Source loading? How does it work?

[mkiijam]

I was playing around with a simulator and made an extremely simple circuit consisting of a INPUT Signal source that fed an OUTPUT. Being a complete newbe I added a resistor to ground right on the wire that led from the INPUT to OUTPUT and noticed that no matter how low I made the resistor in Ohms, the signal on the simulator's scope, would not diminish. I thought that was odd until I thought I must being dealing with Ohm's Law or something, and that the INPUT block must have infinite current? or 0 resistance or both, right??? I know I should have learned this 30 years ago, but I'm just being honest...

So, I'm pretty sure I am on the right path, what basic understand was/am I missing? What governs the input resistance of real world sources? Cause I cannot fathom if I hooked this circuit up on my bench, that a 1 Ohm resistor to ground would not lower my bench top signal generator!

[ledtester]

1) By default most simulators assume that voltage sources are ideal but every real-world voltage and signal source has an output resistance. Therefore your circuit really looks like this:



For instance, most signal generators have an output resistance of 50 ohms. Now the output is connected to a voltage divider.

2) Even wires themselves have resistance which is not usually modelled by simulators.

[mbeldyga]

Your source has 0Ω output resistance at the moment. You can either add it as a parameter to your voltage source (Rser= in LTspice), or as a discrete external element

[Tomas9970]

Don't most PSUs have electronic protection (like a transistor) that gives unrestriced output and then lowers output voltage to keep under a current limit? Or is it just a quirk of voltage regulation happening after an output resistor so you don't see it's voltage drop.

[TimFox]

A linear power supply stabilizer (regulator) has a very low output resistance due to negative feedback from the terminal voltage.  The current limiter usually included is a non-linear circuit that kicks in above a set current value.  An ideal current limiter would give an infinite output resistance, reducing the terminal voltage as required to maintain the output current into a low load resistance, when activated.

[Tomas9970]

Ok. I decided to make a table to calculate output voltages in relation to output load with various types of output protection. I also have a line at 10 volts to show a voltage regulator target. There's just a 50 Ohm resistor in series with the output, then a voltage divider as shown here and then a transistor keeping a 200mA maximum output. Transistor is also shown with different input voltage but more on that later.

There are some things to take away from this (at least for me):
1.) Single resistor in series and a voltage divider don't look as bad as I thought as long as you don't care about efficiency and can spare a significant headroom. They also both scale to the same percentage regardless of the input voltage, which makes them good for AC signals because they will scale the whole waveform instead of clipping it.
2.) The divider is pretty equal to a single resistor as long as the R2 is at least 10k Ohms. It gets even closer with higher R2 values but never beats the single resistor.
3.) Transistor limiter allows for highest voltage relative to load but it's trigger point moves with different input voltages (as you can see on the second line), which makes it good for DC power supplies but bad for waveforms as it would clip them instead of scaling the whole thing.