# voltage divider

What does this mean? i do not under stand 100%. What does rth mean? What does under load mean? What does vioded mean?

and what is that called the whole thing?

The whole thing is showing that a potential divider is rubbish at making a power supply. Rth is the source impedance of the 5V point in the circuit. It is found by working out the value of the two resistors in the potential divider in parallel. Under load means when the 5V is trying to power something. In your case a 50R resistor. Because of the source impedance of the 5V when it is placed under load (has power drawn from it) the voltage delivered to the load is chuff all.

What does this mean? i do not under stand 100%. What does rth mean? What does under load mean? What does vioded mean?

You would have to learn a bit about Thévenin's theorem in order to understand what is going on. Here's a brief outline:

The circuit in the upper left is a voltage divider that is set up to deliver 5V (across the 1K resistor) when supplied with 9V. If you change either of the resistors then the output voltage (the voltage across the lower resistor) will change.

When you try to use this type of supply to power something you do so by connecting that 'something' in parallel with the 1K resistor. This changes the circuit and that in turn changes the output voltage. For instance, if your 'something' is a 50 ohm resistance then the parallel combination of the 1K and the 50 ohm resistor is about 47.6 ohms. When you go through all the math you find that the 5V has dropped to 0.51V which certainly is not good. The real problem is that you have to redo the calculations for this series-parallel circuit each time you connect something different to the 1K resistor.

What Mr. Thevenin came up with is a way to make these calculations a bit easier if you do some work up front. If you correctly calculate the 'Thevenin Equivalent Resistance' called Rth and the 'Thevenin Equivalent Voltage' called Vth you can replace your original circuit in the upper left with the one to the right. In this case Rth = 444 ohms and Vth = 5V. The calculations for Rth are at the top right and the calculations for Vth are not shown.

Now, if you connect your 50 ohm resistance to this new circuit you have a simple series circuit and can easily come up with the output voltage (the voltage across the 50 ohm resistance), which is again 0.51V.

The real benefit comes when you change the load. If you connect a 200 ohm resistance instead of the 50 ohm resistance then the total resistance is 444 + 200, the current is 7.76mA, and the output voltage is 1.55V. This is the same result you would have gotten if you correctly redid the calculations using the original circuit.

The bottom line is, as Mike said, that a voltage divider makes a lousy power supply.

"What does this mean?" This is an equivalent circuit that allows you to replace a series-parallel circuit with a simpler series circuit.

"What does rth mean?" This is the Thevenin Equivalent Resistance". The single series resistance that you use in your new circuit to replace the series-parallel ones in your original circuit.

"What does under load mean?" This means that your power supply is supplying power to an external circuit called the "load". In this case the load was the 50 ohm resistor.

"What does vioded mean?" This should be Vloaded. It is the output voltage of the supply when the load is connected. In this case 0.51V.

Don

Here some sites you should check about “Thevenin” circuit.

http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Source/Source2.html

And here my picture.

why at 3:15 here: http://www.youtube.com/watch?v=XxLKfAZrhbM it comes out to 444 and when i do it it comes to 2000????

You are obviously doing it wrong. Did you do any research on Thevenin's Theorum? How do you come up with 2000?

Don

i got it to work just if the resistors are so much higher then 444 ohm how does it come out that much:) just would like to know.

It comes out to 444 and when i do it it comes to 2000????

How did you calculate the 2000 ?

I did it and it come to 444.444444...

I use a scientific calculator. It have ( ).

So I did that way ... 800 X 1000 = 800 000 / ( 800 + 1000 ) = 444.44444.....

or

800 X 1000 = 800 000 800 + 1000 = 1800 800 000 / 1800 = 444.44444.....

I hope you listen in Math class... ;)

Were can i get one of them? ;)

I didn't write the rules, I just follow them. If you study and then use the technique that Mr. Thevenin described you will arrive at 444 ohms for that circuit. Describe how you came up with 2000 and I may be able to figure out what you did wrong.

Don

I use a scientific calculator....

If you have one that uses RPN then it is easier to use the reciprocal equation -- and it works just as easily when there are more than two resistors in parallel.

800 1/x 1000 1/x + 1/x results in 444.4444...

Don

i got it to work just if the resistors are so much higher then 444 ohm how does it come out that much:) just would like to know.

You need to take a course called Circuit Theory 1 and pay attention when you get to the part about 'parallel resistors'. You seem to ask a lot of questions but you don't answer any.

Don

one more thing is thevenin only for voltage dividers?

one more thing is thevenin only for voltage dividers?

Check the web links in my posts. And my picture.

I guess it time to do "homework" or "study"... ;) This is how you lean on your own.
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