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Old Jun 25, 2009, 12:02 AM
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Converting a Koolflight 5V UBEC to 6V

Has anyone successfully converted a Koolflights high voltage 5v UBEC to 6v output? Wondering how difficult it would be and if you know the component changes necessary.

Thanks

Kevin
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Old Jun 25, 2009, 08:30 AM
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Richard Ingram's Avatar
Birmingham, AL
Joined Aug 2002
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I looked inside that bec a while back and if memory serves it is a switching regulator built around a standard reg chip. I suspect that the feedback resistor network could be changed to provide 6 volts output. The spec sheet on the reg chip would show how to calculate the values. Just open up the plastic cover and see what the number is on the regulator and then find the datasheet.

Update:
Well, memory didn't serve me well. The bec uses the LM2596 regulator but it is the 5 volt fixed version. I don't know if the voltage can be raised or not. We'll need someone with more knowledge. The regulator chip is made in an adjustable version but not used in the one I bought.

Further Update:
Looking at the datasheet on the LM2596 it appears the differrence between the fixed voltage versions of the device nad the variable voltage version is that the feedback resistor network is "built in" on the fixed voltage device. Looking at the variable device increasing resistor R2 in the feeback resistor pair would increase the output voltage. With this in mind adding additional resistance at pin 4 in the fixed voltage (5v device) would increase the output voltage. This would take a little experimentation since the values of R1 and R2 are unknown (at least I did not see the values in the datasheet). Might be worth a try if the board layout was such that a trace could be cut and a surface mount resistor could be added. You could start with say 220 ohm and see what the voltage chage is and go from there.
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Last edited by Richard Ingram; Jun 25, 2009 at 01:21 PM.
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Old Jun 25, 2009, 01:52 PM
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Richard,

Thanks for the info. Before I jump the gun I'm not even sure if I need the 6v version. I was wanting to use it with a Spektrum AR7000 receiver and was worried about receiver drop out at 5 volts.

Anyone using the 5 volt version with 4 100 oz digital servo's on the 7000 receiver?

Kevin
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Old Jun 25, 2009, 07:25 PM
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N.E. USA
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Quote:
Originally Posted by Tipover
Richard,

Thanks for the info. Before I jump the gun I'm not even sure if I need the 6v version. I was wanting to use it with a Spektrum AR7000 receiver and was worried about receiver drop out at 5 volts.

Anyone using the 5 volt version with 4 100 oz digital servo's on the 7000 receiver?

Kevin
Yup... I used one of the 5v versions in a plane of mine that uses five HS-5645MG digital servos (AR7000 and 9000) but please understand that these are not stressed to the extremes and if they were, they could draw up to 1.5-2 amps each so be careful (KoolFlight BECs are only good for 5 amps max, momentarily). Castle Creations is soon coming out with a new "BEC Pro" which will be good for up for 20 amp loads (12s max input) and I plan to use these in my newer models (of course, like the smaller BECs CC sells... you are able to program the output voltage from 4.8-12.5 volts). E-flight also sells a 60amp BEC (6s input max though) as well as 30-40 amp BECs and I use these BECs in my higher performance 3-D models that use 4+ digital servos (160oz-in +).

I've never experienced a "brown-out" situation with Spectrum receivers, and you probably will not if you use a voltage reg vice directly connecting a battery. Just make sure the reg is spec'd properly for your max current draw (I always measure mine just to be sure). Also... Spektrum brown-out (i.e. reboot) happens when the supply voltage drops below 3.5v (actually... mine work fine down to 3.2 volts... the 3.5v spec by Spektrum allows for some wiggle room)
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Last edited by Tomapowa; Jun 25, 2009 at 11:36 PM.
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Old Jun 25, 2009, 11:47 PM
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Ok guys,

I've been using this Kool Flight HV 5v UBEC with 10s1p A123 for 3 years now with 4 JR DS9411's without a problem. This is in a 8 lb Funtana 90, so no big flight loads.

My concern has been if the AR7000 will be ok with the regulation it provides. Sounds like it should be fine since the 5 volts is being regulated down from such a high source voltage.

Thanks much for the input. Feel a little more secure trying it now. Still interested in the conversion to 6V if I can figure that one out later.

Kevin
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Old Jun 29, 2009, 04:23 PM
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Kevin,
I've been meaning to post this... I mod'd one a while ago with no issues.
See attached pics... the Koolflight reg I have uses a National LM2596S-ADJ (the adjustable version)... and two resistors determine the output voltage. The resistors on the board are for the 5v version (yanked this from an older plane) and are 1K (brn-blk-blk-brn-brn) and 3.09K (org-blk-wht-brn-brn), both 1% resistors. I changes mine to 3.9K and 1K, and this gives you an output of 6.027 volts.

Check out the spec page for the output voltage formula:
http://www.national.com/ds/LM/LM2596.pdf

Hope this helps...
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Last edited by Tomapowa; Jun 30, 2009 at 09:00 AM.
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Old Jun 29, 2009, 06:29 PM
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Tomapowa, many thanks, and I hope you didn't go to all that trouble of pulling things apart just for me

After thinking this through a bit more... I've come to the conclusion that changing the Koolflight regulator to 6v output will likely increase the servo's current draw and possibly increase the chance of a Spektrum receiver brownout. My 9411 servo's are pleanty fast as is. What do you think, leave well enough alone and just try the 5V reg with the AR7000?

Kevin
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Old Jun 29, 2009, 06:52 PM
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Actually, that BEC was removed from a plane I sort of crashed this weekend (older electric 3-D that had hundreds of miles on her!)
If you really do not need the extra servo torque, you could stay with the 5v servo supply. It would take considerable draw from the BEC for it's output to get below 3.3 volts... so it should not be an issue. Spektrum brownout issues appear to be related to people using dead or near-death 4-cell (4.8v) battery packs... a big obvious no-no. I hope to test a few BECs (Koolflight for one) and test them under various loads and various input/output voltages. I'll post some data when I get it all compiled. I'm still awaiting the official release of the Castle Creations BEC Pro.
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Old Jun 30, 2009, 08:11 AM
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Quote:
Originally Posted by Tomapowa
Kevin,
I've been meaning to post this... I mod'd one a while ago with no issues.
See attached pics... the Koolflight reg I have uses a National LM2596S-ADJ (the adjustable version)... and two resistors determine the output voltage. The resistors on the board are for the 5v version (yanked this from an older plane) and are 3.09K (brn-blk-blk-brn-brn) and 1K (org-blk-wht-brn-brn), both 1% resistors. I changes mine to 3.9K and 1K, and this gives you an output of 6.027 volts.

Check out the spec page for the output voltage formula:
http://www.national.com/ds/LM/LM2596.pdf

Hope this helps...
I think you have the color codes for the resistors interchanged.

Dave
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Old Jun 30, 2009, 09:00 AM
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Joined Oct 2003
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Quote:
Originally Posted by 376782
I think you have the color codes for the resistors interchanged.

Dave
Yup... good catch... sorry... fixed in orig post... (orange obviously is "3" and goes with the 3.09K resistor, not the 1K resistor)
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