A very intresting read I found about servo's and BEC.

[Charlie-III]

I've been on the road the last month with very limited time home....so no testing done yet.

I figured I would add the following post since it's related to this thread....

I'm just going to get this out of the way and say this was not a real world test - I did not drive my car as I did this.
First up Stall current:

Spikes in the 2.8A range. The servo was not allowed to move and was told to turn. The servo was centered and I tried to turn it.

Then just moving back and forth. I get some spikes in the stall range which means I need to adjust my endpoints

So while it does draw some pretty hefty amps there really should be no problem with brown outs as the MMP BEC is rated at 3amps. The BEC is the culprit, not the servo. Now I am positive under the right conditions up to five amps can be drawn but I did not achieve those issues but even then it would only be for a split second.

The test was very unscientific so take it as you will but it might help some planning their BEC circuits with lights and stuff.

Radio: Spektrum
ESC: BH LT4

 

[YJ_Driver]

I just saw this thread and wanted to share some thoughts.

About current draw vs. voltage, it all depends on the circuit. Most simple circuits like light bulbs and DC motors will draw more current at higher voltage. Our servos use DC motors so this holds true for any given model, and we take advantage of this to increase output torque and speed which requires more input power (which is voltage x current).

Some circuits are designed to adjust their input current draw to maintain a specific power output (be it mechanical, thermal, or electrical). For example switching power supplies like those found in PCs and the CC external BECs will draw less input current with higher voltage to drive the given load such as our servos.

Now if someone were to design a new servo to connect to higher voltage batteries like 11.1V, then they could design it to draw less current while still providing the same output torque and speed. This would work well for those of us on 3S LiPos, but the market may be too small.

I agree that manufacturers should provide the current draw specs for servos as it's pretty straight forward. To reply to a post about specs for ESCs and even BECs though, they're primarily dependent on cooling which in turn is effected by ambient temperature and airflow which varies too much in our uses. Then they'd get into debates such as we've been seeing with LiPos and we don't need any more of those ;-)

 

[Charlie-III]

Re: A very interesting read I found about servos and BEC.

I just saw this thread and wanted to share some thoughts.

About current draw vs. voltage, it all depends on the circuit. Most simple circuits like light bulbs and DC motors will draw more current at higher voltage.

Huh....WTF??!!

So, you're saying the reason AC manufacturers raise the input voltage to larger "window units" is to increase the amp draw??

I don't think so.

For a given amount of work (electrically speaking) you either use volts or amps.
If you raise the input voltage you lower the amps.
If you lower the input voltage, you raise the amps.

Look at ANY electrical device that is "dual voltage" and see that when the input voltage goes up the amps goes down.
This goes for motors, PC power supplies, etc.

No, I'm not an engineer, I'm a long time facilities guy and now senior field service guy.8)

 

[heyok]

Re: A very interesting read I found about servos and BEC.

I think what he is saying is that if you keep the load constant and raise the voltage, the current will increase. This is correct.

Charlie, I think you are talking about a dual voltage appliance and yes indeed, when you run at the higher voltage, the current draw is reduced. Usually this is accomplished by using different windings on the input transformer of the device.

So... you are both right as you are both talking about different things

 

[Hardline]

Yeah, YJ is correct. V=IR

Put 2 volts across a 1 ohm resistor and it will push 2 amps.
Put 3 volts across the same resistor gets you 3 amps.

If you plug your 110V toaster into a 220V outlet, you get more current ...lots of it. And sparks, smoke and probably a kitchen remodel.

 

[Charlie-III]

I think what he is saying is that if you keep the load constant and raise the voltage, the current will increase. This is correct.

Charlie, I think you are talking about a dual voltage appliance and yes indeed, when you run at the higher voltage, the current draw is reduced. Usually this is accomplished by using different windings on the input transformer of the device.

So... you are both right as you are both talking about different things

Yeah, YJ is correct. V=IR

Put 2 volts across a 1 ohm resistor and it will push 2 amps.
Put 3 volts across the same resistor gets you 3 amps.

If you plug your 110V toaster into a 220V outlet, you get more current ...lots of it. And sparks, smoke and probably a kitchen remodel.

I will cogitate on this a bit.

My mind is fried from a busy work day.

If I decide I'm incorrect, I apologize in advance for previous posts.

BTW, I hate being incorrect, but will man up and say so when I am......"thumbsup""thumbsup"

 

[YJ_Driver]

I will cogitate on this a bit.

My mind is fried from a busy work day.

If I decide I'm incorrect, I apologize in advance for previous posts.

BTW, I hate being incorrect, but will man up and say so when I am......"thumbsup""thumbsup"

What you said is correct as long as the device is designed that way. If you read my next paragraph I do mention that PC power supplies operate like you say.

I just wanted to clarify that our servos (at least the ones I've seen) aren't designed to draw less current at higher voltage: if you raise the voltage then the current will go up with it and this is how they generate more torque.

I do look forward to your test results, I use 7955TGs in my rigs. "thumbsup"

 

[Olle P]

Why test servos ourselves when the manufacturers have likely already done it. Can't we just write some emails and ask them to publish specs? ...

Because many of them provide "fuzzy", or at least incomplete, specs?

You get an idea of what I mean by reading this and this posts of mine.