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e-Electric Motors & Controllers > It's working!!! I made my own BEC!
 
 
aerton
Veteran
Location: Longueuil, QC, Canada

I ordered parts from digikey and soldered my own BEC.
It's a true switching regulator that can handle 55V as input, and generates 5.1V output at max current of 10A!
It's the first time I did something like that and I was very glad it worked.
I couldn't find a BEC with similar parameters, every other BEC I saw was max of 2-3A, and I always felt unsure if that's enough, especially since I attached my own navigation lights, which consume a total of about 1A.
What I like about my BEC is it feels solid and reliable.
07-31-2008 09:52 PM
HOMEPAGE  
 
 
MooneyDriver
Veteran
Location: Kent, Ohio

Which switched-mode dc-dc converter did you use? Do you have the digikey part number?

Hey man, why does my lipo get bigger every time I charge it?
07-31-2008 10:13 PM
 
 
aerton
Veteran
Location: Longueuil, QC, Canada

Yes, it's L4970A.
By the way, if you will find anything that is rated for more voltage, I will really appreciate you letting me know, because I couldn't.
Digikey or mouser.
07-31-2008 10:33 PM
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MooneyDriver
Veteran
Location: Kent, Ohio

Higher input voltage? I think you did quite well. They might have something, but I bet you have to buy 1,000 of them.

Hey man, why does my lipo get bigger every time I charge it?
08-01-2008 12:13 AM
 
 
aerton
Veteran
Location: Longueuil, QC, Canada

Yeah, I don't know I couldn't find for more than 55V max. That's like 13S lipo. 63V would make me happier:-) But 55V covers all my needs now.

I really didn't want to buy a ready made one. I wanted to know how it works. And their amp rate felt like at the limit. I run 10S, so 42 volts charged, and those becs are for 40volts.

I was soldering it point to point and didn't even know if it will work :-) I am more in digital electronics than analogue. I took liquid tape and painted the contacts to prevent accidental shorts. But it's really fun to have it working. I got a good idea how it steps down with their inductance and capacitor circuit. Putting 10A rated inductor coil would have increased the weight, cause they are big, so I put 4.5A coil, I get a feeling it will give out 10A in surges just fine. Learning this stuff as I go.
08-01-2008 12:25 AM
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nige69
Heliman
Location: Adelaide, South Australia - Australia

Where did you get the plans and schematics for your bec??

I would be interested in building one, and others might be as well

Nigel
Mini Titan E325
Spektrum DX7
08-01-2008 02:23 AM
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aerton
Veteran
Location: Longueuil, QC, Canada

It's in the datasheet of L4970A. Just go through it and just get a good idea how it works and that's it.
08-01-2008 02:26 AM
HOMEPAGE  
 
 
"Cam"
Veteran
Location: UK

That's really neat I have to try this.
08-01-2008 06:24 AM
 
 
aerton
Veteran
Location: Longueuil, QC, Canada

I am pleasantly surprised of your interest guys. I thought noone would care, apparently I am not the only one excited about safer, more powerful BEC.

Their scheme calls for 3300uf capacitor, it's huge BTW, really big one and wights 20g, so right now I use 2.2uf, since I had it already, but my guess is 330uf will be fine or 1000uf.

I just tested with a load of 5amps for 17 seconds, the voltage just dropped to ~4.57volts, so that's pretty good. In 3 seconds it doesn't drop, it stays at 4.98v. If I were to put a bigger than 2.2uf capacitor, the voltage would drop a lot less. But even now it's good enough.
08-01-2008 08:30 AM
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Rob_T
Elite Veteran
Location: Tualatin, OR - USA

Looking at the specs of the IC your on-load regulation should be much better - it's not supposed to drop to 4.57V. Do you have enough heatsinking for the IC?

I agree that 3300uF is excessive - especially when you're feeding from a battery - the battery pretty much acts as a big capacitor anyway! Also for efficiency make sure you follow the recommendation for the inductor, that's usually the most critical part of a swicthing regulator.


Rob
Eco8, Piccolo Fun, Shogun, HB Elite CP, Trex 450XL CDE, Swift
08-01-2008 08:08 PM
 
 
aerton
Veteran
Location: Longueuil, QC, Canada

As for inductor, that's something I didn't fully understand.
Seams like it's actually the inductor that is primarily responsible for dropping the voltage. It smoothes out the spikes of input voltage, but how does it actually produce the output voltage is not clear to me.

The inductor that I presently use is M8269 also from digikey.
It's 40uH and 3A rated. it weights 8g.

I also bought the toroid inductor of 47uH - M8929 - which is 10A rated, and it's 40g and a 1" diameter, so I kind of resented the idea of putting it in. But I may try just to see what happens. Also I want to increase the entry capactior from 2.2uf to 1000uf, really 2.2uf that I put instead of 3300uf is a way too small, so that can really account for voltage drop too.
08-01-2008 09:44 PM
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Rob_T
Elite Veteran
Location: Tualatin, OR - USA

It's a long time since I really was at the top of my game with this stuff, but one thing I do remember is that you need to watch the current rating of the inductor. Inductors suffer from an effect called "magnetic saturation", where basically if you try to pass too much current through them their inductance value changes substantially (it decreases) and the result is that your switch mode regulator becomes much less efficient than it should be. I'd at least for bench testing try using the 10A toroid and see if the circuit can then deliver 5A constantly without the voltage dropping after a while.


Rob
Eco8, Piccolo Fun, Shogun, HB Elite CP, Trex 450XL CDE, Swift
08-02-2008 12:21 AM
 
 
aerton
Veteran
Location: Longueuil, QC, Canada

That makes sense. I'll try that and I'll buy 1ohm 20W resistors to simulate a 5A load.

In the specs it says Dropout voltage between Vin and Vout relative to the current and temperature, and at 25C it shows 0.8V at 5A.
Fig 23,24 on page 15.

Do they mean that this is the voltage drop of this voltage: V=(Vin-Vout) at 5A?
08-02-2008 01:24 AM
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Rob_T
Elite Veteran
Location: Tualatin, OR - USA

Sort of. What they mean is that it stops working if Vin isn't at least 0.8V above Vout (5.1V) at the specified conditions.


Rob
Eco8, Piccolo Fun, Shogun, HB Elite CP, Trex 450XL CDE, Swift
08-02-2008 05:25 AM
 
 
aerton
Veteran
Location: Longueuil, QC, Canada

Ah! That's not at all what I thought it means. :-)
08-02-2008 06:57 AM
HOMEPAGE  
 
 
aerton
Veteran
Location: Longueuil, QC, Canada

Here are the recent tests:
At 4.5A the voltage drops to 4.53V and stays there for 5minutes unchanged.

At 1A, same thing but the voltage drops to 4.96V

At 8A, it drops to 4.11V stays there for ~2 minutes and shuts down itself, seams like it overheats.

Overall I am glad that at about 5A it keeps the voltage high enough and constant.
08-06-2008 09:16 PM
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