Help Understanding Something Fundemental - Page 1
Forum section: electronics / beginners

author: at2marty
author_rank: Regular Contributor
timestamp: April 25, 2014, 10:34:58 pm
content: I'm working on building an ESR meter based on Jay_Diddy_B's design using through hole components. One thing that I'm not quite getting though is the power supply section. In the attached schematic, I understand the purpose of C9 (a filter). Assume an input of 9V from a battery. It's getting input to a 7805 Voltage Regulator. When looking at the rest of the schematic though, I don't quite understand how the output eventually ends up being +2.5V and - 2.5V.I understand that R13 and R14 form a voltage divider so I would have +2.5V at that junction. I assume that C7 and C8 are there for filtering, but then that 2.5V from the voltage divider all of the sudden becomes signal ground.The +5V from the regulator all of the sudden becomes +2.5V.If you follow the ground from E2, it is connected to GND on the 7805, then becomes -2.5V.I don't get it.Can someone explain how the circuit actually works? I thought of sending Jay_Diddy_B a PM to ask him directly, but I figured that the better way is to ask on the forum so that others might learn.

author: Thor-Arne
author_rank: Unknown
timestamp: April 25, 2014, 10:41:47 pm
content: The 5V is divided by R13 and R14, together with C7 and C8 they create a virtual ground that is used as ground for the rest of the circuit.

author: at2marty
author_rank: Regular Contributor
timestamp: April 25, 2014, 10:45:07 pm
content: Wait a sec.... I just looked at the over all schematic again, and I think I understand what is going on. There is a difference of potential between "input ground" and what becomes "signal ground".The 2.5 volts from the voltage divider becomes the new reference. The "ground" in the rest of the schematic is referenced to the 2.5V from the voltage divider. Am I on the right track?

author: Hideki
author_rank: Frequent Contributor
timestamp: April 25, 2014, 10:48:59 pm
content: There is nothing magic happening in the circuit itself. What may be confusing is that voltages are always measured relative to some reference point.In this case the midpoint of the 5 volt regulator output is CHOSEN as the reference and marked with a ground symbol. The GND connection of the regulator is then 2.5 volts lower than this, so it's marked -2.5V and the output of the regulator is 2.5 volts higher than the reference point.

author: at2marty
author_rank: Regular Contributor
timestamp: April 25, 2014, 11:01:59 pm
content: Thank you both for responding to my question. It's been a long day, and after submitting the question, I looked at the over all schematic again and it kind of "hit me" as to what was going on. What is kind of ironic, I answered my own question with my question.QuoteI guess calling it "signalground" is what threw me off until I thought ofreference.

author: IanB
author_rank: Super Contributor
timestamp: April 25, 2014, 11:28:35 pm
content: Yes. The key thing is that when someone places a "ground" symbol on a schematic, this is the same as making the statement "I hereby declare that this point is to be considered 0 V, and all other voltages are to be measured relative to this". When building up the circuit and measuring voltages, this ground point is where you should place the black test lead from your meter.