6 Turnigy TG9s and a 45amp esc with a separate bec

[Pat (rActive) Harbord]

I'm building the Jamara Beachcraft Bonanza and have the servo count up to 6 including retracts. I understand that TG9s are very power hungry so bought the GT45 with 3amp seperate bec from BRC this is rated to peak at 5amps for 10 seconds. Now when I waggle the aileron servos or the pair for the vtail (even before connecting to control surfaces) the bec unit gets very hot very quickly, ie burns your fingers. I can't belive these servos are drawing that many amps, is there a way I can test the draw? I have a watt meter and a digital multimeter. Any thoughts/advice? I will junk the TG9's if neccessary.

[Tim Mackey]

If the BEC unit is a simple linear device, these can run hot even when unloaded...as the excess voltage is simply turned to heat - this is especially so when powered up by a high cell count battery. The higher the input voltage the more has to be burnt up to drop down the output.

I did some in flight testing of servo current some time back, and found that the levels were actually quite a lot less than most folk expect  - you could just put a multimeter set to 10A range in the positive feed line from the BEC to radio, and see what sort of figures you get. Of course, it will be a little slow on response time to be very accurate, but should show an idea.

After some searching, I failed to find the ESC you mention....I think you meant GX45

 

[Frank Skilbeck]

Timbo, is that correct, the BEC will only heat up if you are drawing some current (Power = Volts x Amps) and if using a linear BEC with a 3s Lipo (say 12v) and stepping down to 5v then the power dissipated will be 12v -5v times current drawn by the user so even a couple of amps will generate over 10 watts in heat. 

 

I'd check the current drawn by the servos by hooking us a regular Rx battery and digital multimeter (i.e. leave the ESC etc out of the circuit for now) you might have a faulty servo. I've used a few turnigy servos (not sure about the TG9 thought) on electric planes without any issues.

[Pat (rActive) Harbord]

Thanks guys, yes sorry timbo GX45. I assume it's a linear bec. I have 10 tg9s and they all cause the bec unit to get hot. I'll try powering with a 4 cell battery with my meter inline and see what figures I get. I'm using a 3 cell 2200mah lipo pack. I've never had more than 4 servos in an airframe before so want to be very thorough before I take to the sky.

[Tim Mackey]

Not sure I follow your answer Frank, sort of sounds like you agreed with the principle, or maybe not  LOL . Whether by design or fault, I do know that some simple linear regs I have usd in the past did warm up a little even unloaded...maybe they were duff

RA - are you sure that the BEC is designed for 4 cell Lipo input...some are limited to 2 or 3s.  Just a thought.

[Steve Hargreaves - Moderator]

A linear regulator will always get warm even without running any servos are they are effectively dissapating the excess voltage as Timbo points out in his initial post. Certainly "too hot to touch" would be worrying to me....anything that hot is more likely to fail than something running cool.

 

As Timbo says check the max allowable input voltage.....the further away you get from the 5volts output the more power the regulator will have to disappate & the hotter it will get!!!!

[Pat (rActive) Harbord]

I'm running a 3 cell lipo so nominally 11.1v

I was surprised when the bec got hot especially as the servos are not under load (not connected to control surfaces)

 

This is the first time ive used a separate bec unit. I've never noticed an esc with bec built in get even warm.

 

Right off to run some tests

[Tim Mackey]

Looking again at the specs.... it claims an input of up to 22V ( 6s lipo ) so you should be well within at only 11.1V.

I must say it looks suspiciously like the "super simple" ESCs that are available from other sources...with a separate BEC simply soldered on via fly-leads

[Pat (rActive) Harbord]

okay, couldn't find a 4 cell receiver battery in my odds and sods box so just tested the current draw with the intended setup

3 cell lipo - ammeter - GX45 esc with bec in line - receiver - 2 servos no load

TX programmed for v-tail mix

 

constant left right on the rudder channel gave me peaks of around 350ma, furious stick twiddling produced 860ma.

 

So even allowing for some error I would think the two servos are drawing 1amp max

 

The bec still got very hot during the test, but cools down very quickly again.

 

The distructions say: the bec unit output voltage 5.5V rated at 3A (5.8A peak)

input: 6-20 Ni-Mh/NiCad or 2S - 7S LiPo

 

 

[Pat (rActive) Harbord]

Posted by Timbo - Administrator on 16/05/2010 17:27:19:

I must say it looks suspiciously like the "super simple" ESCs that are available from other sources...with a separate BEC simply soldered on via fly-leads

 Your description sounds right on the money

 I chose it because I wanted to make sure I would be safely within the current requirements of 6-8 TG9 servos renowned for there high current draw. This seemed to be the perfect answer.

 

 

Edited By rActive on 16/05/2010 18:37:12

[Tim Mackey]

Remember of course that when flying, the control surfaces will be loading the servos and the current will increase a lot further than in this static test. Despite the blurb, I am afraid I would still be more than a little wary of using it - you made your decision and choice on sensible grounds, and obviously aim to ensure as far as possible that the supply will be reliable.

Now this is just me, but I would be returning the unit as unsatisfactory to supplier ( BRC are prettty good usually in these type of circumstances ) and using a proper stand alone UBEC device, rated to 5A or so to give plenty of headroom. These units are not expensive these days, especially when considered agains the price of the model...and a potential crash!

Havent looked lately, but if BRC dont sell these...I know a man who does

Edited By Timbo - Administrator on 16/05/2010 19:12:06

[Pete B]

I'm not exactly knowledgeable on lecky matters but, looking at that photo, the BEC is connected by what appears to be normal 3-core servo wire. My suspicious mind wonders if all that has been done is to move the BEC electronics away from the ESC body, thus losing any heatsink area for the BEC?

 

If you are running several servos through a single servo wire, I'm not surprised something is getting hot!

 

My UBEC's have smaller gauge wire than the 12 or 14SWG on an ESC, but at least I can see the (18 or 20SWG?) wires connected directly to the battery supply and not emerging anonymously from the ESC's heatshrink..........

 

I'll throw in one of these in advance in case I'm talking nonsense..............

 

Pete 

edit: spelling 'cos I'm a pedant...............

Edited By Pete B on 16/05/2010 19:34:51

[Pat (rActive) Harbord]

I'll have a chat with BRC tomorrow, looks like I need a different solution for my particular setup.

[gary w]

That's the problem with GX range "plug and play" very basic..As Timbo said the BEC is simply fly leaded on to the circuit board...all you need is a 3amp UBEC for about 8 quid as your only using 3s it should handle those servo's..

[Pete B]

I've been thinking about my post above and haven't changed my view on being happier to have a heavier gauge wire connecting the UBEC to the battery. However, a normal 4.8V Rx pack harness, using standard servo wire (26SWG?) has to supply sufficient current through the Rx tu run, say, six servos, so perhaps the gauge of the wire isn't particularly critical.

 

Anyone any thoughts on it?

 

Timbo, where are you?

Pete

[Tim Mackey]

Well I have to say that when I first saw your post I agreed, but didnt think it necessary to add anything realy. Now that you ask me specifically, I reckon both ideas are right

Some servo wire sold is heavy guage, and aslo some battery packs use heavier than norm too - It may well help to minimise any voltage drop - especially over longer runs - but I doubt that a servo / multiple servos would really draw so much power that the standard wire would be unable to carry the current. generally speaking, current carrying capacity of the various wire guages is way higher ( short term ) than specified, especially ss they are usually air cooled. I am often surorised at just how small the cables are sometimes on the various LiPos I buy, and on ESCs and motors which carry relatively huge currents.

[Pat (rActive) Harbord]

I ordered a seperate bec unit from giant cod. I'll let you know what happens when it arrives. I understand they will be up and running again tomorrow following their relocation over the weekend.