What is +12V and -12V supply input on OpAmp?

[Nepaliman]

I am reading a book of bachelor, science. I learnt more about opamp but cannot understand a simple input supply. Actually book has no information about it.

+12V on 8th pin and -12V on 4th pin mean total I need 24 volt supply? Then what is GND again? Please tell me in very simple words!

 

[ericgibbs]

hi N,
The power supply that provides the +12V and -12V will have a Common pin, which is considered as 0V or Ground.

E.

 

[Nepaliman]

Oh then total potential difference is 24V from -12 to +12.

- It means do I need 12-0-12 transformer in simple?

- But If I had to use a Battery or 12V two batteries then what can I do? Please!

 

[ericgibbs]

hi,
If you use a 12V 0V 12V transformer [centre tapped] you will require, rectifiers , two voltage regulators, one for +12V and one for -12V, together with the required decoupling components.
The simple way is to use two 12V batteries.
If you do use two batteries, connect the +V on one to the -V on the other, this is the Common point or ground.
Then one battery will be +12V and the other -12V.

In most simple experiments you may find 9V batteries will be cheaper to use.

 

[Nepaliman]

Oh yeah my guess was correct then- two 12v batteries in series then 0V is -12V, connected +12V point is 0V and 24V is +12V. It is just about 12V up and down potential difference, not exactly Negative voltage needed. I understand!

In 12-0-12 transformer (after around 14.5-0-14.5 smooth DC output), should I have to connect 'LM7812' in series of Positive line to get +12V and in another- should I have to use 'LM7912' in series of Negative line to get -12V? I think Yes.

 

[ericgibbs]

Oh yeah my guess was correct then- two 12v batteries in series then 0V is -12V, connected +12V point is 0V and 24V is +12V. It is just about 12V up and down potential difference, not exactly Negative voltage needed. I understand!

In 12-0-12 transformer (after around 14.5-0-14.5 smooth DC output), should I have to connect 'LM7812' in series of Positive line to get +12V and in another- should I have to use 'LM7912' in series of Negative line to get -12V? I think Yes.

hi,
You need the 7812 and 7912 regulators, they require at least 2.5V higher on the input to the regulators at full load, thats 14.5Vdc input

A 12V transformer when rectified will give 12V * 1.414 = 16.9V peak less two Diode voltage drops, say 2 * 1V, so thats 14.9Vdc peak on the smoothing caps.

However when you start to draw current from the 7812, the Vdc peak voltage to the 7812 input will reduce.
If you draw more current the regulator will not be able to hold the output at +12V ,as the input voltage will have fallen below 14.5Vdc.

Do you follow OK.?
E

EDIT:
With a 1000uF smoothing capacitor after the fullwave rectifier I would say that you could not draw more than 100mA from the 7812 before the regulation failed.

 

[audioguru]

Many opamp circuits do not need voltage regulators because an opamp rejects hum and noise from an unregulated power supply.
If you use a 9V-0-9V center-tapped transformer and a bridge rectifier then the peak voltages are 12.7V and the filtered DC voltages will be very close to +12V and -12V.

EDIT: The positive power supply pin on a single opamp is pin 7, not pin 8.

 

[MikeMl]

Also, a lot of opamp circuits do not need dual split supplies. It is usually possible to "bias" an opamp circuit such that all of the "action" happens around a voltage which is near the mid point of a single supply. Lots of opamp circuits operate on just a single +5V supply with the negative supply pin on the opamp connected to 0V.

 

[Nepaliman]

Many opamp circuits do not need voltage regulators because an opamp rejects hum and noise from an unregulated power supply.

What!! So smazing!!! When I used Linear transformer and used bridge rectifier and a big cap, then if I powerd Basic FM Tx or old AM radio, the device plays LOTS of hum and too bad AC noises, but will Opamp reject them all?? Almost what is the rejection percent? 100% completely?

Did you mean if I made an amplifier (mic amplifier) using Opamp and powered the Opamp from linear transformer, then if I fed output amplified audio to FM tx, won't FM tx chatch the hum and AC noises? (Tx using battery, opamp using transformer)

 

[audioguru]

If a signal from a circuit has hum and noise and is fed to the signal input of an opamp then the opamp will not reject it.
Only hum and noise on the power supply pins of an opamp are rejected if the voltages do not drop too low.

You cannot power an opamp from a transformer. A transformer produces AC but an opamp is powered with DC.