Bipolar Power circuit

[stuboy78]

Hi all

I want to design a circuit that will give me +5 and -5 volt of power. i got a schematic as shown here and assembled it.

**broken link removed**

I supplied 12V VAC to the circuit (i hope thats what should be supplied) However, when i power the circuit, the 7905 regulator heats up... it becomes very hot... there is no voltage in either +5 or -5.... the 7805 regulator is normal... whats wrong in this circuit? The 1000u cap are of 24V.

 

[Leftyretro]

Hi all

I want to design a circuit that will give me +5 and -5 volt of power. i got a schematic as shown here and assembled it.

**broken link removed**

I supplied 12V VAC to the circuit (i hope thats what should be supplied) However, when i power the circuit, the 7905 regulator heats up... it becomes very hot... there is no voltage in either +5 or -5.... the 7805 regulator is normal... whats wrong in this circuit? The 1000u cap are of 24V.

Check the pin-out wiring connections for the 7905, they are not the same as a 7805 and it's a rather common mistake to wire up a 79xx regulator as a 78xx regulator is wired. Find a data sheet for a 7905 and look carefully at the pin connections.

Lefty

 

[ezpcb.com]

I think Lefty is correct. Notice that the pin sequence for 7805 is Input-GND-Output, but it's GND-Input-Output for 7905. try to exchange the connection of pin 1 and 2 of 7905.

 

[stuboy78]

oops... yes i think thats where ive goofed up....

the 7905 pins are Ground - Input - Output


Thanks buddies

 

[stuboy78]

but am i doing right by supplying 12 VAC or should it be restricted to 9 VAC

 

[flat5]

More heat and power is wasted using 12v. 9 volts would be a better choice but 12v will work. Use adaquit (sp?) heat sinks and you're ok if thr transformer can supply the current.
Your half wave design will save some power.

 

[Hero999]

How much current do you want to pull?

If it's 1A those filter capacitors need to be 2200µF.

If you reduce the transformer's voltage to 9V, the capacitors need to be increased to 4700µF

 

[puga_trump]

hei i did the same ting to but im nt using transforme, but a pc power supply so i took a -12V supply as the input for the 7905 ic.... so i tink maybe ur supply of AC voltage gives the problem.... n i tell u wat its more easy and the voltage is more stable if u use the PC power supply...

 

[Nigel Goodwin]

How much current do you want to pull?

If it's 1A those filter capacitors need to be 2200µF.

As it's only halfwave rectified I would suggest keeping the current it supplies quite low, I'm dubious 2200uF would be enough for only halfwave rectification.

I would strongly advise getting a centre tapped transformer and using fullwave rectification.

 

[Hero999]

My calculations accounted for that.

2200:mu:F will be enough. With 12V in the ripple can be as high as 8V before the regulator will start dropping out. I simulated it on LTSpice and the minimum valley is 8.7V.

 

[Blatman Bond]

Might be something broken already.
Try it one by one, disconnect both the regulator then place the 7905 only.