What is the advantage of using an H bridge the L298n versus a motor shield?
Im going to be using 2 Dc motors at 3 volts. I will run them continuously for 2-3 minutes.
If i buy an H bridge cant i just configure everything as shown here L298.jpg (image)
instead of using 30 dollars on a motor shield?
Hi there,
It sounds like you're using those small hobby motors... for your application, the L298's are probably overkill (rated for 4A continuous @ 46 VDC). The Arduino motor shield uses L293D's, which are rated for 600ma continuous, and each can independently drive two motors.
[apparently, being my first post, I'm not allowed to paste the link to the datasheet?]
Digikey has them for about 4$ each... quite a bit cheaper than a motor shield. 
~Kailey
L293D Datasheet:
~Kailey
Besides choosing the right H-bridge, I think the advantage is simply that you get a shield which contains everything to drive your motors:
Unless you got very specific requirements or are keen on designing your own board, I would recommend the shied.
The Arduino motor shield uses L293D's, which are rated for 600ma continuous, and each can independently drive two motors.
The SN754410NE seems to be a replacement for the l293d. I found a 10 dollar Motor Driver Shield with the SN754410NE.
@looseleaves728
I found a 10 dollar Motor Driver Shield with the SN754410NE
Would you mind including a link? 
Would you mind including a link?
well its 10.99 and with 5 dollar shipping. The shipping was free for me though, since i had a 5 ( 25 but i spend some) dollar gift card i won.
the comments seem good. its sold by NKC electronics
It sounds like you're using those small hobby motors... for your application, the L298's are probably overkill
Actually, at least one person has posted here expressing frustration and disappointment that the L293D on a motor shield wasn't up to driving the "small hobby motors" in his Tamiya gearbox kit, which can draw over 2A.
People tend to underestimate the amount of current that motors in toys consume, which sometimes leads to broken motor driver chips and broken hearts. 
You can get the L298N below for $3, but some addtional parts are needed. Bare in mind that these types of H-bridges can have a significant voltage drop across them that could impact using them in low voltage situations.
The NKC board does not have a separate motor supply: the motors are run off the Arduino's V+ input. That's okay if you're making, say, a simple robot or other device where the motors and the Arduino are running off the same batteries. It's a problem if you want to use higher-voltage motors to reduce the current requirement.
Personally, I don't like the fact that they chose to use the SPI pins to drive the motors, because I've been doing some Ethernet-based projects recently, but your mileage will probably vary.
The NKC board does not have a separate motor supply: the motors are run off the Arduino's V+ input.
So if i were to use this shield, I wouldn't really be able to power anything else through the voltage out of the arduino. Well, that would be on heavily battery packed car robot thing. haha
How would i power it with an external source?
The website says:
Do not install the Vin pin header to the Arduino / Freeduino board (or install the Vin and GND pins upwards, to plug a connector from where you can supply this shield with a different voltage range than the Arduino / Freeduino board)? and install a socket and supply Vin in the Shield from a different regulated DC power source, using the same GND connection
I understand what im not supposed to do, but i have no idea what its telling me to do :/. I'm bad with the whole electricity part of the arduino.
So if i were to use this shield, I wouldn't really be able to power anything else through the voltage out of the arduino.
The motor is being powered from the unregulated input to the Arduino, so it's not taking anything from the regulated 5V supply (except for a few mA to run the chips on the shield).
The problem is that if you want to run, say, a 12V stepper, you have to run the Arduino off a 12-14V supply, too. Which does wind up reducing the 5V current available: the higher the voltage going into the regulator, the less current you can draw out of it.
yeah, I actually have the NKC electronics motor shield as well. It's not as convenient as one would hope. I tried using two motors from the dollar store fans and was confident that it would work perfectly, but I could never manage to get the motors to start up by themselves. I always had to give them a push start, even on the highest setting.
but I could never manage to get the motors to start up by themselves.
I haven't actually seen one, but it should be possible to cut the V+ line on the motor shield, and feed the motors a higher voltage.
One way to do this would be:
remove the V+ and ground pins going to the Arduino.
Replace the ground pin with an extra-long one, so you have it extending to its normal length both above and below the motor shield board.
Replace the V+ pin with one that only extends above the shield board.
Now you can conveniently plug a higher-voltage supply into the motor shield, and be sure that its ground is properly connected to the Arduino's.
You can use a female header connector to hold the individual pins in a position that's properly aligned with the others as you're soldering them.
The 3 volt toy motors will usually tolerate much higher voltages. The 3 volt number represents power from 2 AA cells. Lower current is almost always better.
