need help on negative power supply

[hippoo]

i use 7805 to build a -5V supply as below:

as the power supply of+12V is floating, will it giving any problems? let's say if it is going to connect to the negative terminal of the op-amp.

if it is going to cause problems, any suggestion to alter the problems or there are some other types of circuit configuration to get -5V at the output with positive supply?

thanks a lot........ :|

 

[10kangstroms]

You have reversed two of the pins, and you didn't show an output. See the attached schematic.
You will need to add capacitors as recommended in the datasheet.

 

[bimbla]

No, the circuit wont work.
The circuit should have a common ground.

bimbla.

 

[hippoo]

dear bimbla,

for your reference, i attach the circuit configuration given by the texas instrument:

i have measured the voltage of V32, it giving me -ve 4.98V (refer to my first circuit diagram)

as per what u hv said that this circuit do not hv the common ground... may i know why it is not working since it is able to give me -ve 4.98V?

this is my main worry, as some of my friends told me the same thing but they do not know what is the reasons and impact when it is going to power the load...

thanks...


:|

 

[bimbla]

Ground in any circuit is the point with reference to which all the voltages are measured.
It is like the origin (0,0) of the number line.

In your case, you are measuring the battery voltage with respect to the negetive terminal of the battery. Where as the negetive of the battery is then measured with respect to the o/p of the regulators. So you are doing measurements with different references. This is not correct.

The multimeter will not know all this. It will only tell you the magnitude of the voltage and which of the two terminals of the probe is at a higher potential. No -ve sign will appear if the +ve (red) terminal is at a higher potential.

I hope that this will clear your doubts. If not, raise further questions.

Regards,

bimbla.

 

[VVV]

If the battery is TRULY floating, then your circuit WILL work.

That is because the regulator is connected correctly as far as the input voltage is concerned, so it will do its job and regulate the voltage between its output and its common to 5V. Just because you CHOOSE to connect its output to the ground of your circuit, which simply means you choose the output of the regulator as the reference point, does not stop the circuit from working. The voltage which you now consider the output will simply be -5V with respect to your chosen ground.

But again, you must make sure that the battery is really floating. Of course, instead of a battery you can use the winding of a transformer with its dedicated rectifier + filter.

 

[hippoo]

i hv try the following circuit practically,

results:

V_AC = 12V
V_DC = -12V
V_BC = 5V

so.... i think the +ve voltage regulator with -ve configuration is working well, rite? (if the node C is going to be the circuit reference node)

with this configuration, of course i will find that my V_2 is floating...

i consider the node C as my circuit SIGNAL GROUND or VIRTUAL GROUND, correct? (kindly giv me your comment...)

when i connect the node C to the EARTH GROUND terminal (from the DC supply or AC supply in the laboratory), the measured results were still be the same with the above values... (kindly giv me your comment...)

finally, i would like to know if my circuit load (instead of one simple op-amp) is going to be very complex, will this kind of configuration going to give me any troubles? or it is well better to use the following circuit involve with the negative regulator....

kindly give me your commentssssssss which circuits will be the better solution ( ++ your reasons)...


thanks A LOT, thank you........

:|

Added after 8 minutes:

sorry....

ERRATA....

my first circuit diagram.... the text "7812 IC" shall be "7815 IC"..... (15V)

actually i simply put it in for your reference as i deemed you may unfamiliar with the LM340T15 of national semiconductor... (& the LM324N of ST).

 

[hippoo]

dear bimbla & others,

do you hv any comments regarding my findings?

thank you.


regards,

hippoo

 

[bimbla]

Hi,
I agree with VVV and Hipoo.
But, In Hipoo's case, I thought we wanted to achieve
it using only one regulator.

regards,

bimbla.

 

[louisnells]

Never refer the source +12v in the circuit.
ie: You said its a -ve power supply, then defenitely there has to be a +ve supply too. But because the +12v is floating there shouldn't be any problem. So as long as +12v supply is floating or its not being referred in the main circuit there shouldn't be a problem.

i use 7805 to build a -5V supply as below:

as the power supply of+12V is floating, will it giving any problems? let's say if it is going to connect to the negative terminal of the op-amp.

if it is going to cause problems, any suggestion to alter the problems or there are some other types of circuit configuration to get -5V at the output with positive supply?

thanks a lot........ :|

 

[hippoo]

dear all friends,

anybody kindly give me comments regarding my latest two figures, which one is better. kindly give me reasons with your choice.

thank you... :|


regards,

hippoo