# Dual Power supply with Opamp and 9V battery

#### naspilla

Joined Jun 8, 2023
9
First of all I would like to say that I am very newbie in this electronic design. So please don't get upset if this question is too stupid.

I am designing a circuit with opamps that can provide +3V and -3V and that can deliver 0.5 Amps. I know I can use the Lm317 and Lm337 but this time I would like to use opamps. This is a prototype of my circuit, could someone give me some advice?

#### Attachments

• 2.8 KB Views: 3

#### ericgibbs

Joined Jan 29, 2010
18,234
hi n,
I have reworked your asc circuit, the way it was drawn made it very difficult to follow, please check to see if I have made any errors.
You say 0.5A, is that for the +Vout and the -Vout.

Add a resistive load to the +3.3Vout and -3.3Vout.
E

@naspilla Where is the Virtual Ground reference point for your +/-3.3V circuit.?

#### Attachments

• 3.1 KB Views: 0
Last edited:

#### Dodgydave

Joined Jun 22, 2012
11,152
It's a bit confusing, you have the Negative ground and +4.5V connected together?
Also have you got a better circuit diagram to follow,..

#### LowQCab

Joined Nov 6, 2012
3,584
It's just about impossible to beat 2-TO-92 packaged Linear-Regulators for your requirements.
Why all the complexity ?????
.
.
.

#### Audioguru again

Joined Oct 21, 2019
6,444
The max allowed output current of a little 2N3904 transistor is only 200mA, not 500mA.
The heating in Q2 is (9V+4V) x 0.5A= 6.5W but only 0.625W is allowed.
The 0.7V or more base-emitter voltage of Q2 was not added to the -3.3V output which becomes about -4V.
The 9V battery voltage will run down and the +4.5V reference voltage will also run down since it is not regulated.

#### Audioguru again

Joined Oct 21, 2019
6,444
WAIT a minute!
Q2 must be a PNP and its collector must be the -4.5V supply for its emitter to produce a negative output.

#### ericgibbs

Joined Jan 29, 2010
18,234
Hi,
This Thread has been moved to the Homework Forum.
Mod.

#### naspilla

Joined Jun 8, 2023
9
The max allowed output current of a little 2N3904 transistor is only 200mA, not 500mA.
The heating in Q2 is (9V+4V) x 0.5A= 6.5W but only 0.625W is allowed.
The 0.7V or more base-emitter voltage of Q2 was not added to the -3.3V output which becomes about -4V.
The 9V battery voltage will run down and the +4.5V reference voltage will also run down since it is not regulated.
I really apreciated your help! Im gonna change Q2 and I need to think how to improve this solution

#### MrChips

Joined Oct 2, 2009
29,850
A single 9V battery can supply ±4.5V.
I see no good reason to change this to ±3V.

#### Sensacell

Joined Jun 19, 2012
3,330
The elephant in the room:

A Duracell 9 volt battery has an internal impedance of 1.7 ohms.
A 9v battery cannot supply 0.5A for more than half an hour at best, the data sheet shows a 2 hour life at 250 mA, it will be much worse at 500 mA.

#### Attachments

• 1.1 MB Views: 2

#### Audioguru again

Joined Oct 21, 2019
6,444
The Duracell datasheet shows that the 9V battery is almost useless at 250mA so guess how bad it will be at 500mA.
I think it will get very hot!

#### Attachments

• 59.6 KB Views: 4

#### crutschow

Joined Mar 14, 2008
33,358
This is a prototype of my circuit,
A schematic should be drawn to make understanding the circuit as easy as possible.
Your schematic definitely does not.
It's okay to use net names to connect several nodes, such as power and ground, but not so much for most other connections.
Please draw them similar to the way Eric modified yours in post #2.
Not only will that help us understand, but it will also allow you to more easily identify any possible mistakes in your circuit.

#### naspilla

Joined Jun 8, 2023
9
This an update of my schematic. Thanks for all the recomendations. If anyone has more advices Im ready to listen it.

#### Audioguru again

Joined Oct 21, 2019
6,444
There are many problems with your new schematic:

#### Attachments

• 198.9 KB Views: 5

#### ericgibbs

Joined Jan 29, 2010
18,234
Hi naspilla,
Ref your latest circuit.
You show two power supplies, +4.5Vcc and -4.5Vee, so why are you creating additional circuitry in order to get +/-3.3V from a 9V battery?

When you test a circuit, you must have a load on the +/-3.3V voltage outputs that the output transistors are capable of driving.

When you use OPA's to split a single voltage into a dual +/-Vsupply you have to consider the placing of a virtual ground Node.

E
When posting simulation circuits, always post your LTSpice asc file

#### naspilla

Joined Jun 8, 2023
9
@Audioguru again @ericgibbs Really thank for your help. Im using this opamp circuit just for learning propurses. In the real application Im gonna use a Lm317 and Lm337, and Im not gonna use a 9V battery.

#### Audioguru again

Joined Oct 21, 2019
6,444
I'm using this opamp circuit just for learning purposes.
The datasheets for all opamps have rules and a list of Absolute Maximum Ratings that you should learn.
Your inverting opamp broke this rule:

#### Attachments

• 69.2 KB Views: 3

#### MrAl

Joined Jun 17, 2014
10,909
Hi naspilla,
Ref your latest circuit.
You show two power supplies, +4.5Vcc and -4.5Vee, so why are you creating additional circuitry in order to get +/-3.3V from a 9V battery?

E
When posting simulation circuits, always post your LTSpice asc file
Hi Eric,

That was my question too. Why have plus and minus 4.5v and then use a 9v battery also when you just need plus and minus 3.3v.
Dang, if that's the case, only need two small regulator chips i would think, two TO92 packages maybe.