# Dual Power supply from AC input

1. ## Dual Power supply from AC input

For getting +5V and -5V output from AC Input,I tried this circuit..
Input to J3 connector will be 9VAC,3A from step down transformer.

While searching,every circuits use an another point to connect the GND path in this circuit...

My doubt is,is there anyway to create GND by using same input to bridge from J3..?
Shall we use another bridge rectifier and can connect GND from bridge rectifier to GND path and POSITIVE voltage to B3 positive side(before 7805)..?

2. ## Re: Dual Power supply from AC input

The circuit will be unreliable because ground will move according to the difference in load current on the two outputs.

A better solution is to connect one side of the AC input to ground and the other side through a rectifier to each of C13 and C16. The drawback is you only get half wave rectification but there isn't much you can do about that.

Brian.

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3. ## Re: Dual Power supply from AC input

Hi,

"GND" is just a name ... and it shoud be the signal where (most of) your voltages refer to.
You may name every signal "GND" .. itīs up to you.

Example:
If you name the upper node "GND", then the center node will be -5V and the lower node will be -10V.
If you name the lower node "GND", then the center node will be +5V and the upper node will be +10V.
At least the designer should know if this naming makes sense or not.

***
In your schematic the problem is that you need about equal currents in the +5V path and in the -5V path to ensure symmetry .... otherwise the voltages may float and the regulators are not able to maintain the specified ouput voltage.
I recommend to
* omit the bridge rectifier
* connect the lower transformer pin with GND
* connect a diode from upper transformer pin (A) to the positive bulk capacitor (K)
* connect a diode from upper transformer pin (K) to the negative bulk capacitor (A)
The drawback is, that the voltage ripple at the bulk capacitors will be about twice as high.
The benefit is you need only two diodes, that generate less voltage drop and less wasted power...

***
A 9V AC, 3A transformer will generate about +/-12.7V peak at full load maybe +/-15V peak when unloaded. Consult the datasheet or test this....

Klaus

4. ## Re: Dual Power supply from AC input

If the load is much higher for +5V, you can generate the unregulated positive voltage "normally" with a full wave rectifier (negative output from the diode bridge to ground). You can then generate the unregulated negative voltage with a "voltage doubler" using a few diodes and capacitors.

5. ## Re: Dual Power supply from AC input

Simulation of method described in posts 2 & 3. Uses 1mF capacitors (as in your schematic), and loads 25 ohms because too much current draw pulls the voltage below a level which is suitable for regulator IC's.

The 1/2 ohm resistor is unnecessary. It represents a certain amount of resistance in the power supply.

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6. ## Re: Dual Power supply from AC input

Is this correct..?

7. ## Re: Dual Power supply from AC input

It would be if you turned both the rectifiers the other way around.

Brian.

8. ## Re: Dual Power supply from AC input

What will happen if 7905 is connected like above circuit..?

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9. ## Re: Dual Power supply from AC input

Hi,

There is no "7905".
If IC6 is a 7905, then obviously "IC6_IN" voltage is positive with respect to "IC6_GND" which is against 7905 specification. Itīs likely you kill the 7905.

Btw: Why the non standard pin naming of IC6? A C G
Why no output capacitor at IC6?

Use a simulation tool like LTspice. It helps you to understand whatīs happening and you don't risk exploding parts.

Klaus

10. ## Re: Dual Power supply from AC input

D17 is now correct.
D18 should go to the input of IC6 and its GND pin to the 0V line.
In other words, the original schematic was correct except for D17 and D19 being the wrong way around.

Brian.

11. ## Re: Dual Power supply from AC input

If you getting 9V from a transformer why cant you use below topology with a center taped transformer

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