high voltage 7.4V servo - RC Groups
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Aug 04, 2010, 03:25 AM
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high voltage 7.4V servo


Anybody know of high voltage small Chinese servos which operate at 7.4V, 2 cell LiPo, and Chinese dealers which stock them?
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Aug 04, 2010, 07:13 AM
60 years of RC flying
Daedalus66's Avatar
The necessary specification for a 2s LiPo is 8.4v, the initial voltage, not 7.4.
Aug 04, 2010, 09:09 AM
A man with too many toys
3.7 * 2 = 7.4



NiCad/NiMh servos use the nominal voltage of the batteries and not the peak hot off the charger. I think it would be less confusing to rate LiPo servos the same way, especially since a 2-cell LiPo is labeled 7.4v.


?
Aug 04, 2010, 09:25 AM
60 years of RC flying
Daedalus66's Avatar
Maybe, but the point is that if you are looking at servo specifications you need one that can take 8.4v as the LiPo declines only slowly from that level.

Presumably "6v servos" already have a suitable margin built in for 5-cell use, but once you go to higher voltages it's not at all clear that there is anything called 7.4v servo that's good for 8.4.

All quite confusing and unfortunately the specifications are often not clear.
Aug 04, 2010, 11:31 AM
A man with too many toys
It would be best if the servo specks just said 2s LiPo and there would be no confusion.
Aug 04, 2010, 11:43 AM
Registered User
earlwb's Avatar
here are some high voltage rated servos here:

http://hobbycity.com/hobbyking/store..._.033sec_/_79g

http://hobbycity.com/hobbyking/store...kg/79g/.118sec



this one is good up to 3 cells or 12v too:
http://hobbycity.com/hobbyking/store..._.030sec_/_79g

http://hobbycity.com/hobbyking/store..._.118sec_/_78g

http://hobbycity.com/hobbyking/store..._.033sec_/_67g
Last edited by earlwb; Aug 04, 2010 at 11:51 AM.
Aug 04, 2010, 03:26 PM
Registered User
The component that usually determines the maximum voltage a device can handle is the electrolytic capacitor. Check the markings on them and see what the voltage rating is. It is best to only stress them to about 80% of their rating, less if there is a high ripple on the voltage they are filtering.
Aug 04, 2010, 03:45 PM
A man with too many toys
Quote:
Originally Posted by Rodney
The component that usually determines the maximum voltage a device can handle is the electrolytic capacitor. Check the markings on them and see what the voltage rating is. It is best to only stress them to about 80% of their rating, less if there is a high ripple on the voltage they are filtering.
Thatís a very strange statement. I always thought that it was the motor windings.


Aug 04, 2010, 09:37 PM
Suspended Account
Thanks for info, but these expensive servos for giant planes are not what I have in mind.

Do you know of any micro or mini servos?
Aug 05, 2010, 06:21 AM
Registered Abuser
Kentucky's Avatar
Dymond D47, D60


http://www.rc-dymond.com/index3.php?categoryID=17
Aug 05, 2010, 10:47 AM
Registered User
Quote:
Originally Posted by RC Man
Thatís a very strange statement. I always thought that it was the motor windings.


True, that is also an important consideration but; if you are not overloading the motor you will not burn it out. However, if you are exceeding the voltage rating of the capacitors, they are very apt to fail. I have spent many years in failure analysis of electronics and by far, the electrolytic capacitor is the most prone to failure if you exceed voltage specifications. Now, capacitor size is probably the biggest factor in building physically small servos and so is usually the item that the designer has the most trouble in size limitations. Since a smaller voltage Capacitor is smaller than a similar capacitance with a high voltage rating, you can guess which decision is usually made in the selection of components.
Aug 05, 2010, 11:53 AM
A man with too many toys
Has anyone used a 6.6v battery with a 6v servo?

According to the instructions you can run a standard servo without a regulator.
http://www3.towerhobbies.com/cgi-bin...&I=LXYAS9&P=ML


Aug 05, 2010, 01:04 PM
Suspended Account
Quote:
Originally Posted by Kentucky
Thanks. That fills my needs. D47, 4.8 - 9V, is what I was looking for.
Sep 05, 2010, 02:51 AM
Registered User
Quote:
Originally Posted by Rodney
True, that is also an important consideration but; if you are not overloading the motor you will not burn it out. However, if you are exceeding the voltage rating of the capacitors, they are very apt to fail. I have spent many years in failure analysis of electronics and by far, the electrolytic capacitor is the most prone to failure if you exceed voltage specifications. Now, capacitor size is probably the biggest factor in building physically small servos and so is usually the item that the designer has the most trouble in size limitations. Since a smaller voltage Capacitor is smaller than a similar capacitance with a high voltage rating, you can guess which decision is usually made in the selection of components.
I totally agree with RC Man. The upper voltage limit for servos is determined by the motor specs, not the capacitors. If you don't believe me, go hook up a standard servo to a 2S Lipo and watch what happens. The motor windings (or MOSFET drivers) will fail because of excess heat generation long before the dielectric properties of capacitors fail. Capacitors used in servo electronics are typically rated at 12V, far above the maximum operating voltage of the servo motor.

Also, to clear up any confusion about whether a 7.4V servo can be used with a 2S Lipo, the answer is Yes. These servos are specifically designed to be used with 2S Lipos and are rated at the nominal voltage of such cells, meaning they can handle the peak charged voltage of a 2S Lipo. Go check the Hitec HV range of servos as examples.
Sep 05, 2010, 10:48 AM
hass-alfed and bass-ackwards
carlsoti's Avatar
Quote:
Originally Posted by Kentucky
Quote:
Originally Posted by John Kim
Thanks. That fills my needs. D47, 4.8 - 9V, is what I was looking for.
I run dymond 47 and 60's in my blaster II on 2S lipo just fine. People have reported the D47's to work @ 1S voltage, as well.


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