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Electric retract power supply


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Most retracts these days sense an overload and cut off.  If they do this and you're using more than the bare minimum receiver battery, I see little risk.  Even less with an electric powered model that uses a uBEC.  An older ESC with a linear BEC will almost certainly struggle.

 

If you can measure the supply voltage at the receiver during operation you will see if there's any significant voltage drop.

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You could us a separate battery pack if the model will stand the additional weight + the more links in the chain the more likely something will go wrong...batteries and connections are probably the most likely causes of problems.

 

I was recommended by a very experienced flyer and owner of a UK business that sells a wide range of batteries, ESC's, motors, UBEC's etc to fit a large capacity UBEC and good lipo. The theory is that if the UC or any other servo fails the the UBEC will supply enough power to clear the faulty item without dropping the voltage on the RX. This of course maintains the keep it simple approach with the minimum number of components.   

 

I have models that use both approaches so it boils down to what you fancy really.

 

PS I don't think a failed retract makes any difference be it in an IC or electric model....won't effect motive power as its the impact on the RX which is the issue.

 

PS - IMHO a far bigger problem is being able to brown out the RX with existing servos under high load and a poorly rated UBEC

Edited by Chris Walby
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Matt, 

 

My 80inch IC twin has a 3S2200 RX battery and 20A UBEC, partly as it needs the weight up the nose and secondly I use 3S2200 as flight batteries in electric models so I keep tabs on their condition. If a 3S2200 can't stand pulling a foamie or small balsa model I won't be trusting it in my prize ic twin. I could go to the effort of having dedicated RX batteries, but then I would need to keep testing them on the off chance that a nice day turns up and be ready.

 

I have had HK 5A UBEC's brown out a RX with three servo's (no mechanical load) on a 38 inch model and it didn't have retracts, other manufacturer no problem.

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11 hours ago, Chris Walby said:

I was recommended by a very experienced flyer and owner of a UK business that sells a wide range of batteries, ESC's, motors, UBEC's etc to fit a large capacity UBEC and good lipo. The theory is that if the UC or any other servo fails the the UBEC will supply enough power to clear the faulty item without dropping the voltage on the RX. This of course maintains the keep it simple approach with the minimum number of components. 

 

Chris, the words 'enough power to clear the faulty item' - what does that actually mean?

 

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BTW personally, on an all electric model, I would go with two BECs as per Trevors suggestion.

 

The BECs weight next to nothing, and provide supply separation for the retracts, which is the main path to avoiding failure on the RX/primary control power. Same practical outcome as the old school approach of having two nicad packs.

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2 hours ago, Nigel R said:

 

Chris, the words 'enough power to clear the faulty item' - what does that actually mean?

 

Retract wiring will melt at some point, then it's problem solved?.  I'd always use a separate supply for servo driven mechanical retracts as a stalled retract servo can keep drawing a high current.  As Martin pointed out,most electric retracts have stall protection so they will switch off after a couple of seconds.   All the ones I've used have done so, even cheap HK ones.  I've run a single supply with electric retracts for many years without issue.  What I do try to do is to use one channel per leg with a delay between each so start up surges don't all come at once.  It works well if you have enough channels available and has the bonus that the staggered retract action looks especially good on a warbird.

One option I've set up but have yet to fly is on an electric twin I have one BEC driving the retracts and the other the sevos.

Edited by Bob Cotsford
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8 minutes ago, Bob Cotsford said:

What I do try to do is to use one channel per leg with a delay between each so start up surges don't all come at once.  It works well if you have enough channels available and has the bonus that the staggered retract action looks especially good on a warbird.

 

Would a cheap gear door sequencer work to separate the timing for two legs?

 

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Probably Nigel, but I have the luxury of being able to run them off an SBUS decoder with a choice of 16 channels and built in servo slow and delay functions (OpenTX?).  I may be overthinking things (not for the first time) but I just like the idea of smoothing the  supply demand when it comes to a potential 3+Amp draw on a trike setup.

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I too always use a separate power supply now for electric retracts: a simple BEC and 2s LIPO works a treat.  There's another reason for doing this besides the risk of stalled retracts momentarily dropping the voltage to the receiver; I found with some retract units - notably the pricier e-flight 120 size ones on my Zero! - that driven from just the receiver power supply the retracts were unreliable, particularly if servos were operating at the same time.  Just not quite enough juice to power them... sometimes they would work, sometimes not, sometimes one would work and the other not... a separate power supply and no more problems.    Remember to cut the red (positive) wire in the connection from the retracts to the receiver though.  Take a moment with pen and paper to work out the wiring harness needed is worthwhile. 

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8 hours ago, Bob Cotsford said:

Probably Nigel, but I have the luxury of being able to run them off an SBUS decoder with a choice of 16 channels and built in servo slow and delay functions (OpenTX?).  I may be overthinking things (not for the first time) but I just like the idea of smoothing the  supply demand when it comes to a potential 3+Amp draw on a trike setup.

 

I've done the same (not S-Bus), but for no other reason that it looks cooler ?

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I've used both single and separate supplies but it always worries me if there is enough juice to run the retracts for the time it takes the overload circuitry to trip in case of a jam with one battery.

My latest project will have a separate small 2s LiPo to power the retracts. I'm going to use a small relay in the retract circuitry enabled from the Rx power. This way I only need to have one switch for everything. Given that the retracts only get used for a few seconds per flight it will probably last all season on one charge.

 

A.

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I guess you can make it as complicated as you want. Dual batteries, two UBEC's additional relays a bunch more wire and connections. Still like the keep it simple approach, but can see the advantages....but only as long as there are no more chances of failure/getting it wrong.

 

Nigel:

Chris, the words 'enough power to clear the faulty item' - what does that actually mean? 

 

It means a battery with enough capacity to operate under fault conditions until the fault has cleared and a UBEC that can provide enough current without the voltage dropping below the min RX voltage.

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