Make your own Linear 5 Volt BEC!

[teslahed]

In order to try and cut down on interference in my FPV system i thought i would use a linear BEC instead of a switching one. Linear BECs run constantly rather than at a high frequency so don';t produce interference which can cause your FPV system problems.

Unfortunately they are quite hard to find as ready made BECs on hobbyking or similar.

First of all, the 5 volt power supply, thats easy, simple and cheap as chips, all you need is a small 3 legged voltage regulator called the 7805, this is an analog voltage regulator and doesnt create noise, and costs about 50p or less, just add a small .1uF cap on the input and output pins. If whatever you want to power, uses less then 100ma, then you can use the smaller and lighter 78L05 here is more info... http://en.wikipedia.org/wiki/78xx

None of that sounded impossible so i though i would give it a go myself.

I got my 7805 voltage regulator from here;

http://www.bitsbox.co.uk/vregs.html
http://www.bitsbox.co.uk/data/vregs/78xx.pdf

And the 100nF (0.1uF) capacitors here;

http://www.bitsbox.co.uk/cercaps.html

I took the ferrite ring off an old ESC but you can buy them off ebay or giantshark if you need to.

This is the circuit;



And the chip itself looks like this;



And here are my results;





I will post up more information when i have had a chance to properly test it.

[Gaza07]

Nice job Chris might have a go at 1 of these to 

[Hands0n]

That';s a really cool and simple circuit.  Amy thoughts on how much heatsink, if any, is required for 3S and 4S LiPo? Those analogue regulators will generate heat in the voltage conversion process, although I';d not imagine you';ll be pulling much current through the BEC

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[barneyg]

Nice ... and for those of you with a Crius v2 ...could you not make it with a 7806 and then add a small potentiometer to act as a voltage divider ... something like :

http://www.maplin.co.uk/square-22-turn-cermet-preset-potentiometers-2202

[powerlord]

I think it';s the 7813 that is variable anyway.  You';ll need an big heatsink to pull an amp out these though - they are really for small currents.

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[pheasant_plucker]

I wouldn';t even bother with the Vero board. Just solder the capacitors directly to the legs.

Gerry

[Hands0n]

I';d start to worry about the mechanical strength of doing that :-) 

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[teslahed]

I';ve made a second Linear BEC. I';ve added a heatsink this time and used a voltage regulator that';s rated to 2 amps.

I used veroboard to add mechanical strength. This will be powering my hexacopter, i don';t want to risk it failing in flight.



I';ve added 10nF capacitors between the inputs and outputs and the ground and a 470nF capacitor between the input and the ground to add extra filtering. The 10nF capacitors would be a good idea and i';ve noticed that most other BECs seem to use a 470nF capacitor as well so i thought this would help.

I will also add a ferrite ring on the input as well as the output.

[powerlord]

I';d advise testing it for heat on the ground,  especially if you haven';t matched the heatsink  to the ic heat dissipation requirements - they do get very hot. It';s far more likely to fail due to thermal shutdown than a broken connector.  Just my 2c. Something to consider is using more than one to dispute the heat (different circuut)

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[teslahed]

I';d advise testing it for heat on the ground,  especially if you haven';t matched the heatsink  to the ic heat dissipation requirements - they do get very hot.

This might be a problem for me. I';ve had it flying and after landing the heatsink only felt moderately warm, but when i run it on the bench it gets to about 95 degrees C. Obviously the heatsink needs a bit of airflow to keep it cool enough.

What temperature do these things start to melt down at? Maybe i need a bigger heatsink.

edit - just read the specs; http://www.bitsbox.co.uk/data/78S15.pdf apparently the voltage regulator operates up to 150 degrees C so the fact it';s almost 100 C on the ground isn';t great but as long as it';s not getting much higher than that i should be ok - in theory. In flight it runs a lot cooler because i have the heatsink exposed and facing into the airflow.

If anyone with more experience than me with these thing can tell me if these temperatures are ok or not i';d appreciate it.

http://youtu.be/IbHudWgM6MM

[pheasant_plucker]

If you use aluminium arms on your frame use one of those as a heat sink.

Gerry

[teslahed]

My frame is made of wood, fibreglass and carbon fibre, so i can';t really use it as a heatsink. That';s a really good idea for anyone who can though.

Here';s a photo showing the FPV Hexacopter and the Linear BEC. The Linear BEC is just to the right of the centre of the picture, the heatsink is positioned to clear the transparent heatshrink and face forwards into the air flow.

[SimonS]

I';m using a 7812 on my new quad to provide 12v for the camera from the 4S Lipo.  I';ve not used any capacitors and all seems ok.

[SimonS]

True. Next time I';m near Maplins I';ll get some.

[Gav]

Going to get a 7812 too now I';ve gone to 4s lipos. Next time in town will get a couple. Just for 12v supply for the accessories like gimbal controller board, leds and camera. Will save my quad carrying an extra lipo in flight albeit a small mah 3s.

[SimonS]

I';ve found that the AlexMos board actually works better with a 4s supply than 3s especially if you want to use something like a NEX-5.  3S is fine for a gopro but stability in agressive flying is improved on 4s.  The AlexMos is listed as 2-4S.  I';m not sure about the various open source bords (Brugi, RCTimer etc).