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Home✈️Aircraft🚁HelicopterHelicopter Main Discussion › bl700h servos on 6.0v or 8.0v
06-01-2015 10:49 PM  4 years ago
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tomtek

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bl700h servos on 6.0v or 8.0v
I am going to install a talon 120 esc on my align 700e dfc. The servos are bl700h and bl755h servos. These servos are hv. According to the specs they are rated at 6.0v or 7.4v. My esc gives me two choices for voltage. 6.0v or 8.0v. Which should I choose?
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06-02-2015 01:34 AM  4 years ago
EEngineer

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two choices for voltage.
If you're not sure about 8V and since they're rated at 7.4V, you could install single diode "stepdown"s in each of the servo leads and have the ESC's SBEC output 8V.

The stepdowns will drop ~1V so you'll be running your servos at ~7V under loads.

Just make sure the diode selected is up to the task power dissipation wise. A 1N4001 won't cut it.

A 5A diode should do it....just use some servo extension leads to install the diode in, so your servo leads aren't modified.
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06-02-2015 09:17 AM  4 years ago
nwmtech

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The servos are perfectly fine at 8v. A 7.4v rating would indicate they're safe for use on a 7.4v Lipo RX pack. When the pack is fully charged your well above 8v so set your BEC to 8v, fly, and enjoy.
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06-02-2015 09:20 AM  4 years ago
EEngineer

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I assume you have data on that.

Have you tested those servos?

Just asking...
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06-02-2015 09:32 AM  4 years ago
nwmtech

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I don't need to go through a 2000 cycle controlled environment bench testing session using simulated loads to know that they're ok on 8v because I have something called common sense.

Take a look at all the Align team pilots and you'll notice that they're all using a straight 2s Lipo RX pack going straight into the FBL system with no step-down diodes, regulators, etc.

We're long past the days of step-down diodes (which are actually closer to .7v under load since we're being so technical) now that HV servos have taken over. Common sense would say that it's idiotic for a servo manufacturer to rate their servos at 7.4v but not have them be rated to run on a fully charged 2s Lipo at 8.4v. That would mean that the consumer has no chance at getting the highest rated speed and torque out of their servos unless they ran a BEC? Just doesn't make any sense as so many people still prefer to run a receiver pack.
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06-02-2015 10:13 AM  4 years ago
WessCo

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Summerville, GA USA

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I'm using a set on a Raptor e700 @ 8v.MikadoUSA
VTeam
OptipowerUSA
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06-03-2015 12:43 AM  4 years ago
EEngineer

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which are actually closer to .7v under load since we're being so technical
At full load, the voltage drop can be ~1V....as can been seen on the technical spec data from diode manufacturers.

Depends on the diode type.

A fast recovery diode will be < 1V, whereas a standard recovery diode will be ~ 1V.

(since we're being so technical)...

Even so, at ~ .7V drop, the OP will be at even closer to 7.4V using his SBEC at 8V.

The OP asked a question, and I gave him an alternate solution.
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06-03-2015 12:45 AM  4 years ago
EEngineer

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idiotic for a servo manufacturer to rate their servos at 7.4v but not have them be rated to run on a fully charged 2s Lipo at 8.4v.
It is....so why do they do that?
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06-03-2015 02:51 AM  4 years ago
nwmtech

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I couldn't begin to tell you why they do what they do. I just know what works and the experiences of lots of other hobbies who bought those servos. I use manufacturer specs as a guideline then ultimately base my judgements on personal experience and the input of other hobbyists.

I can completely appreciate you giving your opinion on how you would do it and I'm not saying that if properly implemented wouldn't work just fine. I just want to make sure the OP is aware that the method you described is far from the norm and that it is widely accepted and proven they're perfectly fine on 8v.

I had zero problems with your recommendations and had no intention of commenting on it until you tried to call me out with your request for data to back-up my statements in a snide way.

Do you have any data to say they CAN'T be ran on 8v? What about the Futaba BLS451's and the Align 610's that were "6v" rated servos that tons of people ran at 8v for years and years with no issues? How is that even possible when the specs said 6v? I think what your'e getting caught up on is the servo spec sheet. They give ratings for 6v and 7.4v. No where in those specifications is a Maximum Operating Voltage. It's simply a torque and speed rating at a given voltage.

I have no desire to argue though so I'll respectfully bow out of this conversation now. Happy flying.
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06-03-2015 04:19 AM  4 years ago
EEngineer

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back-up my statements in a snide way
I didn't mean to appear snide, nwmtech.

If I was, I sincerely apologize....

I'm used to having "solid" specs....and I fully agree that the voltage ratings specs are not standardized...at best...

Thought you might have some test specs. that are not commonly available.

And on the diode subject, we were both right on the voltage drop at full load....

I was thinking of the "standard" schottky...for greater "voltage" drop at load....you were thinking of a "fast" schottky which has less voltage drop.

And either one would work....

But, that's added complexity....it's better not to have to do this.

On passive freewheeling ESCs, one wants a "fast" schottky to "snub the back EMF that the motor coils generate.....as when the motor coils are being "energized" by the ESC, the PFW diodes are reversed biased....and only when the motor coil is "de-energized" does the diode "conduct".

The fast schottky(less voltage drop) dissipates less power(runs cooler)...

Anyway, sorry for the mis-understanding...
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06-03-2015 04:45 AM  4 years ago
nwmtech

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No worries at all. The hobby is too much fun to dwell on stuff. Cheers!
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06-03-2015 05:08 AM  4 years ago
EEngineer

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You, too....cheers

Wish I could fax you a beer....
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10-02-2018 06:16 PM  16 months ago
rnst2

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surrey, BC

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thank you.
8volts it is.
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10-03-2018 01:37 AM  16 months ago
ticedoff8

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Morgan Hill, CA. USA

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You waited 3 years to try this?
I thought I took too much time building my helis.
+ > ÷
Believe 1/2 of what you see and none of what you hear.
Fake News will be the downfall of our Republic!
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