USB power booster with output cutoff

A single USB 2.0 port has max current limit of 0.5A. Thus, when multiple devices (some devices are power-hungry, such as a headphone amplifier) are connected via a USB hub, it can easily hit the max current limit. Fortunately, some USB hubs take external 5V input, which can make up the limited current of the source USB port.

I was one of those who were using a lot of devices, and USB hubs were essential to me (currently, more than 20 are connected to my PC via three USB hubs). One day, I touched my USB headphone amplifier by chance while the PC was powered off, and I was a bit surprised because the amplifier was so warm. That was because the device kept draining power from the external 5V input of the USB hub, even when the host PC was off. It didn't seem right to me, so when I directly connected the amplifier to my PC's USB port, not through the USB hub, no such a thing happened again.

Okay, so this is my analysis. The following table shows how the voltage sources of a conventional USB hub are routed to the output USB ports.

		Ext. 5V
		Connected	Disconnected
USB 5V	Connected	Ext. 5V	USB 5V
	Disconnected	Ext. 5V	0V

As you can see, the external 5V input is routed to the output USB ports all the time, even when the source USB is disconnected (or the host PC is off).

However, voltage routing of a USB hub that I thought ideal was like this:

		Ext. 5V
		Connected	Disconnected
USB 5V	Connected	Ext. 5V	USB 5V
	Disconnected	0V	0V

I spent some time trying to find out USB hubs that work as the above table. I purchased and tested at least 10 USB hubs of different brands, but unfortunately, none of them worked in the way as in the above table.

Well, now it's clear what I should do. Because there is none on the market, I should make it for myself.

Description

Output USB power is cut off when the master USB is unplugged.

- Master DC input: USB Type A

- Slave DC input: 5V DC adatper

- Output: USB Type B

Parts:

Si2323 x 1

STP80PF55 x 1

DC adapter jack

47k x 2

20k x 1

100k x 1

0.1u x 1

USB cable

Packaging

Rebuilding laptop batteries: Lenovo T410

1. Remove depleted batteries from the battery pack.

2. New cells: Samsung ICR18650-32A x 6

3. Desolder the original controller circuit from the original battery pack.

4. Weld anodes of two cells. Be cautious of the polarity.

5. Connect the other ends (not welded yet) of two cells with a resistor and let the cells sit until the voltage gets balanced.

6. After the cells are fully balanced, weld the other ends. Aggregate the welded cells and form a pack. Attach and solder the controller circuit.

7. Repack in the original plastic housing.

9V dummy battery

Never worry about depletion and replacement.

3-pin to 4-pin adapter for DC cooling fans - PWM speed control

About 10+ years ago, mainboard manufacturers made a gradual transition in their cooling fan control method; previously, for the adaptive fan speed control, they had adjusted the DC supply voltage. These days, PWM-based speed control is more common. The biggest change incurred by this transition is the number of pins; from 3 pins to 4 pins.

In the transition period, some makers, such as ASUS, supported both of the speed control methods and allowed users to select the control mode in the bios setting. On the contrary, some only supported 4-pin fans. Unfortunately, even after the transition, some high-end cooling fan makers still provided only 3-pin fans for a while. They would have thought that it would be OK, because 3-pin fans are pin compatible with 4-pin fans. True... at least for the users who were willing to give up the adaptive fan speed control feature.

The following is the 3-pin to 4-pin conversion circuit that I designed 10 years ago for my computer: