BEC circuit

Discussion in 'The Projects Forum' started by jeka616, Feb 20, 2013.

  1. jeka616

    Thread Starter Active Member

    Jan 14, 2009
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    Trying to post here all morning!
     
  2. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    Good day all

    I've built the below circuit, to power up my Receiver in RC airplane from 2S LI-Ion battery. This circuit cuts votage from ~8v to ~5.7V.

    At first, worked ok. However, I started to "load" the servos, by adding weight onto control surfaces. I measured max current draw from the battery, it is about 1.5A at 80% load ( was not able to load more, as I was scared to break the wing/tail control surfaces).

    My two LM7806s are without heatsink for now. And they get hot. Other thing, the voltage drops to 4.3V. I.E, it does not keep straight 5.6v....

    The input voltage is constant, fluctuates between 7.6-8.2V.

    I increased the value of output caps to 100mf/25v, and put one big 2200mf/25 cap at the input of ICs.

    the LM7806 is rated for 1.5A (which I doubt now). The Schottky diodes which I use, FR155, is rated for 3A. But they also get hot...

    I'm planning to add 2 more similar branches, so will have total of 4 7806s in paraler, as shown in circuit.

    Any ideas why I'm not getting regulation even at 50% of components rating?

    Any other BEC circuits that you can suggest me looking at?

    Thanks
     
  3. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
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    Finally made it through......

    here is the attachment. Hope it will appear too...
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    These voltage regulators generally need at least two volts more input than the regulated output. Google "dropout voltage".

    Also, voltage regulators are not perfectly matched for output voltage so connecting them as you propose may cause one leg to handle more current than the other. So much that you get thermal runaway as one gets hot then starts to carry more and more load. I would add a small value resistor between the diode and regulator. Resistor value only has to be big enough to accommodate expected difference in voltage between the two devices at your max current draw.

    For example, if the DATASHEET says 6v +/-2% (0.24v max) and you expect each regulator to put out 0.75amps, then select buffer resistors of 0.24/0.75=0.33 ohms.

    Also, if you are controlling motors which can cause inductive kickback current, you should add a diode between your ground and the output of your schottkey diodes (reversed biased). That way no current will flow through the diode in normal operation but the diode will allow current to flow in the reverse direction as the motors slow to a stop.

    Summary : Thee things. Dropout voltage. Buffer resistors. Reverse biased diodes.

    Good luck
     
  5. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
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    Thanks. Checked on datasheet, it has 2v dropout voltage. I thought 8.2v is enough to be regulated down to 5.6v? In the other hand, if I add one more cell, it will be ~12v to be regulated. And I will have more power dissipation as heat.

    Regarding the resistor, I will add them. Looks like 0.25W should be enough. Also, 7806 is rated at 1A max. So I will plan with 50% safety factor. As you mentioned, it is +/- 0.2V. I will then plan for 0.24/0.5=0.48 Oms. Will see what close value I get. Based on this, resistor power should be 0.12W ? right? 0.25W resistor must work OK.

    one clarification I need is regarding your last point, diodes...is this what you mean should be done? picture attached
     
  6. jeka616

    Thread Starter Active Member

    Jan 14, 2009
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    for bias diodes, 1A rated 1n4007 is available for me. Is this sufficient? Or should I look for more?

    Regarding the other type of BECs....I can plan and build SMPS, based on CD4047. Can step down from 8v to 5.5v exact, with feedback. With 2 power mosfets and small torroid core should be sufficient. But the size obviously increase..
     
  7. jeka616

    Thread Starter Active Member

    Jan 14, 2009
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    or based on SG35 25. I've built one for my car amplifier. It is bullet proof. Although it steps up 12v to +/- 32V
     
  8. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
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    OK, I've built 3 parallel circuits. LEDs in fisrt 2 branches light up, the voltage across them is around 2.4V. The LED on 3rd branch does not light up :) the voltage across it is 1.3V. The resistor value is the same for all 3 branches. I have put actually 270ohm resistors. I may plan to reduce the resistor values, maybe down to 200ohm.

    The IC's get hot without the heatsink. So one must be added.

    I'm planning to build 6 parallel branches. Is there any downside? The original author mentions that number of branches can be increased infinite, to achieve amount of regulation. Well, 6-7 branches should work ok for me.

    In the other hand, I'm also thinking to make simple LM317 regulator, with bypass transistor. Did come across one 12v high current power supply somewhere. Just need to find it....
     
  9. jeka616

    Thread Starter Active Member

    Jan 14, 2009
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    Also came across the regulator on LM317. With a pair of darlington transistors. Seems like handling about 8-10 amps. It is also shown on ICs datasheet.

    Any feedback on this? and also on few posts above?
     
  10. #12

    Expert

    Nov 30, 2010
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    I have a couple of questions, if you don't mind...

    What is a BEC?

    Why would you use 6 regulators instead of using one regulator to control a high current transistor?

    and finally, P=I squared R, so 1 amp through .33 ohms is .33 watts.
    (I said that because I'm having difficulty tracking all this information at once.)
     
  11. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
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    Good day #12, (sorry don't know your name)

    BEC= battery eliminating circuit, this is for RC aircraft. The receivers/control servos can handle 6v max. So if you put bigger battery, you need to cut it down to 6v.

    I came across the 6 in parallel version on the web, and it looked simpler for me. But as you mentioned, and I found in lm317, I can use one IC to control high current with transistor

    Also, for wattage, I assumed for each regulator to handle 0.5Amps. there fore.
     
  12. jeka616

    Thread Starter Active Member

    Jan 14, 2009
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    I'm more inclined now to use LM317 with TIP147. or similar. can I parallel the TIP 147 to get more amps? Theoretically possible...
     
  13. #12

    Expert

    Nov 30, 2010
    16,298
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    Yes. Same advice with the buffer resistors, current sharing.

    (ps, as far as the internet is concerned, my name is number twelve.)

    pss, the 317 is more adjustable than the 7806. Good choice.
     
  14. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
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    Here is it
     
  15. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
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    Here it is. Copied from LM317 datasheet
     
  16. #12

    Expert

    Nov 30, 2010
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    I doubt you can buy an LM195 nowadays. You're going to wind up with some other power transistors. Quantity depends on how fast you can move the heat out of them.

    So...what's the bottom line on the amount of amps you need?
     
    Last edited: Feb 21, 2013
  17. jeka616

    Thread Starter Active Member

    Jan 14, 2009
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    yes, as I mentioned in my earlier post, I will probably will use the replacement for power transistor in my local area.

    All servos together pull about 2amps. But this is in static condition, I do not know how much more they get loaded during the flight, when I do 3D maneuvers.

    Ive got 3x Futaba S3010 servos, and the manual says they can pull as much as 2A at heavy loading. and also have 3x Futaba S3004 standard servos. They pull maks 1A.

    SO...theoretically, worst case I need 9A. with some safety factor
     
  18. #12

    Expert

    Nov 30, 2010
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  19. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The battery voltage will not change if you add the second one in parallel instead of in series.
     
  20. jeka616

    Thread Starter Active Member

    Jan 14, 2009
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    That's correct. However, the point for adding extra cell was not to increase the capacity/amperage, but increase input voltage so delta V is more than dropout voltage.

    Maybe you ment something different, please explain.
     
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