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Topic: difference between a negative and a positive voltage regulator? (Read 9 times) previous topic - next topic

heyarn

Hi everyone,

I accidentally bought a L7905CV LM7905 L7905 Voltage Regulator IC - 5V 1.5A on tayda electronics, when I should have bought a LM7805 L7805 7805 Voltage Regulator IC 5V 1.5A.

I only realized I bought the wrong thing because when I hooked it up to power my breadboard, I was getting 4 volts when I was expecting 5 for my attiny..

So what's the difference between the 2? What is the use of negative regulators?

Thanks! :)

DVDdoug

Don't use the wrong part!

A voltage (or voltage measurement) is referenced to something, usually to ground.  The voltage can be positive or negative (or AC, which swings both positive and negative).  Some circuits require positive & negative voltage.  For example, many op-amp circuits require both a positive and negative supply.    If you are building a "bi-polar" power supply, you might use a 7815 to regulate the positive voltage, and a 7915 to regulate the negative voltage.

If you reverse the leads to a voltmeter, and connect the +input to ground, it will read negative when connected to a positive supply.

In some cases, such as with a battery, you can simply reverse the connections to get a negative voltage.  Some audio circuits use two 9V batteries... one for positive and one for negative.

Your 7905 might be "blown".   You'd need a negative supply to test it. Or you could use a 9V battery, or a power supply with an isolated ground connected "backwards".


Docedison

Under any circumstance the 7905 is unsatisfactory fro a + 5V regulator unless you are a skilled technician. You didn't damage it as it is protected for reverse insertion internally. Get am LM2940 it is a TO220 case regulator just like the 7805 but it will work down to a .2V I/O differential at low current loads... Under a .1 A load it's dropout voltage is 5.2 - 5.3 volts for regulated 5V out. The LM7805 or any '7805 requires 2.5V I/O or the input supply must be 2.5V greater than the output voltage i.e. 7.5V min in for 5V out... Batteries could be smaller (fewer cells) and last longer. 4 D cells would NOT work with a 7805 but would on a LM2940 and let you use nearly 75% of their capacity. With 5 cells you could use 90% of the batteries capacity but about 15 - 20% of the same batteries with a 7805... "From the LM2940 data sheet"...

Doc
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Grumpy_Mike

Quote
What is the use of negative regulators

To provide negitave power supplies. Things like many op amp circuits need a dual rail supply. That is one with both a positave and negitave rail.

heyarn

Thanks for your replies! :)

Is there w way I can salvage my negative resistors to steadily output 5v? Is it as simple as reversing the polarity?

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