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current draw

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john melia 127/02/2015 11:11:40
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how do i check the current draw of an ar8000 spektrum rx , i am using five hitec 5685 hv servos , with a powerbox-systems sensor switch delivering 7.4v to the rx

Steve Balaam27/02/2015 12:21:03
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Get yourself one of these works very well

**LINK**

Cuban827/02/2015 12:34:41
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Same thing here, better price.**LINK**

Phil Green27/02/2015 14:54:07
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Posted by john melia 1 on 27/02/2015 11:11:40:

how do i check the current draw of an ar8000 spektrum rx , i am using five hitec 5685 hv servos , with a powerbox-systems sensor switch delivering 7.4v to the rx

You need to unplug all the servos, all satellites, and unplug the smart switch, leaving just the unconnected receiver. Then take an extension lead, separate and snip the Pos (red) midway, strip the ends and plug one end of the modified extension lead into the rx and the other into the smart switch. Connect a multimeter set to (say) 200mA across the two stripped ends, meter neg to the receiver end. Switch on & read.

Isolating the receiver like this is the only way to measure its current draw. It should be roughly in the region of 100mA.   Servos will not significantly affect the receiver's current draw as their signal inputs are fairly high impedance.

Cheers
Phil

Edited By Phil Green on 27/02/2015 14:56:07

john melia 127/02/2015 15:17:25
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so servos working under load , will not effect the current draw of the rx ? what would happen if two or three servos were working very hard , or one stalls , would the high current not damage the bus in the rx . Is that a different scenario to what the rx actually draws current wise ?

ben goodfellow 127/02/2015 15:19:58
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thats the rx bus rail ,john thats different to the rx draw... I THINK.. fly low

ben goodfellow 127/02/2015 15:20:57
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must off been typing at the same time.....

john melia 127/02/2015 19:12:51
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wow thought a few electronic gurus would have chimed in by now

Phil Green27/02/2015 23:47:31
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Posted by john melia 1 on 27/02/2015 15:17:25:

so servos working under load , will not effect the current draw of the rx ? what would happen if two or three servos were working very hard , or one stalls , would the high current not damage the bus in the rx . Is that a different scenario to what the rx actually draws current wise ?

Entirely different. The question was 'how do i check the current draw of an ar8000 spektrum rx'.
The battery supplies the bus. The receiver is a spur and each servo is another spur off that bus - therefore servos aren't a consideration in determining the receiver consumption. The receiver is a 3.3v device and provided a heavy or stalled servo doesn't drop the bus below the drop-out level of the receiver's internal regulator then the answer is no, servos cannot affect current drawn by the receiver.

I now wonder if the question should have been ''how do I check the overall current draw of a complete installation using an ar8000 spektrum rx and xyz servos' which is a totally different question ?

Posted by john melia 1 on 27/02/2015 19:12:51:

wow thought a few electronic gurus would have chimed in by now

Maybe one will chip in soon John

Cheers
Phil

 

Edited By Phil Green on 27/02/2015 23:52:05

Martin Harris27/02/2015 23:57:52
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Posted by Phil Green on 27/02/2015 23:47:31:
Posted by john melia 1 on 27/02/2015 19:12:51:

wow thought a few electronic gurus would have chimed in by now

Maybe one will chip in soon John

Cheers
Phil

Edited By Phil Green on 27/02/2015 23:52:05

Perhaps you should check Phil's profile, John?

john melia 128/02/2015 00:19:03
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phil that is the question i was trying to ask but didnt know how lol , basically is the ar8000 up to handling five 7.4v servos .

Pete B - Moderator28/02/2015 09:24:59
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I think the question you need to ask is whether your power supply is up to supplying 5 7.4v servos, John, the RX is just another component requiring an adequate supply.

Pete

Ray Mcdermid22/04/2015 18:26:18
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it will handle it using a 7.4 supply,,, but you have to make sure that the amps (not voltage) is sufficient to drive your servos. if your battery is under 1000ma (milliamps) forget it as you will be underdriving your servos, thats if they move at all.

yet a 7.4v battery with a 2000ma (2amps) and above will be a lot livlier due to the strength (not voltage) of the power supplied.

think of a lipo for instance, a 1 cell 7.4 gives out the correct voltage with a relatively low amperage, a 3 or 4 cell doubles the amperage due to the multible cells (plus more storage due to multiple cells)

Trying to explain this more, if you have 5 cheap aa type batteries they give off 7.5v but next to no amps so next to useless as a servo power supply, yet the battery I am using at the moment is a 7.2v NI-MH 2200 which drives everything with ease,,,,,, same power 7.4v but a massively different amperage rating.

I'll try to put something up on Youtube that shows my plane's servos with different amp ratings.

Last thing you need is servo's dropping out or going sluggy during flight.

Hope this helps

Ray

Phil Green22/04/2015 19:00:43
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Sorry Ray I know what you're trying to say but your reasoning isn't quite right!
By 1000mA or 2000mA I assume you mean mAh. This has nothing whatever to do with the battery's ability to supply current whilst maintaining voltage. That is determined by the battery's internal resistance or ESR. Very low capacity batteries can supply huge currents if they have a low ESR. Conversely, large capacity batteries with a high ESR will not. Its not a function of the capacity, in your case 1000mAh or 2000mAh.

Regarding the lipo analogy, one cell isn't 7.4v, its nominally 3.7, but as you mention 7.4 I assume you're talking about a 2S (two cells in series) battery. Extending to 3S or 4S does not double the amperage, which in fact remains at the value of the cell with the highest ESR and hence lowest 'C' rating, since each cell in a pack will be slightly different and in a series connection the current is limited by the worst cell. You are correct in that more series cells increase the energy storage in terms of watt/hours, but not in terms of current.
Paralleling cells of course is a different matter, but I'm assuming you're talking series since you mentioned 3 and 4 cell packs and 1S3P and 1S4P packs are not popular formats.

You're also right in that cheap high-capacity AA cells generally have a poor performance due to their high ESR, but again, thats nothing to do with their capacity.

I know what you were meaning to say Ray and the overall message is the right one, just that your principles were a bit shakey, hope you don't mind me chipping in 

Cheers
Phil

 

Edited By Phil Green on 22/04/2015 19:56:39

Ray Mcdermid22/04/2015 22:01:31
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Sorry for way that I attempted to simplify things.

I only have and build very large models, weight is not really an issue unless the model is for stunt\3d flying

on the large (petrol\ radial engine) warbird under construction at the moment, the customer has a strict spec list and power hd storm servos (over £100 each) had to be used, luckily these do not need to be voltage regulated, but every build is different. Fitting decent BECs is wonderful (when I started modelmaking these just did not extst) but getting the power ratio just right can be touchy (seen Becs pop and catch fire a few times)

Up to a few years ago I used to make a simple voltage dropper and regulate each and every servo with one.

The above warbird (F-4U Corsair with a Moki 250 radial) installation will run quite happily on a 2cell lipo, but as yet not fully decided on.

The above project is at the moment 7 months in the making, just over 3\4 finished.

Unless a model is in front of you on your own bench, all a person can do is give a multitude of different solutions until the correct one has been found.

Cheers and sorry if I misled anyone, I assure you that it was not intentional.

Ray

Phil Green22/04/2015 22:55:38
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Hey no probs Ray, as you say, many ways to skin a cat. Sounds like quite an undertaking you have there. Is there a build thread anywhere?
Cheers
Phil

 

Edited By Phil Green on 22/04/2015 22:55:48

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