This thread is actually a fork from a previous one where some good people suggested I should use a constant current source instead of a regulated voltage to read the resistance from an automotive sender (actually 3, 2 NTC temp senders, 1 oil pressure).
While I wait for the parts I went through the LM317 datasheet, did some math on paper and finally simulated the whole thing.
The resistance of all senders in the working range are great, no issue there. The problem comes when it's -20 degC outside. Any of the two NTC senders will be around 5kOhm. With my current setup, LM317 set to output 27mA, the voltage becomes 135V.
I assumed that can't happen so I did a quick simulation to check the results. And I noticed that at a very specific point the constant current starts to drop and the voltage over the temp sender will never ever get to 135V.
That very specific point is actually when Vin-Vout drops bellow 3V.
Then I noticed the following line in the datasheet: 3 V≤(VIN−VOUT)≤40 V
But I don't know how to understand it. This isn't the drop-off voltage. It's supposed to be 1.5V
So, where does the 3V comes from and what happens when Vin-Vout<3V
The second question is about the zener diode used to limit the voltage that goes to the Arduino. It also solves the issue of a very high resistor. But is there any downside of using it? Is there a better alternative?
And finally, is there a downside to using a constant current source to measure the resistor? It seems much elegant now to me, but when it comes to NTC sensors, the entire internet will advice you to go for a classic voltage regulator and a voltage divider (and theoretically even the ccs is still is a voltage divider).
Thanks,
Andrei
While I wait for the parts I went through the LM317 datasheet, did some math on paper and finally simulated the whole thing.
The resistance of all senders in the working range are great, no issue there. The problem comes when it's -20 degC outside. Any of the two NTC senders will be around 5kOhm. With my current setup, LM317 set to output 27mA, the voltage becomes 135V.
I assumed that can't happen so I did a quick simulation to check the results. And I noticed that at a very specific point the constant current starts to drop and the voltage over the temp sender will never ever get to 135V.
That very specific point is actually when Vin-Vout drops bellow 3V.
Then I noticed the following line in the datasheet: 3 V≤(VIN−VOUT)≤40 V
But I don't know how to understand it. This isn't the drop-off voltage. It's supposed to be 1.5V
So, where does the 3V comes from and what happens when Vin-Vout<3V
The second question is about the zener diode used to limit the voltage that goes to the Arduino. It also solves the issue of a very high resistor. But is there any downside of using it? Is there a better alternative?
And finally, is there a downside to using a constant current source to measure the resistor? It seems much elegant now to me, but when it comes to NTC sensors, the entire internet will advice you to go for a classic voltage regulator and a voltage divider (and theoretically even the ccs is still is a voltage divider).
Thanks,
Andrei
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