I have a question about voltage dropping due to LEDs.
It's a simple program that reads a thermistor via a voltage divider into A0 and using the internal 1.1V AREF. While I can read the ADC fine, very steady and works great, when I add an indicator LED it starts to act funny.
When the LED goes HIGH, it drops the voltage and in-turn causes a false reading at the thermistor. I assume that the voltage divider is now dividing 4.9VDC instead of 5.0VDC is the issue.
Any suggestions on how the best way to approach this? Capacitor on the VCC side?
Any suggestions on how the best way to approach this?
We get this every couple of weeks.
The simplest solution is to use one ground on the Arduino to connect the sensor and the other one to connect to the LED.
The phenomena is called ground lift if you want to look further into the why, and while you might want it in audio circuits you do not want it in measurement circuits.
The simplest solution is to use one ground on the Arduino to connect the sensor and the other one to connect to the LED.
The phenomena is called ground lift if you want to look further into the why, and while you might want it in audio circuits you do not want it in measurement circuits.
Considerable improvement, still drops 2 ADC points (2 deg) but I can compensate.
Thanks!
Grumpy_Mike:
We get this every couple of weeks.
The simplest solution is to use one ground on the Arduino to connect the sensor and the other one to connect to the LED.
The phenomena is called ground lift if you want to look further into the why, and while you might want it in audio circuits you do not want it in measurement circuits.
fmeroney:
Considerable improvement, still drops 2 ADC points (2 deg) but I can compensate.
OK now try in addition a 0.1uF capacitor between the 5V to the ground pin close to the thermistor. That is from the top of the 10K to the ground wire of the thermistor.
Grumpy_Mike:
OK now try in addition a 0.1uF capacitor between the 5V to the ground pin close to the thermistor. That is from the top of the 10K to the ground wire of the thermistor.
Take a look at the placement of the ceramic .1uf CAP, let me know if that is what your thinking.
The Thermistor is represented as the black Ceramic Cap looking component. It is actually a RTD Thermistor with 36" leads, see reference pic.
Question, does the cap need to be ceramic or electrolytic (polarized)?
Grumpy_Mike:
OK now try in addition a 0.1uF capacitor between the 5V to the ground pin close to the thermistor. That is from the top of the 10K to the ground wire of the thermistor.
Well on that Fritzing layout the thermistor is not connected to ground. This is because there is no continuity across the central row. But yes that is where I suggested.
Yes it needs to be a ceramic capacitor for the high frequency response.
You are using the Arduino's 5V regulator to supply power to the thermistor circuit. So any change in the 5V line becomes a change at A0. You might try a separate 5V regulator connected to nothing but the thermistor and R1.
Hi, the reason I have not been adding to this thread is cos fizzing made me assume the foetal position again, and its soooo comfortable.
Tom.....
OP, try a freeware program called ExpressPCB , it has a good easy to use schematic editor, and you can do bmp graphic exports that Windows Paint can convert to jpg, for distribution.
A couple of things. Well, three actually, the third being - why do people post .PDFs for diagrams? They do not display automatically in the Web page like a .gif, .jpg or .png.
Now, the first is that if you want a ratiometric result, you should not use the internal reference because it is in fact, accurate while the 5V Vcc is not necessarily so - as seems to be the problem here. If you use the 5V as reference, then you will be seeing the ratio of the two resistors independent of variations in Vcc. You might however want a bias resistor closer to the thermistor value (though I realise you are trying to minimise heating). Probably even better would be to supply a Vref from a voltage divider similar to that used for the thermistor (just a little higher voltage).
Secondly, you do not need the two 10k pull-up resistors, you just select INPUT_PULLUP for those two pins.
Paul__B:
A couple of things. Well, three actually, the third being - why do people post .PDFs for diagrams? They do not display automatically in the Web page like a .gif, .jpg or .png.
I Will definitely do that from now on if thats the norm.
Now, the first is that if you want a ratiometric result, you should not use the internal reference because it is in fact, accurate while the 5V Vcc is not necessarily so - as seems to be the problem here. If you use the 5V as reference, then you will be seeing the ratio of the two resistors independent of variations in Vcc. You might however want a bias resistor closer to the thermistor value (though I realise you are trying to minimise heating). Probably even better would be to supply a Vref from a voltage divider similar to that used for the thermistor (just a little higher voltage).
So if I get you right I should ditch the 1.1v Internal Reference and create another reference with another divider. For example the max Voltage from the thermistor is .835 VDC. So if i create a divider that provides maybe .900 VDC then call the external VREF that would be better?
Secondly, you do not need the two 10k pull-up resistors, you just select INPUT_PULLUP for those two pins.
Understood, did this as a quick test and realize I have the ability to use the internal pull-ups.
I will take a look at ExpressPCB... thanks for the tip!!
TomGeorge:
Hi, the reason I have not been adding to this thread is cos fizzing made me assume the foetal position again, and its soooo comfortable.
Tom.....
OP, try a freeware program called ExpressPCB , it has a good easy to use schematic editor, and you can do bmp graphic exports that Windows Paint can convert to jpg, for distribution.
