LM317 for BEC

[rubenmoniz]

Hi, i've bought some LM317T adjustable voltage regulators and i will be making a lipo charger with them but, i wanted to know if these regulators are good for a BEC.

I found plans for a bec http://www.dream-models.com/eco/electrics-bec1.html (here) and they use the 7806 regulator, so could i make someting like that but replacing the regulator with the lm317 ? (of course i would have to adjust it to 5V).

[jeffs555]

Would be ok for 3 lipos in series, but not a 2 cell pack. All linear regulators have a thing called dropout voltage. This is the minimum difference between input voltage and output voltage. The LM317T has a dropout voltage of about 2.5 volts(the 7806 is about the same). That means you would need at least an input voltage of 7.5 volts to provide a stable 5 volts output and a 2 cell lipo will drop below 7.5 volts in use. There are low dropout regulators that have dropout voltages less than 0.1 volt.

[earlwb]

Yeah it would work. But they are inefficient voltage regulators, they have to burn off excess as heat.
better is to use a small switching regulator. I like to use National Semiconductor Chips. The IC's are extremely tiny, but easy to solder down though. national also has their webbench online app to help make the design process easier. I found that one small PC board layout works for most all their chips and you simply use different components to get different output voltages and such. They also have switching regulators that will output a set voltage even if the input voltage drops below the output, which can be pretty handy.
http://www.national.com/analog/power
They use something like 1mhz to 2mhz for the switching frequency, so the inductors and capacitors can be really small.

Here is where I used three of the switching regulators to provide voltages for a huge monster motor controller board. the switchers are the three units integrated onto the main board on the left. When I did the PC layout I simply copied and pasted the switching power supplies onto the design. They used different IC's for different voltages and were almost identical unless you looked close at the resistors that set the voltage output.
http://i248.photobucket.com/albums/gg171/earlwbollinger/Motor%20Controllers/100-amp-motor-controller.jpg

[DeUglyOne]

You might want to take a look at an easy DIY switching BEC here (http://www.helifreak.com/showthread.php?t=55349). I just got a 3a and 6a board straight from TI as samples.

[rubenmoniz]

thanks for all the replies.

jeffs555: i would be using it on 3-4 cell models so no worries there.

earlwb and DeUglyOne: those seem a litle bit too advanced for me, i just want to make a simple one and i'm now starting to make these things so i'm kinda noobish at this matter and i'll try to make one with the lm317 with a litle heatsink one it and do some tests to see if it works

[phil_g]

Ruben, something like the LM2940 (http://www.national.com/mpf/LM/LM2940.html) would be just as simple but with .5v dropout rather than 2.5v - so you could use it on 3s or 2s. There are many low dropout linears, this is just one example.
i would be using it on 3-4 cell models so no worries there One more concern is the voltage of a 4s pack at nearly 15v, leaving 10 volts across the regulator. At an amp, thats 10 watts of heat, the same as a small soldering iron. With such an overhead (10v) you really need a switcher. (IMHO).
Phil

[DeUglyOne]

thanks for all the replies.

jeffs555: i would be using it on 3-4 cell models so no worries there.

earlwb and DeUglyOne: those seem a litle bit too advanced for me, i just want to make a simple one and i'm now starting to make these things so i'm kinda noobish at this matter and i'll try to make one with the lm317 with a litle heatsink one it and do some tests to see if it worksCan't get much simpler than 3 capacitors and a resistor on a factory assembled module.

When I said samples, I meant just that. Evaluation samples, straight from the manufacturer. 5 minutes on filling out the forms on the TI page and I had them FedEx 2 days later. Didn't cost me a penny.

If I don't have the caps on hand (I probably do, hadn't checked the ESR for the caps in my bins yet) it'll probably cost me about $15 to complete them both.

[Accu157]

And if you use the LM317, be sure to use a heatsink, and make sure it does not short out anything. I cannot remember if it was Vcc or ground.

[rubenmoniz]

phil_g Thanks for the info.

DeUglyOne The problem for me is getting one of those modules...

I'll do some more research on this and then try to make one

[DeUglyOne]

The problem for me is getting one of those modules...Click here (http://focus.ti.com/docs/prod/folders/print/ptn78060w.html).

Pick the one(s) you want, click on samples.