Voltage divider - not getting what I expect

I have the following cirquit - my goal is to get 4.29V down to less than 3V. I am using online voltage calculator (http://www.ohmslawcalculator.com/voltage-divider-calculator). Arduino is powered through ftdi -usb from my computer.

  • 4.29v (Arduino pro mini pin 3 turned on) | | | R1 (10K) | ----------X expecting 2.86V but get 0.52V | R2 (20K) | ---------ground

newuser: I have the following cirquit - my goal is to get 4.29V down to less than 3V. I am using online voltage calculator (http://www.ohmslawcalculator.com/voltage-divider-calculator). Arduino is powered through ftdi -usb from my computer.

  • 4.29v (Arduino pro mini pin 3 turned on) | | | R1 (10K) | ----------X expecting 2.86V but get 0.52V | R2 (20K) | ---------ground

Remember Ohm's law????

Compute the current through 30,000 ohms with 4.29 volts. How much current in milliamps?

Now, using Ohms law again, compute the voltage across 20,000 Ohms with the milliamp answer you got for the first computation. Bet it is close to 0.52 volts.

Now, using the Ohms law again, how much current, in milliamps is needed to get 2.86 volts across 20,000 Ohms?

With that last answer, use Ohm's law again to compute the total resistance needed to allow that much current in millamps to pass through the resistors.

Subtract the 20.000 ohms from that answer and you will know the value of the top resistor.

Paul

Have you measured those resistors ?

Yours, TonyWilk

P.S. @Paul_KD7HB - His calculation is fine: 10K/20K I'd expect 2.86V, not 0.52V

Your calculations are correct… I don’t know what Paul is trying to say…

What’s connected to the “output” of the voltage divider? Any load in parallel with 20K resistor will lower the voltage. What’s the ~3V for? You can use a voltage divider for a reference or signal voltage but you can’t “power” anything with it.

If you’re getting the wrong voltage with just your mutimeter connected, measure the resistors to make sure they are the correct value. (Your multimeter is meghoms so it won’t have a measurable effect on the voltage.)

Think R2 is 2K instead of 20K,

Then the voltage would be 2/12 * 4.29 = 0.71v

I would check the resistor (twice at least)

TonyWilk: Have you measured those resistors ?

Yours, TonyWilk

P.S. @Paul_KD7HB - His calculation is fine: 10K/20K I'd expect 2.86V, not 0.52V

Sorry. I did not DO the calcs.

Paul

Are these 1% resistors? If you are using 5% resistors then you can potentially be 10% off of what any calculation shows.

LandonW:
Are these 1% resistors? If you are using 5% resistors then you can potentially be 10% off of what any calculation shows.

And how does 10% account for a 900% difference?

I would agree with checking the load on the voltage divider. Make sure the mid point is not connected to anything before you measure.

Check the voltage across the whole divider - is it actually 4.29V?
Check the voltage across each part.

Is the voltage divider connected to the same ground as the arduino?

You will find your missing voltage somewhere.

I was just adding. I'm aware that the difference of what I said and what is doesn't add up in this scenario. My statement wasn't intended to "fix" the problem of the OP. My statement was just a general statement and something the OP can put in his back pocket if he didn't know already.