servo amp draw?

[Greywolf74]

Just curious what an rough estimate is for the average amp draw for a 255oz/in steering servo would be in a Rustler that probably weighs twice as much as a normal one. I'm running that and my RX and a gyro all three off of a 3A (5A max) BEC. The BEC gets very very warm but not so hot that its worried me yet. What I am worried about is it eventually failing because of high Amp draws eventually wearing it out. Should a very warm BEC worry me that I should get a bigger one or is that fine?

 

[slowngreen]

Servo is prolly pulling atleast 5. More than likely it wants to pull 6 to get the actual 255oz.

 

[hamz9561]

I agree with SNG. I'd recommend going with about 7.5 to 10 Amp BEC.

 

[Greywolf74]

OK, I'll switch it out....I have another question too. when I plug in my 6S Lipos into my MMM I always get a huge arcing spark out of it.....it doesn't do that when I plug in my 3S lipo to it....is this normal? I have 2 6S and both of them arc when I plug anything into them....even the charger. New to 6S's are they supposed to do that?

 

[Greywolf74]

Ok, so I pulled out the ol multimeter last nigh and made some wires to connect the two plugs together yet leave a surface exposed to use my multimeter to measure the amp draws. the RX and fan together were only .02A and my 255oz/in .13sec servo only pulls about .95 max under a load. Under normal use it only pulls about .5A so the 3A BEC should be enough. I'm not sure why its even getting warm unless they all get warm when in use? or maybe because its hooked up to a 6S lipo? At any rate I just thought Id share my findings with you guys.

 

[mayrider]

I didn't know what a BEC was so i googled it and found this thought it may help.

This is by the author "GERD." helifreak member with the handle PSILO .




BEC - What is it and why buy another?



I thought I would just post this to try and clear up some of the confusion around what a BEC is, what it does and the differences between them as it seems to confuse a lot of people and is very relevant to those of us using Spektrum 2.4Ghz

What is a BEC?
BEC stands for Battery Elimination Circuit and that is exactly what it is.

What does a BEC do?
Before lipos and advances in battery technology, the normal method of powering a receiver was through a seperate NiCd battery pack providing 4.8v to the receiver and servos. As battery technology progressed the BEC was introduced to replace the 4.8V receiver pack (making the model lighter) by drawing power from the higher voltage main pack and dropping it down to 4.8V for the rx and servos. So quite simply the BEC eliminates the need for a seperate power supply for your receiver and servos.

What is the difference between built in and external BEC's?

Generally the BEC found in an ESC is a linear voltage regulator (or linear mode bec), quite simply the way these work is to take the input voltage (11.1V on a 3S lipo) and outputs 5V to the rx. The problem is that it has to dispose of the excess 6.1V and to do this it converts it to heat. Generally these linear mode BEC's are rated at 2-3 Amps, however what the manufacturers do not tell you is that this rating is only true with a 6V input. Using a 3S (11.1V) input the BEC will only deliver around 0.5A before it starts to overheat. Linear BEC's rarely reach more than 50% efficiency and can run as low as 10-15% efficiency.

Generally external BEC's are switching voltage regulators (or switched mode bec), these do not care about input voltage and can run up to around 30V+ input. A switching regulator works by taking small chunks of energy, bit by bit, from the input voltage source, and moving them to the output. This is accomplished with the help of an electrical switch and a controller which regulates the rate at which energy is transferred to the output (hence the term “switching regulator”).

The energy losses involved in moving chunks of energy around in this way are relatively small, and the result is that a switching regulator can typically have 85% efficiency. Since their efficiency is less dependent on input voltage, they can power useful loads from higher voltage sources.


A quick Comparison

Lets take our beloved belt as an example. It uses an 11.1V input and the Esky ESC has a 2A linear BEC. In order for the BEC to output 5V/1A the current flow into the BEC has to be at least 1A so the power on the BEC is 11.1V * 1A = 11.1W however the useful output power is just 5V * 1A = 5W so the BEC has to convert 6.1W of power to heat causing it to get very hot. This gives it an efficiency of just 5W/11.1W = 45.%

Now lets say we disable the onboard BEC and replace it with an external switched mode BEC like the Hobbywings 3A UBEC. For the switched mode BEC to output 5V/1A the current flow in is just 0.50A (actual test data), so in this case the power on the BEC is 11.1V * 0.5A = 5.6W with a useful output of 5V * 1A = 5W meaning the BEC only has to convert 0.6W of power to heat and runs at an efficiency of 5W/5.6W = 89%

Do I need an external (switched mode) BEC

If you are using digital servos or more than 3S Lipos then yes definitely as the linear mode bec's will overheat trying to provide any useful output.

If you are running spektrum 2.4Ghz receivers then it is highly advisable as they draw significantly more current than a normal receiver and are very susceptible to voltage drops. If your linear bec is trying to provide a lot of current it will overheat and fail to provide the correct voltage causing the spektrum receiver to reboot.

I hope this clears up some of the questions about BEC's and is useful.

Gerd

http://www.helifreak.com/showthread.php?t=72280

 

[Greywolf74]

wow, nice article....Thanks!!

 

[hamz9561]

I just remembered that in order to correctly measure current the meter must be in line not parallel (reading across the two poles of the connector).