Reliable way to measure 24 VDC and have reverse current protection

[KrisKasprzak]

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?





Thanks in advance.

[kd5rxt-mark]

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

[thebigg]

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.

[MichaelMeissner]

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.

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:

• Through hole INA219 sensor
• Featherwing INA219 sensor

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

[carlsuttle]

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

[BriComp]

Yes. Put a BAT54S on the input to the Teensy. Should solve that problem.