UBEC Warning

[John Minchell]

I had a 4 Max 20amp UBEC installed in my large Valenta Thermik XXXL 5m electric soarer for GPS Triangle racing. 

I wanted to save the weight of a separate Rx / servo battery and this looked like a good option, without relying on the BEC in the ESC which would not handle the 10 servos in the model.

It was fine for about a dozen flights, but has now failed.  Instead of failing and not passing any power through, it failed and passed the full 6 cell LiPo voltage (25v fully charged) through to the Rx and servos.  It has unfortunately fried my RX12 DR Pro Multiplex dual receiver (£260) and taken out the two digital servos in the fin (£100+), fortunately I did not have the wing attatched with another 8 servos in it.

The specs of the UBEC are 3 to 12 LiPo input and 20amp output (8 to 12 servos). 

I have spoken with George at 4 Max about it and he says it is the first one he has heard of which has failed. 

 

I just wanted to advise anyone using this unit to keep an eye on it and check its output before connecting it and risking an expensive model or equipment.  Or to do as I will now do and carry the weight of an additional Rx/servo battery.

In closing I have a number of other 4 Max products and they are performing ok, so this is extremely disappointing and expensive when you are on a tight budget. 

Pics show failed component after I removed the heatshrink covering.

John M

 

Edited April 21, 2021 by John Minchell

[Sparks]

Hi John,

 

Sorry to hear of your misfortune. Looking at the photo, the voltage selector link looks as if it is bridging the 6V and 7V selector pins.

Was it in this position when the unit failed? If so, this may have been the cause of the overvoltage as the BEC would have no feedback

reference, so the output would just go to unregulated maximum. The exploded ELCO 270uF certainly testifies to overvoltage.

 

 

Edited April 21, 2021 by Sparks
spelling

[Nigel R]

Ouch.

 

Sparks, I imagine the jumper just bypasses one of the resistors in that ladder next to the pins. A second assumption would be that the resistor between the pins for 5V position is a zero ohm, giving a safe selection for 'no jumper attached'. Difficult to say what failed first but that cap has certainly popped as you say probably overvoltage.

 

[Gary Manuel]

I've been trying to work out how the jumper selection works, having 5 different voltages to select, using just one supplied jumper. Can't find instructions for it but.....

 

My first observation is that the photos show the UBEC upside down relative to the heatshrink cover / label. The jumper is near (second and third from end) the 5v/5.5v end rather than the 7v / 9v end.

 

My second observation is that there are 6 pins, giving the jumper 5 possible positions to be fitted in. These 5 positions "probably" reflect the 5 positions on the label. The jumper is in the second position from the low voltage end. i.e. it is set for 5.5 Volts, which should be OK for the receiver and servos. 

[Simon Chaddock]

John

That is indeed bad luck particularly as it has taken out so much else.

I note it has a substantial 'fingered' heatsink suggesting there is a significant heat output which in turn requires a positive airflow over the fingers.

Just an observation but could the failure have been as simple as overheating at some point?

    

[John Minchell]

Thanks for the replies guys - the jumper is indeed on the 5.5v pair of selector pins and was always in that position in use, as I have some servos which are only 6.0v capable as well as the HV ones that the UBEC took out.  My MPX Rx is safe to 12v but receiving 25v was too much.

The fingered heat sink is totally covered by the heatshrink plastic as supplied - preventing any through airflow and there is substantial carbon dust and "ash" type residue on the inside of the heatshrink which is where the component blew and failed. 

I am now better educated and will take a scalpel to all ESC's heatshrink that I am using the BEC on to power the radio and allow air flow through the fins inside the heatshrink or across the plate type of heatsink.  Whether overheating was the cause of this one blowing I  will never know, but nothing got overly hot (I checked with a temprerature sensor under the canopy and there are fuselage inlet and outlet airflow holes) as the climb was 30 to 45 seconds or so followed by gliding for up to 30 minutes on the task.  And of course the UBEC was powering the Rx servos for the whole flight.

[Denis Watkins]

Find another solution John, exposing the Lipo connections to a dead short is prevented by the heatshrink.

A considered approach with exposing the cooling fins is what you mean to achieve

Edited April 21, 2021 by Denis Watkins
Mis-spelling

[John Minchell]

Denis, all models which are competition ones or have a large bill attached, will not be using a Bec ESC or a stand alone UBEC like this one.  They will all have separate Rx supply battery and the main power LiPo will only get used for the motor.  I will now only trust Becs in cheap foamys and maybe F5RES 2/3 channel glider etc.  I wasn't advovating cutting off all the heatshrink - only a section at both ends to allow airflow through, but maintaining the electrical insulation.