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BEC voltage VS Servo voltage

BEC voltage VS Servo voltage

Old 12-28-2016, 09:46 AM
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Default BEC voltage VS Servo voltage

Hey there everyone. I've got kind of an odd question.

So I have an ESC capable of being set at 7.4v but my servo runs at a maximum of 6v. Although it is a savox servo and I hear they can cause brown-outs if your ESC is set at only 6v.

So my question is: can I safely have the BEC running at 7.4v if my servo is only rated at 6v? The idea being that the servo will only draw the power necessary for operation and hopefully not overload to 7.4v
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Old 12-28-2016, 09:52 AM
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Ohm's law states that the electrical current (I) flowing in an circuit is proportional to the voltage (V) and inversely proportional to the resistance (R). Therefore, if the voltage is increased, the current will increase provided the resistance of the circuit does not change.
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Old 12-28-2016, 10:01 AM
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Originally Posted by Billy T. View Post
Ohm's law states that the electrical current (I) flowing in an circuit is proportional to the voltage (V) and inversely proportional to the resistance (R). Therefore, if the voltage is increased, the current will increase provided the resistance of the circuit does not change.
So if I understand correctly, unless there is a resistor between the servo and the ESC, then the servo will be overloaded with a 7.4v bec?
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Old 12-28-2016, 10:14 AM
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Originally Posted by EbbTide View Post
So if I understand correctly, unless there is a resistor between the servo and the ESC, then the servo will be overloaded with a 7.4v bec?
Basically, running the servo at a higher voltage will cause higher current draw. What is the BEC circuit rated at in amperage?
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Old 12-28-2016, 10:16 AM
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Originally Posted by Billy T. View Post
Basically, running the servo at a higher voltage will cause higher current draw. What is the BEC circuit rated at in amperage?
The BEC is rated at a maximum of 7a

And thanks for the help BTW
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Old 12-28-2016, 10:35 AM
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It is not only the higher amps causing a damage but also the design of the electronics is made for 6v max, only a higher voltage can also burn out the circuits.

Alhough I also have heard and seen 6v Savox servo's running on 7.4v I would not do it. It is asking for a failure.
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Old 12-28-2016, 11:55 AM
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Originally Posted by EbbTide View Post
The BEC is rated at a maximum of 7a

And thanks for the help BTW
I would recommend using the servo at 6V so as to keep the current draw lower than if you ran it at 7.4V. Also, since the servo is not rated for higher voltages, you will not cause accelerated wear on the components. Just my $.02

Brownouts happen when your current draw exceeds the BEC current capabilities, you need to minimize current draw to avoid them.
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Old 12-28-2016, 12:14 PM
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Originally Posted by Billy T. View Post
I would recommend using the servo at 6V so as to keep the current draw lower than if you ran it at 7.4V. Also, since the servo is not rated for higher voltages, you will not cause accelerated wear on the components. Just my $.02

Brownouts happen when your current draw exceeds the BEC current capabilities, you need to minimize current draw to avoid them.
Thanks for all the info. I'll definitely just stick with 6v and hope that my ESC BEC is good enough to take the savox servo. If not I can swap it with my xpert servo which has been rock solid
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Old 01-03-2017, 07:58 AM
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Default BEC/VOLT/AMP problems

Solving the ESC's internal BEC brown out issue is important for the life of your ESC not so much the servo. Regardless of the manufactures rating of 7 amps that number is only true if the input voltage from your battery is the same as the output voltage to the servo. Internal BEC's are pretty much just a step down regulator designed to produce a specified volt and current. When stepping voltage down to 6 volt from a 7.4 volt source you can have the full rated 7 amps. Here's the problem, when your power source is a 4S lipo it supplies 14.8 volts to the step down regulator (BEC) it must get rid of 8.8 volts to make 6 volts and that creates a lot of heat. This higher voltage and heat makes the BEC way less efficient and it decreases the available amp output. So in real world applications it's possible to have glitches (brownouts) with a 4s lipo and not a 2s.
Now more relating to your specific question turning up the voltage to 7.4 does not make the servo draw less amps it helps the ESC produce more. And yes the higher voltage will hurt your 6 volt servo.
Regardless, Savox servo's are known as high amp draw resource hogs but I don't think they will cause a problem with a quality ESC that can produce 7 amps.

Last edited by gmackhurry; 01-03-2017 at 12:44 PM. Reason: Typo
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Old 01-03-2017, 03:15 PM
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There are different sorts of BEC circuits: linear vs switching
A linear regulator does "drop" the difference between the battery and the BEC output across the regulator. Linear regulators are inefficient because the power dissipated in the regulator is then the current load times the dropped voltage.

Switching regulators chop the input voltage into a pulse train and take power as needed to maintain the load current at the programmed voltage setpoint.

The end result is that switching or SBEC's would not get as hot as a linear BEC. I think most newer BECs are the switching type.

Servos are not linear resistive loads so Ohm's law does not apply. The current draw depends mostly on the mechanical load.

All that being said, I would not run a 6V rated servo at 7.4 volts. It is really not clear to us consumers what limits the allowed input voltage. I have read some guys say that 6V rated servos are specified at the 5S NiMh "nominal voltage of 1.2 volts per cell. But when fully charged, 5S NiMh is more like 7.2 volts (a least at the start of a run). I have personally run some 6V type servos with 2S LiFe which s 7.2 when fully charged but is really more like 6.5 volts within a few seconds of use. Never had a problem with those.

I just always try to buy HV rated servos these days so I can use them anywhere from 6V to 8.4. If it is a 6V rated servo, keep it at 6V (or 2S LiFe at the most). You might get away with it at 7.4 volts but there is no telling ahead of of time.

As far as the Savox causing brown-outs. These cars only have one freakin servo. Maybe two for a Nitro. Its a sad BEC that can't drive one servo. In helicopters we have four. If that particular servo is a big current hog, it is probably only for short intervals. You could try getting one of the those electrolytic capacitors like this
https://hobbyking.com/en_us/turnigy-...protector.html
It will source extra current for shsort times and reduce the transient load on the BEC. You can also run a small 4S NiMh buffer pack. The 6V BEC will keep it charged without over charging.
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Old 01-04-2017, 11:30 AM
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SBEC ? Geez I need to get with the times.
That can't be a cheap circuit.
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Old 01-04-2017, 11:38 AM
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Cheaper than you think.

By the way, SBEC is not realy a used name but most of the time the switching BEC is sold as an UBEC.
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