Electronics > Beginners
Talk me through this simple circuit?
SparkyFX:
It is not "referring to", it is "referencing of".
Your reference is whatever you choose to be the base level to reference from.
Yansi:
--- Quote from: rstofer on May 30, 2019, 08:35:25 pm ---
--- Quote from: Mr D on May 30, 2019, 08:24:54 pm ---Hi folks,
I'm really a beginner!
I'm trying to get a foothold, and i found this simple looking circuit, and i was wondering if anyone would like to talk me through it.
So let's start at the beginning with the power supply. The placement of the ground means that the voltage rails to the OSC section are +-4.5V.
But does this matter? What would have been wrong with 0-9v?
--- End quote ---
It matters if the intent is that some signals swing + and - around ground. An audio signal, for example. The signal could as well swing around 4.5V and use blocking capacitors to remove the DC offset. Either way...
ETA: I didn't see the Op Amp on the first go-around. The op amp doesn't care whether you power it with +-4.5V or 0-9V but the datasheet for the 741 shows a minimum voltage of +-10V (20V total) on the power pins. I'm wondering...
--- Quote ---
Also, the LED will get more or less bright depending on the load downstream? How does one think about this or calculate it?
--- End quote ---
Well, sure, the brightness can change depending on load. The LED is across the battery and we assume, when first looking at the circuit, that it is a perfect voltage source. It puts out 9V no matter the load.
But it isn't a perfect source, it has internal resistance and when the load increases, the internal resistance drop more voltage. Also, the battery voltage falls as it is discharged. The arithmetic for determining voltage versus load as impacted by source resistance (inside the battery) is a little more involved than just talking about it.
The entire purpose of R2 and R4 is to create an artificial ground so that signals revolve around 0V.
--- End quote ---
As an interesting side note, UA741 seems recommended minimum 10V, while LM741 is 20V. I have never noticed this either! (have not used 741 opamps much anyway...)
Mr D:
Ok, let's try it another way with a thought experiment:
Imagine one believes in God.
God creates an electronic device.
Because He is all knowing and all powerful, He doesn't need to design or test the device.
So, he never needs to analyze the design or probe a prototype with a multimeter.
He simply creates it fully formed, perfect in every way. He never needs to think about where his ground might be, the circuit just works perfectly.
Therefore this device doesn't need a ground, right?
Well, wrong, i presume.
So what i'm asking is: if one takes the observer out of the equation, what is intrinsic to an electronic circuit that it would need a ground?
Could someone give me an example of a circuit component or element that would need a ground, how it would work, and why it wouldn't work without this ground reference?
LateLesley:
Technically, any circuit doesn't need a ground. It only needs a potential difference of the right amount. A 5V circuit would work, if you connected the negative to 200V, and the positive to 205V. There's only 5V difference, and the circuit would work fine. The problem comes with us humans, we stand on the (sometimes wet) Ground. And to an electric circuit, we are just one big whopping resistor. And if you stick a fair few mA through that resistor, it kills it, it dies. When wet, the resistance goes down, and the current increases. So it's good to Reference the circuit to actual ground, to stop you getting zapped. 5V won't kill you. 205V may be a different story. The circuit only needs the potential across the two (or more ) power points to be within the right range, the problem is us humans can't help ourselves and keep poking at metal things, adding a big resistor into the circuit. And where there's potential difference, current will flow. So the whole idea of ground is to teach you not to let angry pixies (electrons) to flow through the 2 year old that poked a knife into your circuit.
Nerull:
You are probably surrounded by battery powered and double insulated circuits that have no earth ground connections at all.
Referencing a circuit such that ground is 0V is just an arbitrary choice done to set the scale. There isn't any more meaning to it than that.
You can think of it like a thermometer. Unless you're using kelvin, your thermometer references zero to some arbitrary point. There is no special magical property to what we chose to call zero degrees, it just is. We had to pick somewhere, and so we did. You can't say 'the temperature is 5 degrees' unless you define where zero is.
Voltage works similarly, though it lacks any absolute reference at all. When assigning voltages in a circuit, you have to pick a point to make zero. It doesn't matter what part you pick. Nothing about this choice will change how the circuit functions.
For ground referenced circuits, we have conventionally defined 0V at ground potential. Many circuits are not ground referenced, and we just choose a part of the circuit to call 0V. Conventionally its the primary return current path.
A circuit only *needs* an Earth ground connection if it is using the ground as a conductor to pass current. Most circuits don't - their grounds exist purely to give any fault current somewhere other than the poor person touching the device a place to go.
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