Voltage Regulation

willis123

I'm planning on using a Fairchild voltage regulator to feed a solenoid valve instead of using the BS2's internal reg. Does anyone know what the difference is between·Fairchild's LM7805 and their LM78M05 voltage regulators? Also, on·their website, these are shown wired up with capacitors connecting the input and output to ground. Maybe a stupid question, but what do these caps do?

http://www.fairchildsemi.com/ds/LM/LM7805.pdf
(see Page 22/28)

http://www.fairchildsemi.com/ds/LM/LM78M05.pdf
(see Page 9/14)

Thanks in advance.

(Oh, and I'm new to the whole microcontroller/stamp businesss. I did however have a couple of circuits classes.....though I'm not 100% sure why one uses a capacitor other than for starting a motor or something)

Mike Green

I'm not sure in detail of the differences between the 7805 and 78M05, but the specs show a 1A output limit on the 7805 and 0.5A for the 78M05. Also the 78M05 is available in an additional package type. If 0.5A is more than adequate for your needs, it probably doesn't matter which you use. Be careful about heatsinking. Whatever voltage there is between your supply and the output of the regulator gets turned into heat and the regulator requires about 2.5V minimum above the output voltage. If you're drawing 0.5A with an input voltage of 9V, that's (9-5)x0.5=2 watts that has to be dissipated as heat.

The capacitors are necessary. If you're only driving solenoids and there's no digital circuitry attached to the regulator's output, you probably don't need more. Solenoids don't usually care if there are spikes in their power source lasting tens or hundreds of microseconds. Digital circuitry will behave terribly. It's often important to isolate things like motors and solenoids from digital circuitry by providing separate power supplies. Often the solenoids and motors will run off the unregulated supply while the regulated supply is just used for the digital circuitry. If you use a "low dropout regulator", these require a supply voltage only about 0.5V above the regulated output voltage. You can use a 6V battery supply (or equivalent) to run the motors and solenoids and still provide a 5V regulated supply for the digital stuff.

The capacitors on the input and output of the regulators are necessary to provide power for brief periods (fractions of a microsecond) before power can reach the regulator from inches to feet away. Even straight wires have some inductance at these frequencies. If you're using a "wall-wart" for your unregulated supply, it's probably 36" away at least. It takes nearly forever for power to reach the regulator from that far away through kinks in the wire, etc. The capacitor (physically adjacent to the regulator) provides the power during that interval. Similarly, you need to put a 0.1uF capacitor across the supply pins of pretty much every IC in your circuits to supply power for the few nanoseconds it takes for power to come a few inches from the regulator. If parts are physically close or low power and slow (in operating frequency), you can get away with sharing a capacitor between them, but there needs to be one every few inches of power supply wiring.

Chris Savage

This thread is being moved from the BASIC Stamp Forum to the Sandbox Forum.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Kaos Kidd

Mike:
That was a dead on good explnation. Albight I'v never heard it before, it's dead on...
(It even explains some things, of which there are lots of, that I didn't know about..)

THanks...

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Propeller + Hardware - extra bits for the bit bucket =· 1 Coffeeless KaosKidd

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