BEC using a 5v reg chip

[dave morris]

I tried to search but not working. I have a 4s lipo setup and want to make my own BEC as I dont think the one on the ESC will handle the load. I would like to be able to cover a 35 watt load (got several servos) and was wondering if one of the 3 pin 5v reg ic could handle it? I am not an electronics expert so plain english please on how it would go. I have looked into the sbec, and ubec, but would like to keep it a little cheap and light

[Chippie]

the majority of the 5v regs or even adjustable one's wont cater for that sort of load....7amps-ish...(35 watts @ nominal 5v I reckon)
If you dont wanna go the Ubec route then the other option is to use a series pass transistor to carry the extra load, then the project is starting to become involved...unless any one else has any ideas ? ;P

[jeffs555]

First off, I dont think you really mean 35 watt load, if you do, you are talking about one hell of a big regulator. At 5 volts, 35 watts would be 7 amps which sounds very high even for 4 servos, so the first thing to do is to figure what your actual requirements are. You need to know the peak current and the average current for your servos. For a regulator, you first need to make sure it will handle the input voltage of 4 LiPos(16.8volts), the peak current, and the average power disipation. The first two are easy, provided you can find peak current specs for your servos, as the regulator will have a fixed spec for maximum voltage and current. The allowable power disipation is much harder to figure, as it depends on how the regulator is mounted and what kind of heat sinking you use, and the actual average current for the servos depends on how they are being used. For a simple linear regulator, the average power disipation is equal to the average current times the difference between the battery voltage and the output voltage, so with 4 LiPos, 16.8v-5v=11.8v, and 11.8volts x 7 amps = 82.6 watts which would take a huge heat sink and probably a very large external pass transistor on 3-pin regulator. If you are really talking about this much power, you would probably want a switching regulator.

Jeff

[dave morris]

my bad, I just check with the watt meter, I have 5 servos, two for retracts, worst case I stalled both the retract servos, and wiggled the control surfaces, saw about 1.2 amps (hs50's and 55). So looks more on the max of 15 watts should be safe. So with the revised numbers what do you think?

[Mr.RC-CAM]

With a 4S Lipo, worse case currents are going to be tough to handle for a simple series pass Vreg. Heatsinking for 10 watts of heat, inside a small fuselage, is the concern. Also, assuming nominal currents of a couple hundred mA's, you still have a Vreg that has to handle 2 watts.

Have you considered something like the UBEC? Switcher designs are best for "high voltage" BEC apps. Otherwise, add a big heatsink, in the airstream, and avoid stalling a servo.

RC-CAM

[dave morris]

I think I am being penny wise, pound foolish, will stick with the switching regs, and not put my jet in jeopardy, thanks guys.

[vintage1]

I was in the process of looking for switching regulator chips, so we can all make our own SBECS. My PC crashed and I got diverted.

Read Jeffs summary. 15w of servo power does not mean 15w of regulator heat.

On 4s and up you MUST use a switching regulator or a very heavy heatsink!

If I get time I'll do another search. Try googling on DC switching regulators etc.

[clipclop]

I have seen switching regulators at maxum, texas instruments and national , was a while ago but from memory one these with a power tranny and a few caps resistors is pretty easy to do .
Stewart

[dave morris]

I only hope the ESC makers get with the program and start making all the ESCs with swithing BEC's, save us the overhead. Does it cost that much more? take up more board space?

[jeffs555]

The answer to both of your questions is yes. A switcher does take up more board space, and the incremental cost over a simple linear regulator is relatively high compared to the cost of the rest of the components on a simple ESC. Also, there are the interference issues that a switcher generates, causing much more design work and support problems. On a low volume product(most anything for RC is low volume), the design and support costs usually far outweigh the parts cost.

Jeff

[Haldor]

I only hope the ESC makers get with the program and start making all the ESCs with swithing BEC's, save us the overhead. Does it cost that much more? take up more board space?


Take a look at Kontronik Jazz xx-6-18 ESC's - these do just that.

[Chris T]

One could use a standard LM317 at 4s, as long as current is not too high. This device can heat up over 100° C without shutting down. With 3 micro servos and 2 LEDs it should be ok... and one can hook diodes in series with LM 317 input. Then power is dissipated in the diodes (.6V drop per diode). So you can make sure power dissipation in the controller is not so high, that it shuts down rx during flight :)

[raptorhelis]

this site i have put together has a ton of info on regulator and power consumption basically everything you want and need to know and understand about the electrical power systems for r/c aircraft. check it out www.microlitesystems.com email me if you have any other questions. thanks