RE: [beagleboard] Custom Power Supply

Looking at your schematic, I believe that 1K is too large a value. It should be about 240 ohms and R2 should be scaled accordingly. It could be that the LM317 is taking too long to stabilize. You will notice that the application circuit provided by the data sheet uses a 240 ohm resistor.

Yep, a linear regulator will cook with a BBB attached. Assuming the BBB consumes 1A, then you will dissipate (25v - 5v) * 1A = 20Watts. This regulator cannot handle that power dissipation and will die. Better to use a switching regulator which has an efficiency of around 90% or better.


If your source is 25V, you should really consider a switching regulator.

If you go to the Digikey web site and search for “7805 replacement”, you will find a switcher

with the same pin out as a 7805 regulator.

The BBB requires a very stable supply. The spec is 5.0 Volts plus/minus 0.25 Volts under all loads, none to maximum. If the supply, as viewed at the BBB PMC, goes above 5.5 or below 4.6 for even a few milliseconds, the PMC will go into protective shutdown. Put a scope on the power leads and make sure your supply can stay inside the specifications, while the load is jumping around.
— Graham

You need to specify the current requirement for each voltage.


It shouldn't die but it will close down to protect itself, hence the
non-working BBB.

You definitely don't want to use a linear regulator on a battery.
because you will waste most of the battery energy in the regulator. If
your battery is 25V and the BBB supply is 5V, and you're drawing 1A,
20W is being dissipated into heat in the linear regulator, while only
5W is being used productively to run the BBB. A switching power supply
is not 100% efficient, but can attain 80-90% so you will only draw
5-6W from the battery and your system will last 4-5 times longer on
the initial charge.

Quality switching supplies are dirt cheap so save your time and money

and get one!

It is possible to SMPS with a wireless circuitry, but you have to use good design principles. You need to study EMC/EMI/SI and this is a big field so I recommend you consult with an engineer who has experience in this field. In general, you need to isolate various subsystems so that they do not interfere with each other. A good book on this subject is:


I will take a look at SMPS circuits. i would need to design this because this is part of my senior project. I was just concerned about supplying different voltage values to sensors solenoids and motors, plus 3 BBBs.

Suggest to your professor that it is often more reasonable to buy a
pre-made wheel than to invent the wheel along with the new engine for
the car (which is really your project).

You will find that the use and specification of power modules is often
an "off the shelf" item in industry. Those that don't avail
themselves of the design tools at TI, and use a reference design with
a "designer" package. This then devolves the power supply into "lay
out the board, put on the chips."

I have read online that an SMPS is not ideal for circuits that are dealing with radio frequency. our project relies telemetry data that may be sent by radio. im not sure how much of an effect the power supply would have.

Think that there's a switching supply in your cell phone?