Very slow capacitor discharge for USB power

[caspian]

Dear forum members,

Let us consider the circuit of the picture which provides a 5volt power to a USB2 port in a laptop. the power line is called +USB_VCCA. U40 provides this power when the En pin is low. There are two capacitors C974 and C975 that hold +USB_VCCA.
I see a problem in this circuit: when power is disconnected from the laptop, +5VALW reaches 0v immediately, but +USB_VCCA stays high. it takes about 5 minutes for +USB_VCCA to reach gradually from 5v to 3v to 2v to 0v. no USB device was connected when I tested it.
Do you think that C974 and C975 are defective that hold the +USB_VCCA power for such a long time?
when I turn the laptop off, U40 turns off, but C974 and C975 apply a voltage to U40. Do you think this may damage U40?

thanks

[R_J]

Re: Very slow capacitor discharge for USB power

The capacitors are fine. with nothing connected to the USB port there is nothing to discharge the capacitors, this is normal.

[caspian]

Re: Very slow capacitor discharge for USB power

In page39 of the schematics file, +5VALW is generated. +5VALW is connected to a number of ICs. +5VALW seems stable when I measured it.
schematics:
https://www.badcaps.net/forum/attach...7&d=1490606457

When the laptop is off, resistance to GND on +5VALW is about 3KOhms. When I turn the laptop on and then turn it off, resistance to GND on +5VALW is measured 64 Ohms. then after 3 seconds, it reaches 3KOhms.
That means: When the laptop is on, some IC reduces the resistance of the +5VALW line down to 64 Ohms. When the laptop is off, the mentioned IC gets disabled and cannot move the resistance of +5VALW down.

I also measured the +3VALW line. It has such a resistance drop too.

there is something else that is more strange: I also measured resistance on the +USB_VCCA line. It has such a resistance drop too. but this line hold the drop for a long duration.
Since I turned the laptop off, I started measuring resistance on +USB_VCCA. the resistance goes up 1 ohm per second. that means it takes about (3000-64) seconds to reach from 64ohms to 3kohms. whereas resistance of +5VALW takes 3 seconds to reach from 64ohms to 3kohms.

I removed U40 from the mainboard. After that, resistance of +5VALW is the same. but resistance of +USB_VCCA is unlimited continuously.

Now, I do not know what part of the mainboard is defective that the resistance values go up like this.

[caspian]

Re: Very slow capacitor discharge for USB power

are these resistance values normal?

[budm]

Re: Very slow capacitor discharge for USB power

There is nothing wrong with the caps.
1) Get yourself a capacitor, may be around 220uf 6V, 10V, or what you can find.
2) Discharge the cap by touching the two legs of the capacitor together.
3) Set your meter to Ohm mode the connect red probe to + leg of the cap, black probe to the - leg of the cap, keep your eye on the meter when you connect black probe to cap to observe what you are seeing for 3 ~ 4 minutes.
4) Connect capacitor to 3V (two 1.5V batteries in series) Voltage source for 5 seconds and then remove the battery, then set your meter to DC V mode and measure the DC V between the two legs of that charged cap, observe what is going on for 3 ~ 4 minutes.
5) Tell us what you think is going on with what you see on the meter, also look up to learn what capacitor is.

[caspian]

Re: Very slow capacitor discharge for USB power

thanks for your reply. Sometimes, I get confused about details of electronics components.
what do you think of the resistance of the +5VALW line that becomes lower than expected when the laptop is on?
Is that normal?
I thought the resistance of the +5VALW line should always stay in the range of a few kilo-Ohms.

[budm]

Re: Very slow capacitor discharge for USB power

You cannot measure resistance when the circuit is powered.
the power supply provide Voltage to run many components, some of the components may be switching on and off, so it is not going to be steady state.

[caspian]

Re: Very slow capacitor discharge for USB power

So it seems the circuit is OK.