Don't take in account the ratio i know its not right, just assume there's 12v coming in and 3.3 going out
here I've used a 3.6V Zener diode but every time I've used this in practice on a breadboard i lose 2v and it shows 1.6v ish realistically this should float at 3.6 according to multisim ? any ideas or is it just common knowledge to upsize them
this is used to protect an MKR pin
thanks
Zeners need a certain amount of current passing thru them before they will operated as expected. You may need to look and see the particulars for your zener and the current it is passing.
Tparry:
Don't take in account the ratio i know its not right, just assume there's 12v coming in and 3.3 going outhere I've used a 3.6V Zener diode but every time I've used this in practice on a breadboard i lose 2v and it shows 1.6v ish realistically this should float at 3.6 according to multisim ? any ideas or is it just common knowledge to upsize them
this is used to protect an MKR pin
thanks
It's not even that, it's allowing current though when it shouldn't because i know the voltage at the voltage divider is below 3.6V and therefore shouldn't be doing anything but in this case its dropping 2v
Tparry:
It's not even that, it's allowing current though when it shouldn't...
As zoomkat was trying to tell you, zeners already start conducting below their rated voltage.
There is only 3.6volt across that zener at the rated zener current, which could be 100mA.
Tparry:
this is used to protect an MKR pin
Zeners are generally speaking almost useless to protect a pin.
Max pin voltage is VCC+0.5volt, which means 0.5volt (not 3.3volt) if the Arduino is off.
Why do you think it needs protection.
If... you need protection (please tell us why), then a schottky clamping diode to VCC might do a better job.
Leo..
It would take 16.64V at the top of that divider to get 3.3V at the tap, 12V would only be 2.38V. How much current through the divider? You need a 150k for R1.
If this is to measure a voltage (battery/supply), then you should divide to one of the build-in references.
For a MKR board, that could be 1volt, 1.65volt or 2.23volt, but definitely not to 3.3volt.
That is also safer, because of a lower voltage on the pin.
And the only way to get a stable readout.
This advice might not suit ratiometric sensors, so tell us what you're measuring.
If you're measuring a 12volt lead/acid battery, then a 150k:10k divider could be ok.
With a call to 1volt Aref in setup().
Don't use silly resistor values. E12 (or E24) is ok.
You need to calibrate final readout anyway.
Leo..
Wawa:
If this is to measure a voltage (battery/supply), then you should divide to one of the build-in references.
For a MKR board, that could be 1volt, 1.65volt or 2.23volt, but definitely not to 3.3volt.That is also safer, because of a lower voltage on the pin.
And the only way to get a stable readout.This advice might not suit ratiometric sensors, so tell us what you're measuring.
If you're measuring a 12volt lead/acid battery, then a 150k:10k divider could be ok.
With a call to 1volt Aref in setup().Don't use silly resistor values. E12 (or E24) is ok.
You need to calibrate final readout anyway.
Leo..
Ah alright that makes sense makes some of the stuff we were taught useless at uni.
Basically yes its measuring a 12v lead acid bat, the resistor ratio you just gave would give 0.75v at the center? currently its set up to spit out 3.3v at the divider anything more than this the the MKR1300 pin cant take
The battery voltage in itself might rise to +12v after fully charging giving which would give rise to the voltage out of the divider.
This is only being sampled every so often not continuously there is a mosfet and a npn to control which inst in that pic but not relevant.
Its currently going to the analog pin and being scaled to the ADC 1023 and then to the battery which does give accurate read outs.
Basically i want to protect the pin from over voltage
I guess i could scale the the divider down further but still in regard to protection it inst doing anything
Hope this makes sense
Thanks
With that 10k:150k divider, and 1volt Aref enabled in setup(), you have protection to >50volt.
Because the pin can still take 3.3volt while the A/D outputs 0-1023 with 0-1volt.
Above that, you still have protection, because of the build-in ESD clamping diodes (diodes to VCC and GND).
I choose 1:15 (= 16volt with a 1volt Aref), because charging voltage is normally 13.8volt but fast charge and peaks can be several volts above that.
Post the diagram of that fet circuit. Seen several issues with that as well.
Doubt it's even needed, because the ~60mA/month drain of a 160k divider is likely a lot less than the self-discharge of an SLA battery.
Leo..
Wawa:
Doubt it's even needed, because the ~60mA/month drain of a 160k divider is likely a lot less than the self-discharge of an SLA.
Did you mean ~60mAh/month?
Paul__B:
Did you mean ~60mAh/month?
Yes, of course.
Seems to be written at 7:38 am, before the coffee.
Leo..