ProModeler Servo: 420oz, billet case, o-ringed, $79

[maslin]

Originally Posted by Smokin' Joe

I spoke with John today to ask what voltage to set a BEC at for maximum safe performance from the servo. I asked him about setting it to 8.4 volts, but he recommended 7.4 volts. Interestingly, he also recommended running it off of a small separate 2S lipo over using a BEC (I'm running 3S otherwise).

A charged 2s is 8.4v...

[honchojoe]

Originally Posted by maslin

A charged 2s is 8.4v...

But it doesn't stay at 8.4v for very long. It will settle at closer to 7.4v after a while.

Sent from my SM-G930F using Tapatalk

[maslin]

The discharge rate is fairly linear at the kind of load you would see from a single servo. Unless you ran a 400mAh battery or something equally tiny, you would probably never get under 8v.

I don't believe I've ever heard anybody recommend running a separate battery just for the servo.

[WHITE-TRASH]

Originally Posted by maslin

I don't believe I've ever heard anybody recommend running a separate battery just for the servo.

He's an airplane guy, they do things differently.

[mikebecher]

If you decide to use a BEC, John is recommending to set it at 7.4v because under load they will spike 300-400 mV or even more if stressed. He sent me this chart from an oscilloscope measuring the output of a BEC with a light load. Each division is 50 mV so you can see how much it oscillates between baseline and around +200 mV.

The BEC in my Wromber died this weekend and I just got a new Promodeler servo so I'm going to give the dedicated battery a try. At the very least I won't have to worry about burning up another BEC.

AFAIK, the airplane guys do use a battery separate from the motor batteries to run the servos. A small LiPo is way cheaper than a new BEC anyhow. I figure if it's good enough for them, why not give it a go?

[Smokin' Joe]

Originally Posted by mikebecher

If you decide to use a BEC, John is recommending to set it at 7.4v because under load they will spike 300-400 mV or even more if stressed. He sent me this chart from an oscilloscope measuring the output of a BEC with a light load. Each division is 50 mV so you can see how much it oscillates between baseline and around +200 mV.

The BEC in my Wromber died this weekend and I just got a new Promodeler servo so I'm going to give the dedicated battery a try. At the very least I won't have to worry about burning up another BEC.

AFAIK, the airplane guys do use a battery separate from the motor batteries to run the servos. A small LiPo is way cheaper than a new BEC anyhow. I figure if it's good enough for them, why not give it a go?

Attachment 348913

I am considering doing the same thing. How do you plan to connect the battery? I'm a newbie when it comes to this, but I was thinking of using the Holmes receiver bypass and attaching the battery to the BEC connector. Is there a better/easier way of doing this? Thanks.

[mikebecher]

Originally Posted by Smokin' Joe

I am considering doing the same thing. How do you plan to connect the battery? I'm a newbie when it comes to this, but I was thinking of using the Holmes receiver bypass and attaching the battery to the BEC connector. Is there a better/easier way of doing this? Thanks.

Hey Joe - I haven't set one of these up before either but the small LiPos I'm looking at come with a JST connector. I'll probably wire up some sort of harness myself but haven't given it much though at this point. When the batteries get here (I have to order everything online) I'll post an update.

[drzoo2]

Originally Posted by mikebecher

AFAIK, the airplane guys do use a battery separate from the motor batteries to run the servos. A small LiPo is way cheaper than a new BEC anyhow. I figure if it's good enough for them, why not give it a go?

Attachment 348913

Given the frequency of that ripple, 600Hz, that isn't control loop. That's exactly what happens to a switch mode supply "lightly" loaded. They oscillate. This is because the duty cycle of the control circuit it so low, it cannot effectively regulate output. It's averaging against the output fiter. Once loaded, out of the noise, most of that will smooth out.

I would argue the voltage sag from a good high discharge main pack driving a quality BEC under load, would be measurably better than the sag from a low discharge receiver pack battery, under load.

A low discharge pack has a higher source impedance. The higher the impedance, the more the drop at the battery and not across the load.

...just thinking about it.

[maslin]

I don't have a small enough 2s to wire up with my Pico, or I would. The only mini battery I have on hand is 3s. Might give it shot with a 5000mAh just to see.

[WHITE-TRASH]

People still use 2s? Huh, who knew?

[mikebecher]

Originally Posted by mikebecher

Hey Joe - I haven't set one of these up before either but the small LiPos I'm looking at come with a JST connector. I'll probably wire up some sort of harness myself but haven't given it much though at this point. When the batteries get here (I have to order everything online) I'll post an update.

Looks like this is going to be a piece of cake. I just need to replace the BEC with a receiver battery and no need for a power tap on the main battery connector.

[maslin]

That's not powering the servo directly, you'll just be setting it up like every nitro out there.

I'd go this way, nothing to interfere. As long as the servo can take 8.4v.

[Smokin' Joe]

Originally Posted by maslin

That's not powering the servo directly, you'll just be setting it up like every nitro out there.

I'd go this way, nothing to interfere. As long as the servo can take 8.4v.

I'm planning to set it up basically like this, though I'd probably call what you have listed as "Receiver Pack" a Servo Pack as it is bypassing powering the receiver. My receiver can't handle the voltage of 2S so that will have to be powered off the ESC's BEC. I'm planning to use the holmes receiver bypass which does have a shared ground that isn't noted in your diagram. Like I said before I'm a newbie at this, so let me know if this plan isn't sound.

[mikemcE]

Run this servo at 7.4 not 8.4


HANG UP AND DRIVE !

[Smokin' Joe]

Originally Posted by mikemcE

Run this servo at 7.4 not 8.4


HANG UP AND DRIVE !

John recommended a 2S lipo over a BEC. He did state if using a BEC to set it at 7.4.

[maslin]

Originally Posted by mikemcE

Run this servo at 7.4 not 8.4

Originally Posted by Smokin' Joe

John recommended a 2S lipo over a BEC. He did state if using a BEC to set it at 7.4.

So run a 2s lipo, charged halfway to 7.4v and watch the voltage drop the entire time you drive.

Or run a 3s through a BEC at 7.4v and stay at a constant voltage. Give or take a few millivolt ripple.

I'll take the ripple.

[SCREAMER]

I've been running mine at 7.5 volts for their entire life so there!

[84yoda]

Originally Posted by maslin

So run a 2s lipo, charged halfway to 7.4v and watch the voltage drop the entire time you drive.

Or run a 3s through a BEC at 7.4v and stay at a constant voltage. Give or take a few millivolt ripple.

I'll take the ripple.

Exactly.

This is one of the reasons for a bec, to have constant power. Air plane guys run multiple servos, so that's why they run a separate battery. Plus if a bec was to fail, I would rather have it happen on the ground, in a crawler, and not up in the air.

[Smokin' Joe]

Originally Posted by maslin

So run a 2s lipo, charged halfway to 7.4v and watch the voltage drop the entire time you drive.

Or run a 3s through a BEC at 7.4v and stay at a constant voltage. Give or take a few millivolt ripple.

I'll take the ripple.

John recommended a 2S lipo fully charged to 8.4V. If using a BEC, he recommended a 7.4V setting for the BEC.

[redneckracerx]

I replaced a hi-tec7955 with this servo. I ran my hitec with my bec set at 8v. From what I'm reading I should turn it down to 7.4v?

And, sorry to hijack but I recommended this wiring setup to a friend with a servo capable of running 8.4v the only difference was instead of a separate battery he ran the red/black to the main battery. As soon as he turned the truck on, the servo went full left and smoked before he could unplug it. Now I'm kinda leery of running a setup like this myself. Thoughts?