Author Topic: Voltage Divider - What am I doing wrong?  (Read 1477 times)

GreySuits

Voltage Divider - What am I doing wrong?
« on: June 14, 2019, 11:16:25 PM »
I'm breadboard a oveedrive using a TL072.  I need to drop the voltage down from 9v to 4.5v to go into pin 3.  I tried to do it using the voltage devide below (based on the schematic):

But when I plugged it into the board, using 10k resistors, I'm getting  a reading of 0.25v.  Here is the configuration:

And still got a 0.25v reading on the jumper.

Can any one help me with what I'm doing wrong?
« Last Edit: June 14, 2019, 11:19:04 PM by GreySuits »

Rob Strand

Re: Voltage Divider - What am I doing wrong?
« Reply #1 on: June 15, 2019, 12:29:38 AM »
There's many things you can check.  One trick about debugging is checking what you think is true.

Make sure you are getting 9V to the outside terminals (ie. the power to the both resistors).  Measure at the resistors.

Next measure the voltage across the top resistor.

If you are getting 9V across the outside, 0.25V across the bottom resistor but not 9-0.25 = 8.75V across the top resistor then that means the meter is loading down the voltage (as the sum doesn't add-up to 9V).  Maybe the DMM is on the wrong range.

Next remove the power connections to the resistors measure the resistance across each and also across both (it should be 20k).   If the cap is in place you might need to remove it.
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bluebunny

Re: Voltage Divider - What am I doing wrong?
« Reply #2 on: June 15, 2019, 12:29:23 PM »

This picture is back-to-front.  If you put 9V in at the right (currently labelled "4.5V"), you'll get 4.5V out at the left-hand side (assuming R6 and R7 are the same value).
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GreySuits

Re: Voltage Divider - What am I doing wrong?
« Reply #3 on: June 16, 2019, 03:26:34 AM »
Awesome, thanks guys you both are correct.  I figured it out (was measuring it wrong), and one of my set ups was incorrect.