DC-DC Converter for automotive use

Hi Guys

I'm building a circuit to be used on a motorcycle (12v). It will use an Arduino clone (Moteino) and needs an input between 3.3v and 9v. Anyone got a suggestion for a DC-DC converter that will handle 10-15v input and supply a clean output in the above range. I found this one: http://www.ti.com/lit/ds/symlink/lm2825.pdf, but it's $30+ so will make double the cost of the project.



Got a cigarette lighter type socket available? Can get a plug that goes in that for ~$5 with USB-A connecter for a 5V source. Google "car phone charger", even Best Buy shows a whole bunch of them. If no socket, maybe add wires to one of the smaller ones to connect to whatever source you have.

How does one of these grab you?


Between 6V and 23V input, and 5V output at up to 3A. Really tiny, and dirt cheap ($3.77).

Thanks guys

CrossRoads - Unfortunately not many motorcycles have cigarette lighters :). Could rip it apart though

majenko - that looks good will investigate further

Also came across this too

Seems ideal

Designed for vehicular applications

$1.65 each


Why mess with a chip tho when Reply #2 brings the caps, chip, PCB, etc., all together for just a little more? If you open the HobbyKing page for a bit, an offer opens up with an even lower price. The linear regulator dissipates all power from 5V up to 12-13-14V as heat, so a good heatsink will be needed, especially if anywhere near 1A is being drawn.

Adafruit has a good description of UBEC "UBEC stands for "universal battery eliminator circuit" http://www.adafruit.com/products/1385#Technical Details

and what likely makes up the underlying circuit http://www.adafruit.com/datasheets/MP2307_r1.9.pdf

Thanks Crossroads

Was just trying to get everything on a single board. (Guess it doesn't really matter). Everything I read about the UBEC referred to powering it from Li-Ion batteries, etc. and for use in RC devices, etc. The LM2940 says it's specifically designed for vehicular use and can withstand jump charges, and all the other issues. etc. Being a novice, it seems safer. I'm probably be drawing 20-30mA total. (The project is a wireless transmitter, so all the load is on the receiving end which will be powered by "conventional" batteries). I'm assuming at such a low current, the heat dissipation will be minimal. The LM2940 is also rated for -40. Not sure how cold it will get on a motorcycle in winter with the extra "wind chill" factored in :)

I did order a similar UBEC from Amazon to test (just because I have account there already)



Ok, don't forget the 22uF output cap that the datasheet calls for, page 14:

EXTERNAL CAPACITORS The output capacitor is critical to maintaining regulator stability, and must meet the required conditions for both ESR (Equivalent Series Resistance) and minimum amount of capacitance. MINIMUM CAPACITANCE: The minimum output capacitance required to maintain stability is 22 ?F (this value may be increased without limit). Larger values of output capacitance will give improved transient response. ESR LIMITS: The ESR of the output capacitor will cause loop instability if it is too high or too low. The acceptable range of ESR plotted versus load current is shown in the graph below. It is essential that the output capacitor meet these requirements, or oscillations can result. It is important to note that for most capacitors, ESR is specified only at room temperature. However, the designer must ensure that the ESR will stay inside the limits shown over the entire operating temperature range for the design. For aluminum electrolytic capacitors, ESR will increase by about 30X as the temperature is reduced from 25°C to ?40°C. This type of capacitor is not well-suited for low temperature operation. Solid tantalum capacitors have a more stable ESR over temperature, but are more expensive than aluminum electrolytics. A cost-effective approach sometimes used is to parallel an aluminum electrolytic with a solid Tantalum, with the total capacitance split about 75/25% with the Aluminum being the larger value. If two capacitors are paralleled, the effective ESR is the parallel of the two individual values. The “flatter” ESR of the Tantalum will keep the effective ESR from rising as quickly at low temperatures.

Thanks Crossroads

Just been reading about that. Hopefully, I can find something suitable

Shouldn’t be difficult. Digikey has nice filtering capability for finding parts. Mouser too.

From looking at diagram, for a 50mA (max) output current, I need a cap with an ESR of 0.1 to 1 across the entire operating range

Seems this capacitor is going to be a bit more difficult than I imagined. Running the LM2940 at under 100mA output current (I need about 30mA) requires a capacitor with an ESR of 0.1 - 1. From what I can see, that means 150uF, which are pretty big (physically)


For example this one has ESR of 0.9 at 6.3v and I’m going to be running at 5v.


Others are in the $30+ range

Am i missing something, or is this a real hard part to find at a reasonable price (< $6)?



You're pursuing the right path.

Makes that pre-assembled board look pretty nice, glue it to the back of your card.

If you will only be drawing 30mA, then I don't think it's worth using a switching regulator. Dropping 13.6V (the voltage when the battery is charging) to 5V @ 30mA only wastes 260mW of power.

Did you look at the prices tho? By the time you assemble the linear regulator and the caps & stuff, the switching regulator can end up being less expensive.

I've been assuming that the discussion so far was about switching regulators, however I just noticed that the LM2940 is a linear regulator, albeit one that is more fussy about output capacitor ESR than most. I suggest using a linear regulator that is optimized for lower current and less fussy about the capacitor. I would use a 78L05 with an additional resistor between the incoming 12V supply and the regulator, which (in conjunction with the regulator input capacitor) will protect against short transients. Possibly also a TVS diode in parallel with the input capacitor.

Thanks guys.

I thought there was an issue using linear regulators in automotive apps. I seem to recall something about buck/boost being required

The UBEC is looking more attractive. Will it be Ok for automotive use and all the "issues" therein (spikes, drop-out, etc.)

Could I put a big cap on the output of UBEC/input of circuit to help smooth things out or am I being overly concerned?



try ebay, $1 for a switching regulator free postage, 5vdc - 28vdc in and out pops whatever you want… just use a 1000uf cap on the output if you’re a little worried or even a linear regulator on top of that!.. people worry about the alternator causing a lot of voltage ripple, but the fact is the battery buffers it nicely and then a couple of capacitors to finish the job you should end up with a clean power supply, if you got bored you could rev the engine while looking at the output at the voltage regulator and look for any unwanted noise…

Thanks cjdelphi

Maybe I'll start with this approach