Electronics > Beginners

My negative voltage rail doesn't work

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Zero999:

--- Quote from: hsn93 on August 19, 2018, 05:45:21 am ---hello, im noob like you but i think you cant reach more than 35mA on both "+6v" or "-6v" rails (ambient temp)

if im mistaken anyone correct me?


and for my information,



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 ---

--- Quote from: mheruian on August 19, 2018, 01:21:01 pm ---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

--- End quote ---
Yes the LM741 can't output more than 20mA or so, but that's not normally a problem, when it's used in a virtual 0V circuit.

The LM741's output only passes the current through the 0V terminal. The current flowing from the +V to -V terminal makes no difference to the LM741. It's only the difference in currents between the +V and -V rails which pass through the 0V terminal. If the current through the +V terminal is 110mA and the current through the -V terminal is 100mA, then 10mA will be flowing into the LM741's output and it will be fine.

hsn93:

--- Quote ---The LM741's output only passes the current through the 0V terminal. The current flowing from the +V to -V terminal makes no difference to the LM741. It's only the difference in currents between the +V and -V rails which pass through the 0V terminal. If the current through the +V terminal is 110mA and the current through the -V terminal is 100mA, then 10mA will be flowing into the LM741's output and it will be fine.
--- End quote ---

so this means that the load on negative rail should be almost equal to the load connected to the positive rail, if they Rload+ and Rload- values are too different this will burn the amp? right?

Audioguru:
The design of the 741 opamp is 50 years old and it was designed to use ONLY a 30V supply (plus and minus 15V). Some of them work with a supply as low as 10V.
Like most opamps it limits the maximum shorted output current to 35mA when it has a 30V supply which is probably much less with a 6V supply, then it does not burn out. If the supply is 30BV and you short the output then it heats with up to 30V x 35mA= 1.05W which is more than double its maximum allowed heating of 500mW so then it will overheat and be damaged. 

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