SparkFun Forums

[acrs]

I have a 10.8 volt, NI-CD 1200 mAH battery that i want to power my arduino based robot i tried a LM7805ACT
5v Linear Regulator but the battery burns the Regulator and SN754410NEh-bridge
i did some reading on Ohm's Law but iam lost can someone help me

[esklar81]

I have a 10.8 volt, NI-CD 1200 mAH battery that i want to power my arduino based robot i tried a LM7805ACT 5v Linear Regulator but the battery burns the Regulator

What makes you think the battery is the problem? According to the datasheet, the LM7805 should be able to handle an input voltage far above 10.8 V. If you're cooking the regulator, it's probably due to your robot drawing more current than the regulator can handle. This is nominally a 1 A regulator, capable of handling 2.2 A peak. How much current are you drawing?

Even if the regulator survives being powered by something other than that battery, that could because the other power source is not capable of providing too much current for the regulator or because the other power source is at a substantially lower voltage.

It's important to keep the conservation of energy in mind when designing power regulation. If you feed 1 A into a regulator at 10.8 V and out of that regulator at 5 V, the difference in power (energy per unit time) is (10.8 V - 5 V)*(1 A) = 5.8 W (Yes, you really do waste more power in the regulator than you deliver to the robot. )

That 5.8 W gets converted to heat and needs to go somewhere. How good a heat sink have you provided? If the heat isn't transferred to the environment fast enough, the regulator will shut down. The good news is that it's a thermally protected regulator, so you should be able to use it once it cools down (at least several times ).

and SN754410NEh-bridge

Similarly, this is a nominally 1 A device. What motors are you driving? Have you checked thoroughly for short circuits?

i did some reading on Ohm's Law but iam lost can someone help me

Understanding Ohm's Law is critical, but it's far from all you need to understand what's going on here. For example, you haven't said what the ratings of your motors are. Even if the motor is rated for 1 A or less, it may be drawing excess current because of mechanical load or voltage.

Here're a few other things to consider:
1) If the motors will run at 10.8 V, you could feed battery power to the H-bridge motor power input directly and through the regulator to the Arduino and the H-bridge logic power input.
2) Is an external regulator even necessary? If you're powering only the Arduino and the H-bridge logic, you should be able to feed the 10.8 V directly to the Arduino and use the Arduino's regulated power for the H-bridge logic. (Be careful! Trying to use the Arduino's 5 V regulated power for the motors is likely to have unpleasant results. )
3) If your motors are limitted to 5 V, a DC-DC converter (For example, this) is likely to be much more energy efficient than a linear regulator.

Have Fun,
Eric