Caution: Savox SB-2271SG Cyclic Servo Bench Test Results

[irishmatt18]

Originally Posted by Rollin_3z

Damn it! I hate spending more money on a wrong purchase

Well then don't go to my hobbyshop. They tend to do that to me all the time. I don't know how many times I had bought the "right" thing and it wasn't going to work. Heck, I started to bring it back and just say, nope not the right thing and they do return it now.

[joe1l]

Originally Posted by akamullen

The more voltage you put in to a bec the less amperage it puts out. I have never ran a bec off of 12S, there is no reason to. just wire it up so one pack powers it. also as a bonus, you can plug one 6S pack in and just run the electronics with out turning the esc on until you plug the second 6S pack in. its not as hareyd on bec's and they put out better amperage. win win.

I believed this to be true too when i first started experimenting with external switching BEC's so I purchased the 80$ WR Hercules at the time rated for 3s to 22v 10amp cont. at the time i wanted to use it like a Regulator on a flybar heli but didnt' want the heat issues. When i tried powering the BEC with a 3s 3000mh pack, the servos would move in a jerky motion...not smooth at all. Plugged in 6s lipo and the servos movement was normal. It all depends on the design of the bec...turns out that lower voltage input would cause current ripple. Granted this isnt always the case with every BEC. So far most of the 14s+ becs i have used worked fine at 6s but also worked fine at 12s....with the exception of the BEC PRO which didnt work at all but i'm not going there since that was a long time ago. I wish more companies would publish their current ratings for the different input voltages like Jeti does.

[akamullen]

Originally Posted by joe1l

I wish more companies would publish their current ratings for the different input voltages like Jeti does.

BEC PRO


- Adjustable Output Voltage: 4.8V-12.5V
- Max Output Current: 20 Amps peak

- Max Continuous Current:
-- @ 16V Input = 15A Continuous
-- @ 24V Input = 13A Continuous
-- @ 32V Input = 11A Continuous
-- @ 40V Input = 9A Continuous
-- @ 48V Input = 8A Continuous

[Rob Cherry]

Originally Posted by akamullen

BEC PRO


- Adjustable Output Voltage: 4.8V-12.5V
- Max Output Current: 20 Amps peak

- Max Continuous Current:
-- @ 16V Input = 15A Continuous
-- @ 24V Input = 13A Continuous
-- @ 32V Input = 11A Continuous
-- @ 40V Input = 9A Continuous
-- @ 48V Input = 8A Continuous

[akschu]

Those numbers don't mean much unless you want the bec to be able to handle the servos burst current continuously, otherwise what matters is the ability for the bec to deliver the burst current since there is nothing continuous about servo current draw.

Anyway I did some testing tonight and will share some thoughts, but I can't post pictures until tomorrow.

1. Using the BEC pro on 6s with these servos is NOT safe and should be discontinued. On my scope, fast cyclic movement without blades or any other resistance is enough load to pull the bec pro down to 2.8v from 8v, then it takes 12ms for the regulator to recover. While it appears that the spektrum rx/beastx has enough capacitance to avoid a reset, it can't be by much.

2. Adding a 1500uf cap to the rx pins helps a lot. This cap is a low resistance cap so it helps tremendously. The voltage sags to 4.3v using the cap in the same test, but more importantly, the regulator recovers instantly and the whole event is only 2ms long.

3. Adding a second 1500uf cap gives it even lower resistance and causes the voltage drop to sag to 5.6v for not even .5ms.

I would be very comfortable flying it like this as that is probably pretty close to what it would look like using a receiver pack, but I do think this could be even better.

I plan on getting 4 470uf low resistance caps and putting them in parallel as it seems that 2000uf is plenty of capacity, we just need more caps to react to the current demands.

I'll post my final solution when I have it, but a quick way to get around this is to purchase the cc cap pack they sell for the esc's and wire it to the bec pro output. That would surely make these problems go away.

Schu

[akschu]

Here are the pictures. Please notice how long it takes for the BEC to recover without any additional capacitance. That can't be safe.

I'm going to order 4 of these to try:

http://www.mouser.com/ProductDetail/...sGGggNig%3d%3d

They have the ability to deliver 2.8Amps each rms at 100khz so I think that this capacitor pack will bring the bec up to snuff, and perhaps even perform better than a battery.

schu

[Rob Cherry]

This is awesome info. This data clearly shows that there isn't a BEC out there than can handle these servos. They only appear to work OK with beastx because it uses a stronger buffer, internally, yet the BEC is probably being stressed considerably every time the servos demand a huge spike.

I'd love to try out the capacitor idea. If you say 4 will work, I'm putting in 8!

so the trick is to have capacitors fast enough to send out the power before the BEC drops? What specs do you look at to fulfill that requirement?

[akschu]

4 will already be vast overkill.

Anyway, if you look at the data sheet:

http://products.nichicon.co.jp/en/pdf/XJA043/e-hz.pdf

Look at the rated ripple for the 1000uf cap size 8x20. It shows 2880 mArms which is mill amps rms. So 4 of them should have a low enough resistance to deliver 11.5 amps of current in .01ms which is plenty of time for what we are dealing with. Also, 4 in parallel is 4000uf which is A LOT of capacitance.

I'll order a set today and put them on the scope and see what happens. I really hate running RX packs I don't want to deal with charging them, it's just so much easier to put in a flight pack and be done with it. So I'll use capacitors.

I should note that this will take a lot of the load off of the BEC, so it should be even more reliable.

If you want to order a set of caps to test, I would be very interested in if your v-bar issues go away.

schu

[Rob Cherry]

Thanks man. Whether or not you're an electrical engineer, it's really nice to have someone like you (with the appropriate tools and knowledge to use them correctly) chime in to help with issues like this. Otherwise, the rest of us are just guessing and blaming one component or another without any basis for our claims. I will order up a few and hopefully try it out this weekend.

Thanks!!!!!!!

[akschu]

Yea, I'm not an electrical engineer yet, but I read a lot and have a pretty good handle on the basics. One of these days I would like to get a masters in electrical engineering, but I already have a great job so I'm not sure if I'll ever find the time.

Anyway, I'm placing my order now, so hopefully I'll have something to post later this week, but I'm very confident this will take care of it.

schu

[irishmatt18]

You do that for a hobby...... Man, I need to pick up a book.

[akschu]

I need to retire so that I have more time for my hobbies.

[Rob Cherry]

Originally Posted by akschu

I need to retire so that I have more time for my hobbies.

LMAO. I just ordered mine too, anxious to try something other than watching of us guess at what the problem is.

[Nasscar]

Has anyone tried using / testing the Western Robotic Super BECPro ?

[akschu]

Originally Posted by Nasscar

Has anyone tried using / testing the Western Robotic Super BECPro ?

Mail one to me, I'll test it on the scope, then I'll mail it back.

schu

[Rollin_3z]

Originally Posted by akschu

4 will already be vast overkill.

Anyway, if you look at the data sheet:

http://products.nichicon.co.jp/en/pdf/XJA043/e-hz.pdf

Look at the rated ripple for the 1000uf cap size 8x20. It shows 2880 mArms which is mill amps rms. So 4 of them should have a low enough resistance to deliver 11.5 amps of current in .01ms which is plenty of time for what we are dealing with. Also, 4 in parallel is 4000uf which is A LOT of capacitance.

I'll order a set today and put them on the scope and see what happens. I really hate running RX packs I don't want to deal with charging them, it's just so much easier to put in a flight pack and be done with it. So I'll use capacitors.

I should note that this will take a lot of the load off of the BEC, so it should be even more reliable.

If you want to order a set of caps to test, I would be very interested in if your v-bar issues go away.

schu

Thanks for taking the time to do this..simple and cheap solution.

[Chris Sexton]

Ok so since I have converted my rig to a RX pack, am I good or should i isolate the power away from the vbar for the servos to be safe?

I will be setting up my new mini vbar on the x7 and these servos tonight. Planning to do another test to see how it fairs.


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[Rob Cherry]

Originally Posted by cdsexton

Ok so since I have converted my rig to a RX pack, am I good or should i isolate the power away from the vbar for the servos to be safe?

I will be setting up my new mini vbar on the x7 and these servos tonight. Planning to do another test to see how it fairs.


Sent from my iPhone using Tapatalk

The vbar power bus can handle the current, no problem. Mines been running awesome on an RX pack with everything hooked up conventionally through the mini vbar.

[Chris Sexton]

Originally Posted by Rob Cherry

The vbar power bus can handle the current, no problem. Mines been running awesome on an RX pack with everything hooked up conventionally through the mini vbar.

Cool. Thanks. Makes for a lot cleaner build.


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[Matt Nasca]

This is another solution for those that want to add capacitors. This solution is already made and plugs right in, it's called DC-UP Mark 2. I've tried this myself (with JR8917s) and it really makes a difference in keeping the voltage up. The caps are 1.2 farad, which is huge!



http://www.fromeco.org/products/05frcdcupm2/