GuppyHJD
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Joined: Sat Jan 20, 2018 3:37 am

Protect over voltage on Voltage Divider ciruit

Sat Aug 10, 2019 12:57 am

Hello,
I am trying to monitor a remote 12v battery. When I do the math for a voltage divider R1 = 1000ohm, and R2= 330 ohm, gives me 3.49 v if the battery is at 14v. If it is at normal 13.1v to 12.8v the output voltage drops to 3.25v to 3.177v. I am wondering, is there a way to protect the Raspberry Pi pins from anything over 3.3v? I don't want to fry the Raspberry Pi.

PhatFil
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Re: Protect over voltage on Voltage Divider ciruit

Sat Aug 10, 2019 1:40 am

You do have some leway below 3.3v the pi will register a high with voltages over +1.8-2v referenced to ground? (look that up or test it..)
http://www.mosaic-industries.com/embedd ... ifications


or you could employ an optically linked indicator, https://shop.pimoroni.com/products/spar ... AeEALw_wcB

or a logic level shifter

dl324
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Location: Pacific Northwest, USA

Re: Protect over voltage on Voltage Divider ciruit

Sat Aug 10, 2019 1:46 am

You could use a voltage comparator like LM393. Just connect the pull-up resistor to 3.3V. You can operate the comparator from 5V or your battery voltage.

drgeoff
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Re: Protect over voltage on Voltage Divider ciruit

Sat Aug 10, 2019 8:41 am

Or put a 2.7 volt zener diode across R2.
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davidcoton
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Re: Protect over voltage on Voltage Divider ciruit

Sat Aug 10, 2019 8:48 am

GuppyHJD wrote:
Sat Aug 10, 2019 12:57 am
I am wondering, is there a way to protect the Raspberry Pi pins from anything over 3.3v? I don't want to fry the Raspberry Pi.
The resistor in the voltage divider should be enough protection, up to 4V on the GPIO pin (so over 16V at the battery). Note this is outside the recommended operating conditions but should give adequate protection for an unusual condition. Beyond that you could clamp the GPIO pin via a diode to 3V3, or use a 3V3 zener diode to ground (also gives reverse polarity protection).
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GuppyHJD
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Re: Protect over voltage on Voltage Divider ciruit

Sun Aug 11, 2019 1:57 am

Thank you for the replies. I have ordered a few Zener diode.

trejan
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Re: Protect over voltage on Voltage Divider ciruit

Sun Aug 11, 2019 2:05 am

Use bigger value resistors as you'll be dissipating ~10mA across your voltage divider with your current selection. You may need a 10-100nF ceramic cap from the ADC input to ground as well.

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Burngate
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Re: Protect over voltage on Voltage Divider ciruit

Sun Aug 11, 2019 9:27 am

I'm not convinced about using higher value resistors.

With 1k & 330Ω across 14v, it's only dissipating ~150mW, well within what a standard ¼W through-hole resistor can handle, though a bit much for a standard 0603 Thick Film SMD Resistor ±1% 0.1W.

The OP states
I am trying to monitor a remote 12v battery
To me that means there's a long distance between the battery and the Pi, with all the problems of interference along the way.
As the resistor values go up, so do the problems, so I would be considering reducing the values, not increasing them.

drgeoff
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Re: Protect over voltage on Voltage Divider ciruit

Sun Aug 11, 2019 9:57 am

trejan wrote:
Sun Aug 11, 2019 2:05 am
Use bigger value resistors as you'll be dissipating ~10mA across your voltage divider with your current selection. You may need a 10-100nF ceramic cap from the ADC input to ground as well.
What ADC? No mention in this thread of an ADC until trejan's post.
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trejan
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Re: Protect over voltage on Voltage Divider ciruit

Sun Aug 11, 2019 10:19 am

drgeoff wrote:
Sun Aug 11, 2019 9:57 am
What ADC? No mention in this thread of an ADC until trejan's post.
If they want to know the exact voltage then they'd need one. If they're happy with knowing they're just over some preset voltage then no.

trejan
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Re: Protect over voltage on Voltage Divider ciruit

Sun Aug 11, 2019 10:23 am

Burngate wrote:
Sun Aug 11, 2019 9:27 am
I'm not convinced about using higher value resistors.

With 1k & 330Ω across 14v, it's only dissipating ~150mW, well within what a standard ¼W through-hole resistor can handle, though a bit much for a standard 0603 Thick Film SMD Resistor ±1% 0.1W.
I was more concerned about unnecessary drain on the battery than the resistors themselves. It isn't huge but it may make a difference if they're trying to maximise battery life. I think OP needs to post more details about what they're trying to do.

drgeoff
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Re: Protect over voltage on Voltage Divider ciruit

Sun Aug 11, 2019 10:24 am

trejan wrote:
Sun Aug 11, 2019 10:19 am
drgeoff wrote:
Sun Aug 11, 2019 9:57 am
What ADC? No mention in this thread of an ADC until trejan's post.
If they want to know the exact voltage then they'd need one. If they're happy with knowing they're just over some preset voltage then no.
The threshold voltage on a RPi GPIO is not sufficiently well defined to know with certainty if above or below some preset voltage.
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trejan
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Re: Protect over voltage on Voltage Divider ciruit

Sun Aug 11, 2019 10:46 am

drgeoff wrote:
Sun Aug 11, 2019 10:24 am
The threshold voltage on a RPi GPIO is not sufficiently well defined to know with certainty if above or below some preset voltage.
Add an ADC then :roll:

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