Arduino Uno with voltage divider not working as expected

Members

Im attempting to build a voltage divider (using two resistors) onto a board. The max voltage is set at 16V. Five volt to drop across ATMega pin A1 (to read the max voltage supplied) and the rest to supply the board.

When I connect the power supply I get a faint light on the yellow led. Any help would be appreciated. Ive attached a screenshot of the .sch file as its easier to view.

Thanks in advance

Your screen capture makes it difficult to see anything.
Use the Eagle File:Export:Image feature to make it clear what you are referring to.
I downloaded your picture, cropped it, zoomed in, and it is still not readable.

Or, use the Windows Accessories:Snipping Tool and capture the screen area in question, and attach that image.

Please try again.

Thank you.

R1 = 8.2k, R2 = 3.3k, attenuation factor = 3.485, 16V in, 4.6V out.
https://ohmslawcalculator.com/voltage-divider-calculator

Five volt to drop across ATMega pin A1 (to read the max voltage supplied) and the rest to supply the board.

Does this mean you are trying to power the AT Mega with a voltage divider?

You can't do that.

Could you please post a better screenshot? I can barely make it out. Is that R4 and R5? Why is it grounded between them? The ground would make the node you labeled "voltage divider" output 0. It's very hard to read and something doesn't look right but once you can post a better diagram we can reply. It looks like its being fed to one of the analog inputs.

Hi.

Here's a tip to use when making screenshots on your dual monitor windows setup, and in case you need a single instance to be shown:

Press ALT+Print-Scrn.
That will save a screenshot of only the active window to the clipboard.
Now create a new bitmap on your desktop, and right click to edit it.
Press CTRL-V to paste the screenshot into that bitmap, and save it.

Of course there's better formats and better programs to create pictures, so feel free to use any of those.
The forum software seems to like the png format, so that would be a good choice.

Members

Im attempting to build a voltage divider (using two resistors) onto a board.

When I connect the power supply I get a faint light on the yellow led.

Yes true. I agree ------ the circuit diagram does show a sort of voltage divider, with a couple of diodes nearby, with the node between the two resistors connected to 'ground'.

And, there is another part of the circuit that says 'yellow' for yellow LED.

So we can assume that at least the voltage divider isn't being used to drive a yellow LED, right?

Apologies for the poor picture quality.

fnb111:
Apologies for the poor picture quality.

The pic quality is good now.
Your bottom-left hand side of the pic has a voltage divider. The middle of that divider --- the mid-point --- is grounded. So that point is zero volt. But you also have another node ----- bottom-left, which is also at zero volt.
The mid-point of a voltage divider is normally connected to a component that has a high resistance (or high impedance) ..... not connected to a 'short circuit' to ground.

On the other hand, we then have to think about what the purpose of the mid-point of that divider is for ...... if it were not connected to ground. When that mid-point is grounded though ------- current is just going to flow with the 510 ohm resistor ------ how large that current is (or how small) will just depend on what '+V' is.

And .... depending on how big +V is ......... may need to check the 510 ohm resistor power rating, and check how much power it dissipates.

Hi,

  • What is your project?
  • What is it supposed to do?
  • What is the 16V input?
  • Is the 16V input variable or fixed?

Thanks.. Tom.. :slight_smile:

TomGeorge:
Hi,

  • What is your project?
  • What is it supposed to do?
  • What is the 16V input?
  • Is the 16V input variable or fixed?

Thanks.. Tom.. :slight_smile:

  • What is your project? Measuring the voltage on a AGM battery (sits around 15.3 max)
  • What is it supposed to do?
  • What is the 16V input? DC input
  • Is the 16V input variable or fixed? Between 10V and 16V max

Assuming 15.3 volt across 510 ohm ......... gives approximately 0.46 watt. Hopefully that resistor has at least half watt rating.

That's the resistor in the upper part of the 'voltage divider' with the centre node grounded, which means not operating as a 'voltage divider' as such.

One more comment. Very minimalistic answers and even no answer(s) to questions intended to assist ----- is not going to help much (or at all).

Southpark:
Assuming 15.3 volt across 510 ohm ......... gives approximately 0.46 watt. Hopefully that resistor has at least half watt rating.

That's the resistor in the upper part of the 'voltage divider' with the centre node grounded, which means not operating as a 'voltage divider' as such.

One more comment. Very minimalistic answers and even no answer(s) to questions intended to assist ----- is not going to help much (or at all).

Thick Film Resistors - SMD 0805 510ohms 0.5W 5% AEC-Q200

"Assuming 15.3 volt across 510 ohm ......... gives approximately 0.46 watt. Hopefully that resistor has at least half watt rating."
The max Voltage across R510 should only be
Vmax - R5 voltage drop (which is about 5V max)
so it should be around 16-5=10V across R510

It still does not explain the yellow led lighting up?

Your yellow led are lighting up sometimes because it’s on the serial clock pin and there’s traffic. Your board makes no sense.

I'm pretty sure ATMEGA chips HATES unregulated power. Your yellow LED is probably the byproduct of the chip powering up very slightly and freezing at startup sequence. If you want to power a chip, just skip the voltage divider and go with a regulator.

Also, I might be very wrong here, but I think you're dividing your VIN with what I assume is an OP-AMP circuit on top left also? I don't know, I might be very wrong here. You should try using a multimeter to measure voltage, to be sure.

Edit: I whipped up my arduino uno in curiosity, and tried powering it up. Pin 13 LED (Which is the SCK pin) lights up on start up. So, it really might be the chip freaking out at startup sequence.

wolframore:
Your yellow led are lighting up sometimes because it’s on the serial clock pin and there’s traffic. Your board makes no sense.

A nRF module is connected to D6 & D7 of the ATMega.

Would it be better to separate the voltage divider from the supply to the board? Example: place the divider in parallel with the board supply?

fnb ------ could you circle the voltage divider that you've been referring to? Put a circle around it. At the moment, I can see on the bottom-left of your diagram a circuit that has two resistors joined end-to-end, but has the middle point (between the two resistors) grounded. The grounding probably wouldn't create any issues, but just interested to know what 'voltage divider' you're referring to when you mention voltage divider.

If I change this (attached) what would be the new values for R8 & R7 ?

Southpark:
fnb ------ could you circle the voltage divider that you've been referring to? Put a circle around it. At the moment, I can see on the bottom-left of your diagram a circuit that has two resistors joined end-to-end, but has the middle point (between the two resistors) grounded. The grounding probably wouldn't create any issues, but just interested to know what 'voltage divider' you're referring to when you mention voltage divider.

R4 & R5 is the divider. Its been suggested that this is not the best way to do this.