# Converting ~5V DC to 12V DC safely

We have an Arduino device that is using a sensor requiring ~12V. Are Step up devices safe? Are there any dangers in terms of current etc?

To elaborate; we plan to run the 5v supply from the arduino board through http://uk.rs-online.com/web/p/dc-dc-converters/0389287/ in order to power https://www.sparkfun.com/datasheets/Sensors/Proximity/SE-10.pdf as the 5v has proved insufficient. 9v batteries have worked so far, but haven't lasted long enough.

Will this work, and how might the circuit look?

5 to 12V step up devices are safe to use. Two things to keep in mind:

• efficiency
• current

Power efficiency: Say you have a

• 1W load at the 12V end and
• say you have a 80% efficiency.
• The 5V end would see a $\dfrac{1\text{W}}{80\%} = 1.25\text{W}$ load.

Current: For the same 1W load:

• at 12V side you have a current $I=\dfrac{P}{V}=\dfrac{1\text{W}}{12\text{V}}= \boxed{83\text{mA}}$
• and at the 5 volt side $I=\dfrac{P}{V}=\dfrac{1.25\text{W}}{5\text{V}}= \boxed{250\text{mA}}$
• Sorry for some reason I wasn't send notification of your reply, I'll expand the question above. – Dan Steele Feb 18 '14 at 9:17

You can't do much to hurt a micro by applying voltages within its power supply range, so one does not have to be all that careful.

On the other hand, applying 12V directly to a 5V micro will greatly foreshorten its useful life, sometimes with entertaining sound and odor effects.

• Unless the pin is meant to have 12V applied to it, e.g. AVR nRESET. – Ignacio Vazquez-Abrams Feb 12 '14 at 18:17
• @IgnacioVazquez-Abrams Yes, an exception, as with /MCLR on (some, not all) PICs, but the fact is it's a lot easier to damage things by a probe slipping or whatever with voltages that are > Vdd on a board. It's safety of the micro and other chips, not personal safety, of course. – Spehro Pefhany Feb 12 '14 at 19:02

If you have a 12V bus available, then it's as easy as connecting the output pin to a transistor that connects the 12V to the sensor. If you don't have 12V available, then you can build your own boost converter or use an off-the-shelf part such as MAX771. Microcontrollers aren't meant to deliver power, so if the pin-end load is more than about 30mA you would need to provide some buffering. Drawing too much power from a pin could permanently damage it. You'll also make the chip run hotter.

Yes, DC/DC step-up (boost) converters are safe. You'll need to look at current, efficiency and noise. The converter is a switched converter which usually works at around 100kHz levels, which will generate electric noise. The better the converter, the better this noise is filtered (cancelled) out.