concerns regarding dx5e module with Vreg only

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mbeets
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concerns regarding dx5e module with Vreg only

Post by mbeets » Wed Jan 09, 2013 4:27 pm

Guys,

I have been reading. A lot. And there is something that keeps bugging me.
I have recently hardwired the dx5e module into my 9x in this way: http://www.rcgroups.com/forums/showthread.php?t=1803096
Basically using the Voltage regulator from the dx4e I convert the 6.4-8V from my double 18650 LiFe 3.7 2400mAh batteries to the required 3.3V that the Dx5e module needs.

Here's the thing: If you calculate the amount of Watt that the Vreg needs to dissipate when a voltage drop of 8-3,3=4,7V is applied and the module asks 200mA it amounts to quite a lot. 0,94W to be precise.

I was thinking that I could decrease the Vin of the Vreg to 5V by harvesting the NPN NEC D882 P transistor from the V911 Transmitter (http://sunrising.member.seekic.com/prod ... D882P.html).
But how do I wire it up?

8V --> collector pin
emittor pin --> 5V in on Vreg
Base pin --> ground

or something like that?

Cheers

(P.S.: great reading material supplied by all of you.)

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ShowMaster
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Re: concerns regarding dx5e module with Vreg only

Post by ShowMaster » Wed Jan 09, 2013 5:17 pm

Some thoughts,
The life's resting voltage after the surface charge wears off sits at 3.3v/cell and as used slowly discharges to a safe 2.8v/cell before needing a recharge.
So my thinking is that with your 2 cell after a few minutes on time, will have a pack voltage of 6.6 volts.
Now the regulator only has to drop 3.3 volts for most of the batteries discharge curve,e en less towards the end of charge.
Your 3.3v regulator if it's a low headroom type, only needs about .5 v over 3.3 to regulate or 3.8v input.
Your life 2 cell if discharged to 2.8v/cell will still be 5.6v so plenty of headroom to work with. At 200 Ma/hr drain that's a lot of flying time to drop to 5.6v.
Maybe a single 1n4001 diode drop of .6v could be added in series with the input to drop another .6 volts the regulator doesn't have to deal with?
Hopefully my thinking is correct and this is of some help.
SM


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mbeets
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Re: concerns regarding dx5e module with Vreg only

Post by mbeets » Wed Jan 09, 2013 6:03 pm

Thanks for the quick response!

Quick calculation of the new numbers you provided: 3,3V x 0,2A = 0,66W or 660mW to get rid of in terms of heat.

According to the datasheet the thermal resistance of the SOT 223 package is 15 dgr C/W....

Sooo that would mean around a 10 degrees celsius increase in temperature?

That's actually quite impressive for such al little component!
:geek:

mbeets
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Re: concerns regarding dx5e module with Vreg only

Post by mbeets » Thu Jan 10, 2013 6:01 am

Disregard my previous post. It's wrong.

Using this as source material (http://www.ti.com/lit/an/slva118/slva118.pdf) I think the AZ117 3.3 Voltage regulator is very much up to the task.
If you calculate the power needed to dissipate (Pd) by using Vin - Vout x current draw you get 0.66W.
In the attachment the Maximum power dissipation is shown which shows that the little Vreg can handle substantially more Watt's to dissipate.

Any thoughts?
Attachments
Afbeelding 1.png
max power dissipation of the sot 223 package

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MikeB
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Re: concerns regarding dx5e module with Vreg only

Post by MikeB » Thu Jan 10, 2013 1:25 pm

That's the junction to case thermal resistance, you need junction to ambient, I found this:
"If SOT-223 is used, the junction-to-ambient thermal resistance is 174˚C/W"
So at 0.66W, the junction will be 114 deg. above ambient (without heatsink). Maximum junction temp appears to be 125 deg.
So you are OK in the winter as long as ambient temperature is below 11 deg.
You NEED a heatsink.

Mike.
ersky9x/er9x developer
The difficult we do immediately,
The impossible takes a little longer!


mbeets
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Re: concerns regarding dx5e module with Vreg only

Post by mbeets » Thu Jan 10, 2013 2:55 pm

Thanks!!

I thought I did. I just couldn't believe that such a small component could handle a voltage drop of 3.3V so easily.
Heat sink is one option but maybe putting a transistor between the battery current and the Vin of the Vreg could also work.
Hence back to my first question: How to wire a transistor in place?

Cheers MikeB. Thanks a lot!

mbeets
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Re: concerns regarding dx5e module with Vreg only

Post by mbeets » Thu Jan 10, 2013 6:29 pm

Just bought a 3A switchable (between 6V and 5V out) UBEC and wired it up. Works like a charm.
I touched the Vreg before and after I installed the UBEC and ran it for 5 minutes each. Using the UBEC it was comfortably warm to the touch whereas when we full 8V was passed directly to the Vin on the Vreg it was much warmer.

I'm afraid it's not much of a test but I feel a lot better using the UBEC.

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