Electronics > Microcontrollers

Voltage Divider Not Behaving

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yanir:
Hi, I'm am driving a 8x2 Character LCD with backlight using one of pic24 parts (pic24fv32ka). Everything works except for the backlight. When I power the backlight directly (it's just an 3V green led on the lcd board) it works. But when I try to power it from the microcontroller it doesn't. The voltage coming out of the pin is 3.24 (should be 3.3, not sure why it's 3.24) And I pass that pins output to a voltage divider (r1 = 10000K, r2 = 100000K). With 3.3V as input you should get 3.0V output from the divider. I get around 2.45V, which is too low to turn on the LED.  Even with a 3.24 input the output would be 2.94.

Why would I get such strange output from the divider?

Notes: The output pin on the micro is set for open drain.
My voltage regulator fixed at 3.3V.
I have powered it from a pickit3 and from it's normal power supply with the same results.

Any insight would be appreciated.

Yanir

ejeffrey:
Voltage dividers only work when driving a high impedance load.  The LED draws current, dragging the voltage down.  Instead, you need a current limiting resistor.

alm:
Two issues: when using a voltage divider, the impedance of the divider should be much lower than the load impedance. This makes them impractical for low impedance loads, as ejeffrey writes.

The other issue is that you should keep the current constant for an LED, not the voltage. The forward voltage drop will vary with temperature and between samples of the LED, and due to the exponential relation between current and voltage, this will have huge effects on the current that will be flowing.

Zero999:
I agree with all of the above. Another thing: the output impedance is equal to the equivalent of the two resistors in the divider connected in parallel.

deephaven:
You don't need a voltage divider, just a series resistor to limit the current flowing through the LED. Something like 100 ohms would give you 10 mA. You commented on the output being not quite 3.3V, this is normal IC specs, you will never get exact voltages out of a micro. If you look at the specs in the data sheet it will show you what voltages to expect.