Model A Rev 2 Power Issue

vuqnguyen · Sun Oct 20, 2013 8:33 pm

Hi,

I am only measuring about 3.5V across TP1 and TP2 on my RPi model A rev2. The voltage between TP2 and the polyfuse side towards the board edge is 3.75V. The voltage between TP2 and polyfuse side towards board inside is 3.5V. The polyfuse appears to be bad since the difference is > .1. However, why is the overall voltage only 3.5V? Shouldn't it be close to 5V? I tested with 2 different PSUs and mini USB cables which have been tested on a working RPi.

Thanks,
Vu

TrevorG · Sun Oct 20, 2013 9:38 pm

I received a Model A on Friday and it is now waiting to to be posted back to RS. I could not even get 3.5V. There was a dead short across TP1 &TP2.

Maybe there is a QA problem on some Model A's. The Model B that arrived at the same time is fine.

vuqnguyen · Sun Oct 20, 2013 11:05 pm

The problem is getting worse. The voltage across TP1 and TP2 is only 2.5v now. Is my board pretty much dead?

redhawk · Sun Oct 20, 2013 11:41 pm

vuqnguyen wrote:Hi,

I am only measuring about 3.5V across TP1 and TP2 on my RPi model A rev2. The voltage between TP2 and the polyfuse side towards the board edge is 3.75V. The voltage between TP2 and polyfuse side towards board inside is 3.5V. The polyfuse appears to be bad since the difference is > .1. However, why is the overall voltage only 3.5V? Shouldn't it be close to 5V? I tested with 2 different PSUs and mini USB cables which have been tested on a working RPi.

Thanks,
Vu

What's the voltage across the F3 polyfuse at both ends ??

Richard S.

vuqnguyen · Sun Oct 20, 2013 11:59 pm

I am only measuring about 3.5V across TP1 and TP2 on my RPi model A rev2. The voltage between TP2 and the polyfuse side towards the board edge is 3.75V. The voltage between TP2 and polyfuse side towards board inside is 3.5V. The polyfuse appears to be bad since the difference is > .1. However, why is the overall voltage only 3.5V? Shouldn't it be close to 5V? I tested with 2 different PSUs and mini USB cables which have been tested on a working RPi.

Is the highlighted information above what you are looking for? Essentially, the voltage drop across the fuse is only about .25V. It appears that the incoming voltage is in the 3v range so the fuse does not appear to be the issue.

redhawk · Mon Oct 21, 2013 12:10 am

Are you measuring with a digital or analogue voltmeter??

As for F3 the voltage drop looks about right with my Pi I have TP1/TP2 at 5.0v but PSU input voltage is 5.2v (a drop of 0.2v across F3).
Either your measuring equipment is faulty of you have a significant load on your Pi, perhaps power hungry USB devices??

Try powering the Pi with nothing connected to the USB ports and measure TP1/TP2, then repeat the same measurement without the SD Card inserted (effectively preventing the SoC from starting).

Richard S.

pluggy · Mon Oct 21, 2013 12:12 am

Check the battery in the DMM.

vuqnguyen · Mon Oct 21, 2013 12:17 am

Why do you think my multimeter is faulty? I used the same multimeter to measure a working RPi using the same PSU and USB cable and got 4.98V across the TP1&TP2.

After removing everything (SD, USB devices, etc.) and redoing all of the measurements, I am getting about 3V across TP1&TP2 and a voltage drop of about .25V across the fuse. Is it possible that my 5V+ rail is shorted and "defaulting" (not sure if this is valid) to using only 3.3V?

vuqnguyen · Mon Oct 21, 2013 12:20 am

pluggy wrote:Check the battery in the DMM.

Can you elaborate on what this means? What battery?

redhawk · Mon Oct 21, 2013 12:33 am

When powering the faulty Pi are there any components getting warm, i.e. D17, RG2 or the SoC ??

As for DMM batteries low power can cause erroneous readings but this is obviously not the case since you've compared readings with another working Pi.

Richard S.

vuqnguyen · Mon Oct 21, 2013 2:30 am

The DMM is brand new and tested with a working RPi.

The SoC seems to be a bit warm. I have bypassed the polyfuse and now the voltage across TP1&TP2 are consistently around 3.3-3.4V. Is it possible that the 5V rail is not working anymore and the power goes to the 3.3V rail?

vuqnguyen · Mon Oct 21, 2013 4:58 am

After further tinkering without it working, I have removed D17 and replaced the polyfuse with a wire. The voltage across TP1 & TP2 is still around 3.3V. The board gets really hot. I'm pretty sure it's from the SoC. I can't touch it after 5-10 seconds of plugging in the power.

Is there a short somewhere?

Thanks,
Vu

Mon Oct 21, 2013 7:49 am

vuqnguyen wrote:After further tinkering without it working, I have removed D17 and replaced the polyfuse with a wire. The voltage across TP1 & TP2 is still around 3.3V. The board gets really hot. I'm pretty sure it's from the SoC. I can't touch it after 5-10 seconds of plugging in the power.

Is there a short somewhere?

Thanks,
Vu

That sounds like a short. The SOc should rarely get too hot to touch (heavy load for some time might do it)

Model A Rev 2 Power Issue

Model A Rev 2 Power Issue

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Re: Model A Rev 2 Power Issue