ricardo.santos
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GPIO 5v outputs 5.5v

Fri Jun 08, 2018 9:39 am

Hi

I've measured the 5v pin against GND pin of my RPI3B with a voltmeter and it's showing 5.45v +-0.05v.
Is this normal? I'm afraid that this may damage my ADC (ADS1115) that is limited to the 5v.

What should i do to limit the output to 5v? can i use a zener diode? I'm civil enginner. not very good with eletronics

help is appreciated

thanks

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RaTTuS
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Re: GPIO 5v outputs 5.5v

Fri Jun 08, 2018 9:42 am

1) What is your input voltage ?
2) have you checked the battery on your meter
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ricardo.santos
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Re: GPIO 5v outputs 5.5v

Fri Jun 08, 2018 9:56 am

I've tested with an official rpi power supply and i get 5.19v from 5v pin. Is this normal?

I should not restrict it to 5v with something before connecting to sensors or micro-controllers

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Z80 Refugee
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Re: GPIO 5v outputs 5.5v

Fri Jun 08, 2018 10:40 am

You need to understand that the RPi runs from 3.3V, which is regulated on-board from the "5V" input. The 5V pins on the GPIO are connected directly to the power input, so whatever you put in is what you get out.

Because of the regulation, it doesn't matter to the RPi that the input voltage isn't exactly 5V. Often, a supply is set slightly high to compensate for voltage losses down thin connecting cables (due to the current passing through a significant resistance). If you need 5V exactly on the external circuits, you need to supply 5V exactly into the RPi (or supply the external circuits separately). You probably don't need to be too picky about it, look at the voltage specification for your external circuits and they may well say something like 5V ± 0.25V.

Be aware that you must not connect the RPi's GPIO data lines to 5V circuits. You need to have something in between to convert the logic levels.
ricardo.santos wrote:
Fri Jun 08, 2018 9:39 am
I'm civil enginner. not very good with eletronics
Electronics engineering isn't that different from any other engineering - as an engineer, you should be familiar with reading specifications and ensuring every single one is adhered to.
Last edited by Z80 Refugee on Fri Jun 08, 2018 10:41 am, edited 1 time in total.
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mahjongg
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Re: GPIO 5v outputs 5.5v

Fri Jun 08, 2018 10:41 am

ricardo.santos wrote:
Fri Jun 08, 2018 9:56 am
I've tested with an official rpi power supply and i get 5.19v from 5v pin. Is this normal?

I should not restrict it to 5v with something before connecting to sensors or micro-controllers
Yes, that is normal.
In fact any voltage within 5% of 5.00Volt is okay, so anything from 4.75V to 5.25V is acceptable.
Note that we are talking about the voltage ending up at the PI, and that cable losses of a tenth of a volt are common (the cable has resistance, so if there is a current running through the cable some voltage loss will occur in the cable).
That is why well designed RPI power supplies are designed to compensate for that loss, also it means that what ends up on the PI can vary slightly depending on how much current is used, so measuring on the PI isn't a static thing, and should be done with an oscilloscope.

Obviously putting too much voltage on any device is bad, but for 5V devices the absolute maximum voltage they can withstand is often something like 6.0 Volt, that is why most PI's contain a kind of Thyristor (Over-voltage-protection OVP) device that shorts out the 5V input when you put 6Volt or more on the 5V micro-USB input, this then should blow the PI's poly fuse (to in turn protect the over-voltage-protection diode from burning up).

hippy
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Re: GPIO 5v outputs 5.5v

Fri Jun 08, 2018 11:44 am

ricardo.santos wrote:
Fri Jun 08, 2018 9:39 am
I've measured the 5v pin against GND pin of my RPI3B with a voltmeter and it's showing 5.45v +-0.05v.
Is this normal? I'm afraid that this may damage my ADC (ADS1115) that is limited to the 5v.
The voltage out on the GPIO should be roughly what you are putting in via through micro-USB connector.

The Foundation doesn't appear to say exactly what the supply voltage should be these days, only that it's 5.1V for a Pi 3B, but moderators have previously said "the voltage needs to be 5.0 +/- 0.25 Volts". Your voltage is above that. It would be recommended to reduce that.

The datasheet shows the ADS1115 is rated for 2V to 5.5V operation, absolute maximum of 7V, so you should be okay with that though you may want to choose a power supply which ensures it stays below 5.5V.

Or alternatively run the ADS1115 off the GPIO 3.3V supply which is probably what you should be doing anyway.

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Z80 Refugee
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Re: GPIO 5v outputs 5.5v

Fri Jun 08, 2018 4:24 pm

hippy wrote:
Fri Jun 08, 2018 11:44 am
Or alternatively run the ADS1115 off the GPIO 3.3V supply which is probably what you should be doing anyway.
Yes, that would solve the logic level compatibility problem as well.
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