Project safety:  Fire?  Too paranoid?

While I understand the basics, electricity still seems like magic to me. (I’m a software guy). I would like to use an Arduino to control some fans that cool my entertainment center cabinet. However, I don’t want my lack of knowledge to end up starting a fire.

Feel free to ridicule me if I’m being needlessly paranoid, but here’s my situation:

I have an AC/DC adapter soldered to a 2.1mm plug. It is regulated, rated at 800 mA, and UL listed. I’ll use this to power the Arduino 24 hours a day. Three 120mm case fans will be attached to the digital output pins. The fans are rated at 80mA (@12V) and will be switched on or off (no need to PWM) based internal temperature of my cabinet.

What would happen if I accidentally caused a short to the power supply? What level of safety does the UL listed mark imply? Should I put in a fuse? Buy insurance? :wink:

Any other standard safety procedures for electronics projects?


Assuming your adapter complies with UL guidelines (many things carry fraudulent UL and CE markings) then when you draw an amp or so the AC/DC adapter should safely fail. Probably by melting a filament somewhere in the transformer, those things are too cheaply made to have fuses. The plastic case should stay intact and no high voltage should be exposed.

You will want to check the output voltage of the power supply to ensure that “regulated” means “close to the set voltage”. AC/DC adapters are notorious for running well over their labeled voltage at low current.

If you have plenty of cooling capacity at 12v, you could switch your adapter down to 10 or 9 volts. The fans will turn a bit slower and be quieter.

You could put a 500mA fuse in the +12v line for the day when you move a piece of metal gear, mash the power lines to the fan with an edge, and short the wires. Maybe the fuse would blow before the transformer fails. Which goes first in a short circuit has many variables.

Thanks… the adapter (on the 9V setting) puts out about 9.2 V. This seems to stay constant with varying loads from the Arduino. I’m assuming that the Arduino board can easily handle a few 80mA fans.

One last question… does it really matter what kind of wire I use to connect the fans? I thought about using some spare Cat-5 Ethernet wire. Too thin? Is it better to use single-conductor (single-strand)?

I get paranoid about things I don’t understand. Hence the need to understand them… :wink:

i would not worry about fire unless there is something around to burn… the plastic and the metal of the electronics is not going to catch on fire unless it’s immersed in something flammable.


CAT-5 is what… 24 gauge? Check a table like this one, Now, which number to use… If it is a short wire, not in a bundle you can use the chasis wiring column. If the wire is in a bundle you will want to use the transmission number. If it is in a big bundle, even less.

As a sanity check, Power Over Ethernet moves 400mA, but that is over two wires and is spec’d to survive in bundles of 100 wires. 200mA/wire is well within the power transmission column.

In general, you will want to compute the resistance of your wire (remember to count the length of the wire coming and going) and decide if the power loss is tolerable.

Stranded wire holds up better. Flexing won’t cause it to fracture like a solid wire. But that might not be problem in your application.

I think 80ma is too much current to be delivered directly by the AVR. I think the specs have a limit for sourcing/sinking current of around 40ma. You should use a couple of small transisters (or opto couplers) to actually drive the fans. In addition, you should considered the potential EMF voltage feedback from the fans (inductive loads) when the current is shut off. The opto couplers would be the best best.

This is so much a safety issue as it is a longevity issue. Attempting to provide to much current with the AVR can cause the chip to fail and the EMF from inductive loads such as fan motors can cause hard to diagnose failures.

I knew a simple project would still expose a bunch of stuff I didn’t know! With optoisolators (or a transistor) in place, can I drive the motors off of the +9V pin on the Arduino? What about a capacitor across the wires to the fan? My newbie-sense tells me this could smooth spin-up current demands.

I wish I had a good beginner’s set of components on hand, but I will start off playing with a couple transistors to drive the motors. I’d need an NPN transistor, right? Is there any reason current could flow back through it, requiring a diode somewhere? (I’ll read up on inductive loads this weekend).

Not to give you any more options BUT ( ;)) Im a big fan of MOSFETs personally. They are fast enough that if you were to choose to do some PWM to slow the fans down (to make them quieter) at a later point you could. Here’s a schematic that would work for you:
You would want diodes as shown for back-emf protection, the fans are ‘noisey’ like that. But with its higher current you could easily run all of the fans off a single MOSFET. The fans would be wired in parallel, or all the reds connect together and all the blacks connect together, with the reds going to the +9v on the arduino and the blacks going to the ‘drain’ pin of the 'FET. (Keeping in mind the diode has to go across the + & - sides of the fan.) So yes, you could use the 9v pin on the arduino, since it is really just a pass through, whatever goes in to the plug is what comes out there. (ie If you were to use a 12v supply it would be 12v at that pin instead of 9.) Otherwise regarding safety, use a good regulated switching power supply (my preference) and nothing should go wrong.

I’ll never complain about excess information… thanks for the schematic! Any chance you could explain the diode in parallel with the motor(s)? I suppose I don’t understand it because I really don’t understand inductive loads either. Please point me to a place to read up on this if it is too much to explain.

Also, electrons flow from s (source?) to d (drain?) in the MOSFET, but conceptually, I can think of current going from + to -, or drain → source → ground, right?

Yes you can power the motors from the +9V supply if you use a transistor. You could use MOSTFETS, NPN or PNP transistors for this and the circuit diagram provided should work for just about any common one (NPN or MOSFET). If you want to try a mosfet you can pick up an MPF102 transistor from Radio Shack for about a buck a piece.

The diode in the schematic is reverse biased–meaning the cathode is tied to the +V. In this configuration it does not normally conduct; however, when the motor is turned off the inductor (motor) typically dumps a large voltage spike (can be several hundred volts) for a very brief period. The diode conducts during this spike and suppresses it.

PS if you are just staring a great book is “Getting Started in Electronics” by Forest Mims