By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by CML

Servo current consumption

More data.

All Topics | Latest Posts

Search for:  in Thread Title in  
Tim Mackey05/10/2010 20:13:37
avatar
20920 forum posts
304 photos
15 articles
I have covered this subject before , and as this is relevant to all powered models not just leccys, I am posting this latest stuff here.
I have just finished doing some "work" for a friend who is thinking of moving over to Lithium power for his radio gear ( mainly gliders but that's irrelevant ).
I conducted several experiments to demonstrate the differing power scenarios of using either a standard 4 cell nickel battery, a 2s lipo and switch mode 3A 5v output regulator, or a 3s lipo and regulator for on-board radio supply. 
We may have the facility to upload Excel docs and similar soon, so if that happens I can upload the data spreadsheet, until then this summary of by-product data may be of interest.
 
Single analogue standard sized budget style servo ( HiTec HS-311 )
Load was applied by hanging 1.5lbs lead weight from servo arm.
Unloaded current draw was approx 3m/a and loaded as above was 140 m/a.
 
Loaded current draw on the  particular power source was as follows.
4 cell nickel battery = 140 m/a
 
Incidentally on the lipos it was.....
2s and regulator     =  80  m/a
3s and regulator     =  60  m/a
 
I think a 1.5lb lead weight hanging vertically from the middle hole on the servo arm represents a fairly high load for a "typical" sports model, and simply multiplying this by four, would give a very worst case scenario of typical in-flight loads of said model of approx .5A   (  4 x 140 m/a. )
In reality, its unlikely that all servos would be under full load simultaneously, and for much of the flight I expect it would be considerably less.
Interestingly, this also agrees pretty much with what I found when doing actual "eagle tree logger" experiments during an actual flight in a WOT4 some time back as shown on this OLD THREAD HERE
 It does always amuse me somewhat when we hear talk of theoretical very high power requirements for our radio gear - the reality appears to be far less demanding than the marketing men ( or some club "gurus" ) would have us believe
So.... has anyone else any "real world style data" on in-flight radio power consumption?
Martin Harris05/10/2010 20:22:44
avatar
9558 forum posts
258 photos
All I can offer is that I would be wondering what was wrong if I put more than 270 - 300 mA back into an average 5 servo sports model after 3 10 minute flights and that would include any fiddling time either side of flying and a few days of inactivity.

Edited By Martin Harris on 05/10/2010 20:23:53

David Ashby - Moderator05/10/2010 20:39:24
avatar
Moderator
11094 forum posts
1748 photos
621 articles
Yes it's surprising how little goes back into an RX pack after a session even if flying a model with digital servos.
 
No data to offer Tim but that's very interesting. Just  out of interest and curiosity but do you know what a standard switch harness is rated at for current consumption?
Myron Beaumont05/10/2010 20:43:53
avatar
5797 forum posts
51 photos
Timbo
That is extremely interesting . One query- I see the current consumption is the lowest for the lipo / V -regulator scenario .Is that because it is more capable of supplying the power easier I wonder .I for one am quite supprised at the low values anyway . I've always been a bit worried as to how much my high mAh instants can produce if required .I don't ever put the sort of load you are talking about on  the servos at once ( I think )

Edited By Myron Beaumont on 05/10/2010 20:44:21

Biggles' Elder Brother - Moderator05/10/2010 21:07:58
avatar
Moderator
15748 forum posts
1460 photos
Interesting data Timbo. Makes sense - as David and Martin have said it is a constant surprise to me how little an on board battery takes in even after a all day session with maybe 6 flights with the model. Very rarely more than 5-600mAh.
 
But it doesn't stop me putting a 2200mAh rx pack in every model! I like to feel really really safe!
 
BEB
Myron Beaumont05/10/2010 21:53:33
avatar
5797 forum posts
51 photos
BEB
I agree -But are we sure that the high mAh batteries can push out the power needed if every servo is being pushed hard . Several threads have commented on the thinness of the plates inside for max capacity which it seems makes the thing vulnerable to damage & as well unable to kick out the power when needed .I must say that I personally have had no issues, but then again ,I'm not a 3-d type of flier
Tim Mackey05/10/2010 22:10:33
avatar
20920 forum posts
304 photos
15 articles
Posted by David Ashby - RCME Administrator on 05/10/2010 20:39:24:
Yes it's surprising how little goes back into an RX pack after a session even if flying a model with digital servos.
 
No data to offer Tim but that's very interesting. Just  out of interest and curiosity but do you know what a standard switch harness is rated at for current consumption?
 
Not for sure David - but I would guess around 5A or so at least ?
Tim Mackey05/10/2010 22:15:42
avatar
20920 forum posts
304 photos
15 articles
Posted by Myron Beaumont on 05/10/2010 20:43:53:
Timbo
That is extremely interesting . One query- I see the current consumption is the lowest for the lipo / V -regulator scenario .Is that because it is more capable of supplying the power easier I wonder .I for one am quite supprised at the low values anyway . I've always been a bit worried as to how much my high mAh instants can produce if required .I don't ever put the sort of load you are talking about on  the servos at once ( I think )

Edited By Myron Beaumont on 05/10/2010 20:44:21

 
The reason is because the higher voltage lipo has to supply far less input current to the regulator in order to match the required power by the servo on the regulator output ( Watts )
Ohms law dictates that ( to keep the example simple ) a device using 1 watt of power with a supply voltage of 10V requires 100m/a   ( W=V*I )
If the voltage is doubled to 20V then the current required for the same wattage is halved.

Edited By Tim Mackey - Administrator on 05/10/2010 22:17:42

Myron Beaumont05/10/2010 22:55:58
avatar
5797 forum posts
51 photos
So from what you're saying,the voltage regulator device will produce the required current flow easier from a much higher supply voltage? In other words ,it is not like having a simple resistor to bring down the voltage supply to the Rx or have I got it wrong ?
Peter Beeney05/10/2010 23:24:48
1593 forum posts
59 photos
  
   With the greatest respect I wonder if I might add a small rider here?

   This is why the switched-mode power supply system is so popular. The transformer or induction principle requires very little power to operate. Although I suspect that as the input voltage goes up the transfer of power ratio does tend to fall off a bit.
   Whereas the linear regulator unfortunately does not have this advantage. In Tim’s example, using a linear reg, if the input was 10V at 100mA, the output would be 5V at 100mA. 1W in, 0.5W out. 0.5 watts of power would be dissipated across the reg as heat. And we have 50% efficiency. If we increase the input voltage to 20V we still have 0.5W out, but now it’s 2W in, and 25% efficiency. But more importantly 1.5 watts of power are dissipated across the reg as heat. Increase the output to say 1A and we can soon see why the regulators on some ESC’s can soon be made to resemble a central heating system.
   If we kick these figures around a little, we will soon see too, that in these circumstances it doesn’t matter if we use a 10V battery or a 20V battery, if they are the same capacity they will both last the same amount of time. There is no advantage in raising the battery voltage.

   I hope this is at least partly understandable!                          PB
Myron Beaumont05/10/2010 23:43:19
avatar
5797 forum posts
51 photos
Peter
Yes -partly understandable I was going to saymore but the web has slowed to next to stopped
Tim Mackey05/10/2010 23:49:55
avatar
20920 forum posts
304 photos
15 articles
The example I used did use a switch mode regulator, as Peter states, they are far far more efficient than a linear mode. perhaps the only potential downside is their potential tocreate some interference - hence most of them come fitted with a ferrite ring on the output lead.
I must say that as an exclusive 2.4ghz user I tend to remove the ring as I dont expect this to be an issue.
Wingman06/10/2010 07:27:43
avatar
1153 forum posts
405 photos
Nice bit of work there Timbo. Fairly recently on a thread on here someone worked out the actual force required to move a control surface against airflow resistance and it was actually pretty miniscule compared to the force the servo arm was capable of producing - well below the 1.5lbs you used as a test - can't remember where it was now 
Biggles' Elder Brother - Moderator06/10/2010 09:20:32
avatar
Moderator
15748 forum posts
1460 photos
Hi Wingman,
 
yeap - I think that was me that did that calculation in an answer on servo torque. I seem to remember it worked out that for the typical flying speed of a sports model, given an aileron 0.5m long (I can't remember the width but it was a typical sort of value) at full deflection the force would be about 2N - approximately half a pound. If memory serves me correctly. So well inside Timbo's test regime.
 
BEB
Biggles' Elder Brother - Moderator06/10/2010 09:24:15
avatar
Moderator
15748 forum posts
1460 photos
I checked my bookmarks and the thread in question is here
 
My memory was slightly inaccurate! The force was 1.15N - so that's about 1/4lb - even smaller!
 
BEB
Steve Hargreaves - Moderator06/10/2010 09:45:50
avatar
Moderator
6765 forum posts
199 photos
Good stuff Timbo & a subject close to my heart as I think you know!!!
 
I agree with the above posts regarding how much goes back into an Rx pack after a session......its rarely more than 500mAh....2-300mAh being more common.
 
If we take the 500mAh figure & say this represents 5 x 10 minute flights then this represents 100mAh per flight or an average current draw of 600mA.....not a lot...
 
I remember reading that the maximum current capability of a typical Futaba/JR servo plug is 3A..... 
Tim Mackey06/10/2010 09:58:17
avatar
20920 forum posts
304 photos
15 articles
Good stuff guys....and all seeming to broadly be in line with the testing.
Of course, we also need to remember that the integrity of a properly installed radio system also depends on potential "burst" currents that may occur.
For instance if a control surface locks up/binds, or perhaps suffers from flutter. Flying in extreme conditions, and with huge control surfaces and deflections as in 3d models and aerobatics etc can cause momentary large bursts of currrent, and the system needs to cope with those " just to be on the safe side".
That figure of 3A seems a tad low to me Steve, and I think I may well spend a little time today to actually find the figure at which breakdown occurs. Of course, again a lot depends on installation methods and cooling etc. If the wiring is run inside a tube etc, then it will heat and fail quicker. The reason our overhead mains electricity supply cables appear to be so relatively thin is down to the cool atmosphere in which they operate.
More tests later
Myron Beaumont06/10/2010 10:25:53
avatar
5797 forum posts
51 photos
Anybody remember "Copperslip " ? I have had a tin since I don't know when (used to smear wheel studs and nuts ) I use the mearest smidgeon with care on high current connections in all sorts of applications .I'm pretty sure it reduces resistance whereas WD 40 can melt or soften some plastics .
It's probably been banned by H & S come to think of it
Steve Hargreaves - Moderator06/10/2010 10:41:03
avatar
Moderator
6765 forum posts
199 photos
The 3A figure came from a note on the Component Shop website....obviously no idea where they got it from (see here). I agree it may be a tad low....maybe its a continuous rating.....I'm sure it would take more for a short period.
 
We need to be aware (as I'm sure most are) that the FULL current supplied flows through the battery/switch harness AND the positive/negative bus connections within the Rx...so if each servo is taking 500mA then thats a total of 2A through these components (in a typical model)....I would think it doubtful that each control surface would require 500mA to move it at the same time, although those who have seen me fly might disagree...
 
It's also worth remembering that not all servos/extension leads/switch harnesses are created equal as some have significantly chunkier wiring than others (apologies for the technical term there...) & these will flow more current without the voltage collapsing as I recently found  (thread here if you're interested). I wouldn't call what I did a "test" as such but I did find that thin extension leads can cost you a significant voltage drop along their length when currents start to increase.....
 
Does anyone know what size our servo leads are typically? They look around the 24-28AWG size to me but I can't be sure....I'm sure I've got some current tables for AWG wire somewhere.....
Danny Fenton06/10/2010 10:45:20
avatar
9755 forum posts
4556 photos
Very interesting Tim, so we are all lugging more weight around than we need to........

All Topics | Latest Posts

Please login to post a reply.

Latest Forum Posts
Support Our Partners
Slec
CML
electricwingman 2017
Sussex Model Centre
Advertise With Us
Sarik
Latest "For Sale" Ads
NEW POLL - has the pandemic altered your event safety perceptions?
Q: Has the covid pandemic deterred you from attending shows and events in 2021?

 No, I'll be attending just as many as I usually do
 No, but I'll choose my event with greater care
 Yes, I'll attend fewer events going forward
 Yes, I wont attend any where previously I have

Latest Reviews
Digital Back Issues

RCM&E Digital Back Issues

Contact us

Contact us

Magazine Locator

Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!

Find RCM&E!