Electronics > Beginners

My negative voltage rail doesn't work

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mheruian:
Hi hsn93, i guess your right.

though my target why i want a negative voltage rail was to use it on another op amp as a -v supply. I guess 35mA is more than enough for that since if I am not wrong also, supply current of 741 is around 3mA right? noob for life me  :-DD

james_s:
Why are you using two separate 7805 regulators? You have two identical +5V supplies referenced to the same ground, there is no difference here between this setup and using just one regulator and connecting the same output to both nodes. If you want a negative supply why don't you make a proper negative supply? If you have two isolated supplies you can wire them in series and use the midpoint as ground.

mheruian:
Hi James :D thanks a lot for the response. I didn't know that would work out. Would try that. Thanks :)

james_s:
It will work provided they are *isolated* supplies. For example if you have two separate "wall wart" type power supplies, or the ones that are like laptop chargers, something of that sort then you can wire them in series. Otherwise batteries also work. I've used a pair of 9V batteries in series to get a bipolar supply for playing with op amps.

Audioguru:

--- Quote from: hsn93 on August 19, 2018, 05:45:21 am ---when using this virtual ground does it mean that both sinking and sourcing transistors (Q14 and Q20) will be closed at the same time?
--- End quote ---

No. if they both conduct at the same time then they would be shorting the supply and they would burn out. They both conduct only a very small current when their output voltage is near half the supply voltage. Then when the output of the opamp needs to go positive, Q14 turns on and Q20 turns off (a door closes, a transistor turns off). When the output needs to go negative then the opposite occurs.

Lookup Class-AB Amplifier Output.

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