Tim Mackey Posted December 19, 2007 Share Posted December 19, 2007 As you probably know, I like me 'lectrics and have had one or two discussions about BECS and the like. My next "Santa borne" project involves electrifying a new scale WW1 Bipe, with a projected AUW of around 8.5lbs, 4 digi servos, and a 20 volt fuel tank . I was going to use a standalone 6v out BEC which I have lying around unused, but thought I would do some testing of the system and here's the results for anyone interested........ I plugged in 4 X DS821 digital full size servos to an AR7000 Spekky rcvr, and mounted the servos on a board and suspended weights from the arm of each one. The weights were 1.6lb each. I figured that this would represent a reasonable load on each servo, and probably exert more pressure on them than in normal flight, after all it is not exactly a 3d aerobatic machineThe weights were arranged to be just off the floor, and could be pulled straight up by the servo arm. I used a 4 cell Lipo for the experiment rather than a 5 cell, as I MAY be using a 4 cell in practice, and a 5 cell would give even lower current consumption into the UBEC, for a given output. Off load readings ( receiver disconnnected ) were..... Input V 16.3 I @ 7m/aOutput V 6.0 I @ 0On load, with servos still.......Input 16.2V I @ 34 m/a ( .55 Watts )Output 5.89V I @ 85 m/a ( .52 Watts )Moving one servo gave............Input 16.1 V I @180 m/a peak ( 2.89 Watts ) Output 5.86 V I @ 455 m/a peak ( 2.66 Watts )Moving all servos gave............Input 16 V I @ 280 m/a peak ( 4.48 Watts )Output 5.86 V I @ 677 m/a peak ( 3.96 Watts )Fully stalling one servo by holding the output arm with pliers whilst moving the tr stick showedInput 16V I @ 290 m/a ( 4.64 Watts )Output 5.85V I @ 725 m/a ( 4.24Watts ) So I reckon that even with a fully stalled servo, and the others all working hard, the current drawn from the battery should not exceed .75A and that is allowing a high margin. The output current of the UBEC @ 6v nominal should be around 1.5A, which is well within the rating of 2.5A continuous.In normal flight conditions, with good non-binding control runs to flying surface controls, I expect the current output from the UBEC to average around .5 to .75 Amp.I found the current draw of the digital servos surprisingly low against what I expected, and the experiment also proved the benefits of using a switch mode regulator with a good high voltage input, in so much as the current required to be supplied by the battery is much lower than it would be if using a normal flight pack 4 or 5 cell battery. I feel this also further enhances the case for using a 2 or 3 cell LiPo with a suitable regulator to power onboard radio gear - not withstanding some peoples reservations about using LiPos generally Quote Link to comment Share on other sites More sharing options...
Former Member Posted December 19, 2007 Share Posted December 19, 2007 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 17, 2008 Share Posted July 17, 2008 Timbo wrote (see)So I reckon that even with a fully stalled servo, and the others all working hard, the current drawn from the battery should not exceed .75A and that is allowing a high margin. The output current of the UBEC @ 6v nominal should be around 1.5A, which is well within the rating of 2.5A continuous.In normal flight conditions, with good non-binding control runs to flying surface controls, I expect the current output from the UBEC to average around .5 to .75 Amp.I found the current draw of the digital servos surprisingly low against what I expected, and the experiment also proved the benefits of using a switch mode regulator with a good high voltage input, in so much as the current required to be supplied by the battery is much lower than it would be if using a normal flight pack 4 or 5 cell battery.I feel this also further enhances the case for using a 2 or 3 cell LiPo with a suitable regulator to power onboard radio gear - not withstanding some peoples reservations about using LiPos generally are these figures representative of standard servos too?I've just ordered one of these-http://www.giantcod.co.uk/product_info.php?products_id=402342&osCsid=9a6ee020012f6409c87ce3eb1679bc8dto run the 3 primary controls in the York, and also flaps and 2 retract servos-though unlikely they will all be moving at the same time!Then I will need to disable the BEC input from the ESC, you said-by cutting the red (middle)wire? Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 17, 2008 Author Share Posted July 17, 2008 The figures will vary from servo to servo and of course were only relevant to my particular test setup - binding and friction etc will paly a part in the overall current consumed. If your ESC has a BEC unit onboard, then YES you MUST disconnect the +ve supply from this BEC to the receiver - best perhaps to remove the pin from the plug and tape it back up out of the way, thais means it will be still availbale later if required for a different application.Must admit would probably have gone for higher capacity ubec than 3A for a 6X servo set up - I hear what you say about simultaneous op, but the things are so cheap these days, and considering the whole model's integrity is dependant on these little gizmos...a few quid extra on a 5A unit or so would give me the peace of mind I think . Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 17, 2008 Share Posted July 17, 2008 thanks Timbo-will nip back to giantcod a minute and have another look....can you please confirm red middle wire?ta Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 17, 2008 Author Share Posted July 17, 2008 Well depends on your radio - Jr .... red is positive / brown is negative / and orange is signalFuby.... red positive / black negative / white signal.ESCs of course may use different colours again....but usually follow the fuby codes. Middle ? - Yes.PS if you ring Rob at Giant Cod he will change your order I am sure Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 17, 2008 Share Posted July 17, 2008 JR/Spektrum, but middle is the important part!pondering on the UBEC-the next one up-the Hobbywing 5-7.5A is 3x price (big deal-12 quid) but also 3x the weight...? Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 17, 2008 Author Share Posted July 17, 2008 well even 3 times the weight should be no big deal for the model concerned surely? As for the price - well as you say 12 squid to keep your pride and joy aloft is small price to pay surely? Incidentally heres the other thread you were looking for Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 17, 2008 Share Posted July 17, 2008 Timbo - forum moderator wrote (see)well even 3 times the weight should be no big deal for the model concerned surely? As for the price - well as you say 12 squid to keep your pride and joy aloft is small price to pay surely? Incidentally heres the other thread you were looking for no-but the idea at the start was to dump as much weight as possible..and the grams are dribbling back on..and the final bill is creeping up...thanks for the link Quote Link to comment Share on other sites More sharing options...
Frank Barcello Posted July 18, 2008 Share Posted July 18, 2008 cant get enough of it Quote Link to comment Share on other sites More sharing options...
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