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Author Topic: difference between a negative and a positive voltage regulator?  (Read 7735 times)
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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! smiley
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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".

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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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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.
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Thanks for your replies! smiley

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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no you would have to reverse everything all grounds would become + supply and all +5 would be ground as the regulator requires a Negative, with respect to ground input. Under certain circumstances it Might work but there will always be those reversed polarity issued there. That is the reason why I recommended the other part... basically it's a better 7805. a 7905 reversed is something I might contemplate IF I WERE lost in a desert only. Very much the last choice you should make. AS I said it is a poor one to be used only in dire circumstances and only as a last choice. I've done it once in 50 years and it was a bad move then,, never again.

Doc
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Lol,

thanks for your reply doc, I'll take note of that. smiley

However, just to be sure I have a good understanding of what it is for: It's there to ensure that no more than 5 volts gets sunk into ground.. So, if I have my entire circuit connect to this before it connects to the ground, then I'll be ensured that the flow of electrons from +, to my circuit, to ground, will never be more than 5.

I may not comprehend the idea of negative voltage.. is that like negative water pressure, where it sucks out electrons rather than pushes?

Thanks again!

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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

Sir,Prof.Docedison
i am new here i dont know how to ask question from you directly.
So i am posting here. P lz help me. and reply me on my mail id or here plz.
i am doing a project. I am getting induced emf but it is too small  and AC what should
i use to get DC 5V.
(rectifier,step up transformer and regulator???)
plz name the suitable material with its market number.
Plz help me.
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worst case you blew it.

but i doubt it, thermal regulation would have kicked in on a linear regulator, connect it up properly and get a nice -5v signal, when you do swap the + for the - end, and you have a nice clean 5v supply to power your projects (confirm it with a multimeter)

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I may not comprehend the idea of negative voltage.. is that like negative water pressure, where it sucks out electrons rather than pushes?

A voltage source is said to be negative or positive only when relating it to the circuit common or 'ground' connection(s) of the circuit it is being used in. Some circuits and devices require that there be both a positive and a negative voltage sources available at the same time and some circuits only require a single polarity voltage of either negative or positive depending on the requirements of the circuit it's powering.

 Typical one looks at the polarity of the voltage source terminal that is wired to the circuit common to determine that the other terminal is supplying a negative or positive voltage for the circuit.

 So a common nine volt battery can supply either a positive 9 vdc or a negative 9 vdc to a circuit depending on which battery terminal is wired to the circuit common (or ground connection if you like).

Does that help?
Lefty
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@cjdelphi, retrolefty, you're answering the year old post.

Discussion has been re-started by post # 7. Have at it with that one.
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@cjdelphi, retrolefty, you're answering the year old post.

Discussion has been re-started by post # 7. Have at it with that one.


The subject is timeless!  smiley-wink

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And more comprehensible than post #7....
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And more fun than post #7;-

first OP case is fine. as long as wire it correctly, the circuit is working just fine. same case if you have only have 5.0 v positive voltage regulator on hand and negative 5.0 v is needed, wired it correctly and it will be worked.

in early date, since the performance of the LM317 is better than the corresponding counterpart negative regulators, as well as availability and price. people use 2 positive voltage regulators to supply both positive and negative output by separate secondaries and add one more diode bridge.

now fun part is coming;-

say you need +5v/2A and -5v/1A power supply, but only have -5v/2A, +5v/1A voltage regulator.
you could wire positive voltage regulator to output negative and negative voltage regulator to output positive.

All of above should not be happened at new design, however for service, R/D, DIY ...

The answer for difference between a negative and a positive voltage regulator ...


electronic is art, here wire=draw.
« Last Edit: April 25, 2013, 08:56:03 am by sonnyyu » Logged

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You have to be careful, because most regulators are designed to only source current, they can't sink much. If you wire up a negative voltage regulator backwards won't it have to sink current?
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