to yellow wire

I have a multimeter. Once everything is soldered, how should I double check polarity and voltage? Sorry for the dumb question, just want to be 100% sure before I hook up my Fatshark TX.

Thanks again...
 

xuzme720

Dedicated foam bender
Mentor
#20
Thanks for the detailed explanation xuzme720. This has been very helpful. If its not obvious I have no clue how the balance lead is connected to the 4S battery. I have made my own 2S bats so I understand the basic concept... but 4S seems a little more complicated.

Maybe I will stick with my first wiring diagram:
-connect red wire to red wire
-connect blue wire to blue wire
-connect black wire to yellow wire

I have a multimeter. Once everything is soldered, how should I double check polarity and voltage? Sorry for the dumb question, just want to be 100% sure before I hook up my Fatshark TX.

Thanks again...
That will work. The fatshark is made to work by just plugging in the balance lead so it's probably only pulling from the red and black lead anyway. The 3S port is likely run through a voltage regulator and works differently.

Once you have the 4S lead attached to the 2S lead, you should get 8.4v (fully charged batt) across black and red. Polarity should be red positive+ and black negative-. For fun, you can test black to blue and then blue to red. you should get 4.2v with black negative- and blue positive+, then go red and blue. Now red is positive and blue is negative...

Once you understand how it works, even 6S or 12S are wired the same way. First cell has first wire on one terminal (+), last cell has last wire on (-). The cells in the middle have their terminals connected + to - and a balance wire coming off the join. You start to see why the color doesn't matter, only the order once you know the polarity...and the order is done for you on the plug.
 
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