Main Site Documentation

Too high 5V out on USB


#1

I noted that when I run the Panda II on the USB port, the 5V out shows actually something like 5-7 V, when measured with an external voltmeter. Is this normal?


#2

When plugged into USB the voltage is coming directly from USB passing through a diode so it should be around 4.8V.

The problem is either the voltmeter, the computer or not measuring relative to ground. Plug it into a completely different computer and check the voltage again. If it’s still that high then it’s a problem with the voltmeter or some way you are measuring it, otherwise it has to be a problem with the computer USB port.


#3

I think the voltmeter works ok. I have the Panda II plugged into a 9V external power supply and
the 5 V output reads 4.71 V.

This stays constant when I plug into a USB port on a computer.
Then I unplug the external power and here is what I get:

Laptop (off battery): 12.2 V

PC: 7.95 V

I use the GND and the 5 V out pin on the Panda.
Any ideas?


#4

The 9V external runs through a 5V regulator with a 0.2V drop from a diode so 4.71V is in the normal range.

It’s not even logical that it would be that high with USB only because that’s just directly from the USB port. It would be slightly less than 5V if anything because of cable length and resistance but not higher. Try measuring the USB voltage direct either from the cable or just between ground and the resistor next to the USB port labeled R21. Use the USB jack as ground so it’s right at the source.


#5

Ok, I found the solution…the USB power supply is really noisy (see figure, which shows the spectra of the 5V out voltage for USB (blue) and external power (red). Apparently this voltage is not regulated anymore and there is some 1 kHz noise on top of it (see spikes). This fools the voltmeter, which only measures DC, so it measures an apparent higher DC voltage.

I dont recall this much noise on the arduino 5 V out…Is this a problem of the USB or the Panda ?


#6

It shouldn’t have interference that causes the voltage to change like that. Maybe there’s a short somewhere either with the cable or board.

Unplug it and check the resistance to ground of all the USB pins to see if something is shorting or maybe the cable is bad and causing it.

You can look at the schematic and see the key test points.
http://www.ghielectronics.com/downloads/FEZ/Panda_II/FEZ_Panda_II_sch.pdf


#7

I measured on both sides of R21 and D4

R21 on one side ( assume this is the one on the USB connector) has 4.8 V, which is ok
R21 on the other side has noisy voltage, i.e. DC voltmeter shows 10-12 V (with the short USB cable)

According to the schematic D4 should be connected on the “good” side of the USB connector, so I should get 4.8 V on D4 somewhere. What I get is noisy voltage, i.e. 12 V, on both sides of D4.

I assume this means the board is broken somewhere?


#8

If you get the same problem with 2 different computers and different cables then it would have to be something on the board because there really isn’t anything left. I put a scope on mine and it’s completely clean voltage even right next to the D+ and D-.


#9

Would you have any shield connected to your panda ? I remember having similar problem with an arduino ethernet shield v5, where another 3v3 regulator was in // with the 3v3 regulator on the panda. I had unexpected behaviors untill I cut a strip on the shield. This was later corrected on the arduino ethernet shield v6… ::slight_smile:


#10

No I disconnected everything to check.
I am bit confused, because it seems that the noise is on the USB side of things.

After R21, i.e. where the line goes to UBUS/F36*, it is a stable 4.8V. When measuring between ground and the USB Vcc (on a different cable), I also get stable 4.8 V. It almost seems as if the noise is
coming from the connector itself???

In any case, I wonder if this is a problem. I can put a capacitor between GND and +5V out and it stabilizes nicely.