Reliable way to measure 24 VDC and have reverse current protection

KrisKasprzak

KrisKasprzak

Well-known member
All,

I have a data logger that measures a batter voltage where the battery will start at 24 VDC but degrade to 21 VDC.

In order to protect from accidental backward connection I have a simple diode to prevent reverse current (which would probably damage my precious Teensy). A voltage divider drops the input volts to a safe measurable amount fed into pin A0. Computing the supply voltage from A0 measurement is pretty easy--and works. Kinda. See attached circuit diagram.

In testing the circuit, I noticed the voltage drop across the diode varies based on voltage. Hence why I'm never able to calibrate the unit across a large range of voltages. I'm using and RL207.

Anyone know of a better way measure 24 VDC with reverse polarity protection?
1. better diode
2. some op amp circuit
3. other?


RPP.jpg


Thanks in advance.
 
Kris: If you put the diode after the voltage divider, will the fact that it will see less of a voltage fluctuation work in your favor ??

Mark J Culross
KD5RXT
 
Hi kris

Like you, I fought calibrating a simple voltage divider with a rectifier diode for ages and eventually gave up and bit the bullet and added one of these to my motorcycle instruments project. Its been rock solid and as a bonus I can read real-time current draw.
 
thebigg said:
Hi kris

Like you, I fought calibrating a simple voltage divider with a rectifier diode for ages and eventually gave up and bit the bullet and added one of these to my motorcycle instruments project. Its been rock solid and as a bonus I can read real-time current draw.
Click to expand...

I was going to mention something like an INA219 sensor. Adafruit sells 2 versions, one in a feather wing package, and one with separate through hole connections:

Adafruit has said the INA219 sensors have been hard to get during the chip shortages, and those two boards will use either INA219A or INA219B chips (the INA219A is 1% accurate, and the INA219B is 0.5% accurate), depending on what they can get at the moment. I've built a little monitoring station using a Teensy 4.0 with a featherwing adapter, featherwing display, featherwing tripler prototype board, and featherwing INA219A
 
Does this measurement break Vdd + 0.5v

Just this week I was looking at the same kind if circuit shown at the top of this thread. I read in other thread posts that a voltage applied to an analog pin cannot be higher than Vdd + 0.5v. If the Teensy power is off then any voltage above 0.5v applied to an analog pin, from the external supply, is going to cause smoke.

What did I get wrong.

Carl
 
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