Dec 27, 2011

EE Fundamentals: Voltage Divider Quiz

There are a couple things you need to remember to be able to do this quiz. Luckily, everything you need we have covered in this blog.

First: This circuit is a simple comparator, which I have done a whole entry on in the past. The comparator will output a high voltage when the non-inverting(+) input voltage is higher than the inverting(-) input. When the output goes high, the LED will turn on.

Second: There are two voltage dividers setting the voltages on both input pins of the comparator.

Third: If you watched the video on schematics I posted, please realize that wires only intersect in a schematic where you see a dot. For instance, the voltage divider setting the voltage on the non-inverting pin is not connected to the inverting pin in any way even though there is a wire going from the 4.7k resistor to the mystery resistor that passes over the negative input.

Quiz Question: What is the minimum resistance necessary on the ??? resistor to turn on the LED? In other words, what is the lowest value resistor you can place in the ??? spot to make the non-inverting input voltage higher than the inverting input voltage?

Good Luck!


Joel Ryan said...

Can you explain the number "390" on the right side of the diagram? It is confusing because it is unit-less. If I understood that, I'm sure (read- "not sure") that I can get an answer easily.

Chris said...

The 390 represents a 390 ohm current limiting resistor. It limits the current delivered to the LED when the output of the comparator goes high. I put it in there for completeness/correctness but it really has nothing to do with solving the problem. All you really need to care about are the 4 resistors and the voltage source.

Joel Ryan said...

I'm going to go out on a limb here and say 1.56k? Is that anywhere close?

Noah Ryan said...

Okayokayokay. I'm going to go with 7.05K volts. I hope so much that I'm correct, and my soul will be crushed if I'm wrong.

Noah Ryan said...

So clearly I meant ohms, not volts as I absent mindedly wrote. I was thinking about the voltage at the pins of the comparator so much I forgot the units I was actually interested in.

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