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Old Jun 18, 2012, 11:03 PM
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A123 Rx pack..... add diodes?????

Folks,

Wish to use 2s A123 pack for receiver, but 3150's are really not suitable for > 5.4V.
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Can I just add 2 x diodes in series to drop the 1.4V (suppose fresh off charge A123's will be 7.2V for short while, dropping to 6.6-6.8V for most of time)
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If so, which diodes would be suitable, considering may need max 3-4 Amps??
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Please advise.
-
Many Thanks, Steve
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Old Jun 18, 2012, 11:34 PM
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Yes you can. IMO, buy some 3A diodes and double up in parallel (6A) so you have a huge window as we don't know the spikes your system sees.
I have no idea where you live, USA or otherwise, as your location is void under your user name but Radio Shack may have what you are looking for. An EE Will disagree with me but in this aplication a diode is a diode is a... Look for the black color, white stripe...

Rick
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Old Jun 19, 2012, 11:53 AM
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+1,
I would also like to mention that you can also use a decent switching BEC, such as the 10A BEC from Castle Creations, but the diodes are simpler, cheaper, and very reliable. Unfortunate that those servos are only rated for 4 cell operation
Pete
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Old Jun 19, 2012, 12:49 PM
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Originally Posted by rampman View Post
Yes you can. IMO, buy some 3A diodes and double up in parallel (6A) so you have a huge window as we don't know the spikes your system sees.
Agree that diodes will work to drop voltage but using two in parallel will not increase load handling as this requires both diodes to have the exact same forward voltage drop, which is almost never the case. As such, one diode will be carrying most of the current load.

Use a 6A (or greater) diode to provide some margin. Also consider adding a heatsink if your servo current load is anywhere near constant as the voltage drop will be dumping more heat.

IMO, an inexpensive switching BEC may likely prove to be less of a hassle and may be more efficient and dump less heat. Also, a BEC will allow you to use a lipoly pack, which have much greater energy density than a 2S A123 pack...

Mark
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Last edited by mrforsyth; Jun 19, 2012 at 01:00 PM. Reason: added heatsink recommendation
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Old Jun 19, 2012, 03:02 PM
Marion
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One good reason to parallel diodes is for better reliability. Yes, I have had diodes fail mechanically due to poor packaging. .
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Old Jun 19, 2012, 04:19 PM
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Thanks for all your replies/interest.
-
Switching or linear BECs do not seem to be an option for a 2s A123 source.
-
I tried a couple of Turnigy ( Hobbywing) 3A becs, supposedly rated down to 5.5v i/p, and even at 6.2v the o/p dropped to 4v for 2.5A load.
-
I have built many linear regs in the past, as follows:-
-
http://www.dream-models.com/eco/electrics-bec1.html
-
They are fine for i/p voltages 8v up, but I have just tested one at
6v input.................it gives a 3v output, so you really do need a >2V overhead for these linear 78XX regs. to work correctly. (don't really want to add another cell)
-
I wonder if anyone has had any success with certain makes of BEC
-
Rgds, Steve
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Old Jun 19, 2012, 07:14 PM
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Quote:
Originally Posted by SteveM View Post
Switching or linear BECs do not seem to be an option for a 2s A123 source.
-
I tried a couple of Turnigy ( Hobbywing) 3A becs, supposedly rated down to 5.5v i/p, and even at 6.2v the o/p dropped to 4v for 2.5A load.
From where was the loaded voltage measured? e.g. - Are you certain that the drop was not due to wire and connector losses and did you verify that the output voltage was higher with same load but with higher input voltage?

Not refuting your claim, merely trying to better understand.

I have had nothing but perfect success with the two following BECs from 'the King':

http://www.hobbyking.com/hobbyking/s...Reduction.html

http://www.hobbyking.com/hobbyking/s...23v_Input.html

Admittedly, I use 3S lipoly or 4S A123 input. Also, at 3A rated output, both are inadequate for your needs.

Have you tried this one:
http://www.hobbyking.com/hobbyking/s...or_Lipoly.html

Also, are you opposed to using 2S or 3S lipoly rather than your A123s? I understand this will require an additional purchase but it will absolutely solve your issue and lipolys are silly cheap these days.

Mark
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Old Jun 19, 2012, 08:21 PM
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Quote:
Originally Posted by mrforsyth View Post
From where was the loaded voltage measured? e.g. - Are you certain that the drop was not due to wire and connector losses and did you verify that the output voltage was higher with same load but with higher input voltage?

Not refuting your claim, merely trying to better understand.

I have had nothing but perfect success with the two following BECs from 'the King':

http://www.hobbyking.com/hobbyking/s...Reduction.html

http://www.hobbyking.com/hobbyking/s...23v_Input.html

Admittedly, I use 3S lipoly or 4S A123 input. Also, at 3A rated output, both are inadequate for your needs.

Have you tried this one:
http://www.hobbyking.com/hobbyking/s...or_Lipoly.html

Also, are you opposed to using 2S or 3S lipoly rather than your A123s? I understand this will require an additional purchase but it will absolutely solve your issue and lipolys are silly cheap these days.

Mark
Thanks Mark,
-
I'll repeat the test tomorrow, checking voltage very close to BEC pcb.
-
Lipos....yes, obvious option.....just wanted to bury A123's in the nose, topping up in situ at 3-4A.
-
will post result...rgds
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Old Jun 19, 2012, 10:45 PM
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Quote:
Originally Posted by SteveM View Post
Lipos....yes, obvious option.....just wanted to bury A123's in the nose, topping up in situ at 3-4A.
Gotcha Steve. Makes perfect sense now.

The more I think about your specific application, the more I personally would discourage the use of diodes to drop voltage.

Reasons for my though process as follows...
  • As you stated in post #1, two diodes will be required to drop voltage into a usable range for your application. Junction drop will be ~1.4 volts. Two A123's in series will have loaded voltage of ~6.2 volts toward the end of discharge. Ignoring wiring and connector resistive losses, this puts you in the 4.8 volt range. Additional drops due to current spikes from heavy servo transients as well as wiring resistive losses may put you in the 'brownout' zone for many 2.4Ghz radio systems.
  • 1.4 volt drop across the series diodes will dissipate nearly 6 watts at 4A current draw. This will absolutely require some airflow across the diodes or other form of heatsinking to ensure reliability.
IMO, using an additional A123 cell (3S pack) or a 2S lipoly in conjunction with a BEC would be a far more reliable alternative. I understand the caution about charging a lipoly within your model but this concern is largely mitigated by simply performing a balance charge with a decent quality charger. (I have performed nearly 5K balanced lipoly charges over the past 6 years without a single incident and would have little hesitation about charging inside any of my planes.)

Really curious to see your BEC voltage drop testing to know whether HobbyKing's input voltage specifications are optimistic.

Cheers,
Mark
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Old Jun 20, 2012, 12:32 AM
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It may surprise you to know that the above is for a Hotliner, on 5s 4Ah pack.
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I also bought the 5-7.5A Turnigy (Hobbywing) BEC, to use with the full 5s lipo, but planned current draw is 120A, and I recently read all of the following:--
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http://www.rcgroups.com/forums/showthread.php?t=1668258
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It made me very concerned over using main flight pack on a $1500 aircraft, at that current,.
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Thanks for all your suggestions.....think may add extra cell.
Will re-test the BEC also.
-
Rgds, Steve
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Old Jun 20, 2012, 02:46 AM
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3A BEC Pics!!

OK.... these pics show 6v SLA input to the 3A Turnigy BEC.
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Left meter shows output, stabbing leads close to BEC....it can't even hold 4.2v!
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Last pic confirms we are just subjecting to a 2.4A load.
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Input leads are clamped between the right hand meter probes, and the SLA terminals with croc clips.
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BEC becomes hot very quickly.
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Rgds, Steve
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Old Jun 20, 2012, 03:17 AM
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Note....just tried an old tx pack input, measuring 7.4V under load..... output holds at steady 5.2v no pbm, although bec becoming hot.
-
More.... just tried the Tgy 5-7.5A one.
6v in, 4.7v out @ 2.6A, 4.4V out at 3.3A load.
Only becomes warm
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Last edited by SteveM; Jun 20, 2012 at 03:32 AM. Reason: more.......
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Old Jun 20, 2012, 05:39 AM
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Rectifiers are better than diodes. They are more durable, have a better package, and you can get them from defective (PC) power supplies.
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Old Jun 20, 2012, 03:20 PM
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Interesting data Steve.

Since your application is a hotliner, I personally would oversize the BEC as servo loads on hotliners that are moving at high speed can be very high due to dynamic loads on control surfaces. Servo current draw can be surprisingly high even when flying flat and level.

Cheers,
Mark
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Old Jun 20, 2012, 03:34 PM
ltc
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Quote:
Originally Posted by SteveM View Post
Thanks for all your replies/interest.
-
Switching or linear BECs do not seem to be an option for a 2s A123 source.
-
I tried a couple of Turnigy ( Hobbywing) 3A becs, supposedly rated down to 5.5v i/p, and even at 6.2v the o/p dropped to 4v for 2.5A load.
-
I have built many linear regs in the past, as follows:-
-
http://www.dream-models.com/eco/electrics-bec1.html
-
They are fine for i/p voltages 8v up, but I have just tested one at
6v input.................it gives a 3v output, so you really do need a >2V overhead for these linear 78XX regs. to work correctly. (don't really want to add another cell)
-
I wonder if anyone has had any success with certain makes of BEC
-
Rgds, Steve
As an EE (40 yrs experience ... still learning and having fun with electronics every day), there is nothing challenging or difficult to design a BEC (linear or switching) that would work for your input/output requirements.

If an off the shelf BEC is unacceptable (and there are many readily available units) and if you want to do more DIY work, I would suggest getting a regulator evaluation board from TI or Linear Technology. These are readily available and allow you to regulate with minimal dissipation / minimum input - output differential.

The 78xx, while venerable and useful, would not be my first choice for this application.
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