Hey folks. I'm a longtime computer programmer but a noob to electronics, trying to build a little robot to get myself started. I have some questions about power management.
I'm trying to understand the 7805 voltage regulator. I've been following the standard circuit in the 7805's datasheet, connecting a 9v battery to input, including a couple of capacitors, and hoping for 5v output. But I get a few weird effects. First, my multimeter tells me the 9v battery is actually producing more like 18v. Is that normal? Second, my 7805 only brings the voltage down to something like 10v-- which is still too much for the component I want to power. Can someone explain what's going on here and what I'm misunderstanding?
A follow-up question: Is it ok to power other components off of the Arduino's 5v output pin? In this case I'm using a Pololu Low Voltage Dual Serial Motor Controller. One pin of the MC wants to be connected to the microcontroller's "control voltage," so this clearly should be hooked up to the Arduino's 5v pin. Another pin wants to be connected to the power source for the motors themselves. Can I also connect this to the Arduino's 5v pin? Or is there some reason it would be bad to pull power for the (potentially power-hungry and erratic) motors from the Arduino's output?
Any thoughts or help will be appreciated. Many thanks!
Sounds like your test meter is toast. There is no way a 9volt battery should measure 18volts. Are you sure you have it set for DC volt function and not some other function?
As far as using the regulator 5vdc for powering off board components, it can be done but special attention has to be given to how much extra current will be drawn. Also some loads like motors are notorious for generating lots of noise and grunge causing problems. It is almost always better to power off board loads like motors, solenoids, servos and high power LEDs with an external power source, but remembering to wire the ground connection between the internal and external circuits.
Sure enough, after some expermentation, some of the settings on my multimeter look just plain wrong. Stupid cheap radshack meter. >:(
Is it safe to run motors or etc off of the same battery as the Arduino, so long as it's not actually running out of the Arduino's output power? Or is it the case that those amount to the same thing, and one really ought to run noisey elements off of an entirely different battery than the Arduino?
Is it safe to run motors or etc off of the same battery as the Arduino, so long as it's not actually running out of the Arduino's output power? Or is it the case that those amount to the same thing, and one really ought to run noisey elements off of an entirely different battery than the Arduino?
It really depends on how much current is being drawn and how "noisy" the load. Best is independent supply, but powering loads from the external power output pin, Vin, often works fine. Last choice is using the internal 5vdc power, however using that voltage is fine for powering low current ICs, pull-up resistors, standard LEDS (if not too many), etc.
By the way, standard 9vdc batteries usually can not supply the current requirements for things like motors and servos, unless they are of exceptionally small current draw types.
my multimeter tells me the 9v battery is actually producing more like 18v. Is that normal?
No.
Second, my 7805 only brings the voltage down to something like 10v
Not normal either! Sounds like you meter is reading double what it should do! Is it an analog or digital meter? And have you got it set to AC or DC Volts?
Sounds like you meter is reading double what it should do! Is it an analog or digital meter? And have you got it set to AC or DC Volts?
I figured this one out, its scale is just way off. It's one of the cheap analog ones with a needle and a cardboard backing with numbers. The scale is accurate on the 0-10v range, but not on the 0-25v range. I think the cardboard backing just has the wrong scales written on it. Maybe whoever assembles these things for Radio Shack was getting their models confused. My use of it just wasn't sophisticated enough for me to notice until now.
On meters like this, the scale usually has multiple markings for the various ranges. Make sure you're reading from the proper row of numbers on the scale! It's easy to make mistakes with this sort of meter, and that's one of the reasons why digital voltmeters were invented, back in the 1950s (they were huge and expensive back then).