Got a short and sweet guide on Voltage dividers. A classic that we somehow didn’t have a guide on. This is a very beginner-orientated guide where we go over the fundamentals of a divider, how to calculate the needed resistors, and take a look at using Falstad to simulate one: “Getting Started with Voltage Dividers | A Practical Guide for Makers”
In this guide we will be taking a practical look at Voltage dividers - one of the most fundamental circuits in the world of electronics and this guide will hopefully be the only guide a maker ever needs to read. We will be taking a look at both the …
Thing is… it still works as a voltage divider, just not a well engineered voltage divider.
I’ve google that Awg14 has 8.28Ω per 1000m.
So let’s imagine I have a football field, a 10v Power Supply, and I lay out 1000m of awg14 cable.
I put a probe in the center point of the cable, such that I have 500m of cable on each side, and each 500m of cable has ~4Ω of resistance. The probe has a reference to ground from my power supply.
I’m expecting 5v at that probe? ??
I have created a … voltage divider? ??
Thing is, every circuit is just made of wires, and every wire has some amount of ohms.
(Resistors are sometimes just wound copper after all)
The wires I use for circuits I build at home have a tiny, minuscule, imperceptible amount of resistance… but not zero!
I understand that, when i start applying a load the voltages respond to ohms law.
I can’t just find the midpoint of a cable and then throw a lamp at it and expect half the voltage.
The falstad simulator says the below circuit is a perfectly reasonable voltage divider… but in reality land I’ve basically grounded that lamp. It’s a short.
But if I had a sufficiently long wire, 10v, and a quality probe, would I be able to find any arbitrary voltage between 10v and 0v across the wire or does this logic break down at when the ohms get small enough?
I would have guessed 0v everywhere because I’ve essentially made a short but maybe I’m wrong.
Hi Pix
Of course ant length of wire tapped part way along could be termed a potentiometer or voltage divider.
If you want a discussion on the voltage divider effects of long wires you should talk to the telephone people, or the older ones as I know nothing about a modern system or for that matter not much about the old. they had to deal with a vast mix of different lengths and resistances and there is a limit. Particularly if you are using the phone lines for switching other equipment like 2 way radio base stations and the like.
The clue here is if you are designing anything that is to be connected to such a variety of values you design that equipment to handle it. This type of thing is a reality and it won’t go away. Up to the user to do something about it. Speaking of telephone systems the humble phone is an example of engineering things to handle many different scenarios. One phone might be 500 metres from the exchange and another 10km. They both have to perform in a similar manner.
Speaking of voltage dividers you can use capacitors and inductors in the same manner for AC systems. Capacitor dividers are not so common but inductive are more so. Some are continuously variable like the mains voltage Variac which is an Auto Transformer.
You can use a combination of resistor / capacitor or resistor / inductor but calculations are not quite so straight forward. For instance if you had a resistance of 50Ω and a reactance (Cap or ind) of 50Ω with say 10VAC applied the voltage at the junction would be 7.07VAC NOT 5V. There will be a phase shift here which causes this.
Anyway that is enough confusion for one night. Relax and don’t have any nightmares about it.
Cheers Bob
The short answer is that you can’t get 0V or 10V from that voltage divider, because in order to do so one leg would have to be zero Ohms, and then it would no longer be a voltage divider. So it’s not a matter of the logic breaking down - it simply becomes invalid when you have a circuit that doesn’t meet the conditions required for that particular calculation.
If I am interpreting this right then I think you are correct.
If you had 1000 meters of wire laid across a field, connected to a 10 volt battery, you would be able to measure halfway and get 5 volts. Assuming this is a perfect ideal world though as the battery would need to be an ideal voltage source that can supply the high current this low resistance wire would draw (assuming that the wire doesn’t overheat and destroy itself as well).
With this set-up, you should be able to measure 10v at the +tive terminal of the battery, run along the wire and measure every voltage in between down to 0v. Essentially you have created the world’s largest and least efficient potentiometer.
As for connecting the lamp, this is stressing my brain for a Monday morning. In the guide, we quickly look at powering a motor with the divider. It doesn’t work here because we are using a 1k resistor and the motor is only a few ohms. These 2 effectively are placed in parallel and their combined resistance drops close to only a few ohms. This changes our ratio of resistances in the voltage divider and gives us nearly 0v for the motor.
However, I think your example there works a bit better as the resistor sizes used in the divider are already very low, so when you put a lamp or motor in parallel with R2 it doesn’t drop very much. This is terribly inefficient though.
These are both great thought experiments but as always, if you translated them into the real world you would quickly run into a plethora of problems.
Ah yeah,a it’s a potentiometer, that’s the insight I needed.
Yes I think I understand. zero ohms is meaningless.
Insightful history as always.
thanks everyone for your responses.
If you look through my edits on my original post you can kinda see the slow process of me rubber ducking my way out of my own question. I think the potentiometer keyword really settled me into real understanding of a few key concepts.
Now I have new questions like “what even is a short circuit?” and “why ohms law but?” but I’ll spend some time googling and reading my way through that answer over Easter.
