Highlander88
Posts: 20
Joined: Fri Aug 22, 2014 4:20 pm

5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 11:24 am

Hello, I have a Raspberry model B rev.1. the very first batch of raspberrys made, and I have this issue, I've tested TP1 and TP2 on the board and my multimeter reads 4V, on the pins I read 3.3V instead of 5V and 2.5 V instead of 3.3V.
If I change the power supply should I resolve the issue? there is something else that I can try?
The Raspberry 3 power supply will work as a replacement?

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joan
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Location: UK

Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 11:37 am

Does the Pi boot?

If it does your meter needs a fresh battery or you are not measuring correctly.

Highlander88
Posts: 20
Joined: Fri Aug 22, 2014 4:20 pm

Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 12:24 pm

Yes, it boots and works as expected

I've changed the battery of the multimeter and the result is the same.

I've tried testing the outputs of the 5V pins, 3.3V and GND and they gave me the same, lower than expected, result. TP1 and TP2 gave me always around 4V, I don't get what could I be doing wrong if I power on the multimeter, set it on 20V DC (nearest value to 5V) and using the red and black probes to test the voltage... Someone has any ideas?

gordon77
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Joined: Sun Aug 05, 2012 3:12 pm

Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 12:29 pm

I would suspect the meter, leads or probes. Try measuring a new battery or other known good voltage.

Does the meter have a resistance measurement to check out the meter leads ?

If the Pi is working and no low voltage indication it must be about the correct volts.

Highlander88
Posts: 20
Joined: Fri Aug 22, 2014 4:20 pm

Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 12:41 pm

I have checked some batteries and the voltage is correct.
Yes, I have a resistance meter and it shows .006 at 200 Ohms that is the lowest value that I have.

Raspberry is working fine, I'm using it headless with 1 USB device connected without issues.

I have tested the multimeter even with my wall outlets and it works fine.

alphanumeric
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Joined: Tue Jan 19, 2016 2:17 pm
Location: Sydney, Nova Scotia, Canada

Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 12:54 pm

What are you using for a ground reference for the meter?

Highlander88
Posts: 20
Joined: Fri Aug 22, 2014 4:20 pm

Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 12:56 pm

In the case of the raspberry Ground coming from GND pins or directly from the board when using TP1 and TP2

drgeoff
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Joined: Wed Jan 25, 2012 6:39 pm

Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 4:48 pm

gordon77 wrote:
Sat Oct 19, 2019 12:29 pm
If the Pi is working and no low voltage indication it must be about the correct volts.
@gordon77
The OP wrote "I have a Raspberry model B rev.1. the very first batch of raspberrys made". No low voltage detection on those.

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Burngate
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Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 6:21 pm

Interesting.
TP1 and pin 2 of the header are directly connected, as are TP2 and the ground pins on the header.
So reading 4v across TP1/TP2 but 3.3v on the header means you're possibly miss-identifying the header pins.

Booting from only 4v instead of 5v is quite possible - on that early Pi, 3v3 was derived from a LDO linear regulator. If the input dropped that low, 3v3 would drop to out of regulation, but may work well enough for the Pi to boot.

Reading 2.5v somewhere on the header possibly means you're reading some unidentified GPIOs, unless the 3v3 rail really has dropped that low.
In which case, where is the 3.3v reading coming from?

Highlander88
Posts: 20
Joined: Fri Aug 22, 2014 4:20 pm

Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 6:47 pm

The readings were right, I've switched power supply and it works as expected, lower than 5V, at about 4.7V but it works

gordon77
Posts: 4242
Joined: Sun Aug 05, 2012 3:12 pm

Re: 5V pins output 3.3V and 3.3V outputs 2.5V

Sat Oct 19, 2019 7:29 pm

drgeoff wrote:
Sat Oct 19, 2019 4:48 pm
gordon77 wrote:
Sat Oct 19, 2019 12:29 pm
If the Pi is working and no low voltage indication it must be about the correct volts.
@gordon77
The OP wrote "I have a Raspberry model B rev.1. the very first batch of raspberrys made". No low voltage detection on those.
OK. I forgot that.

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