USB to 3v 100mA. What did I do wrong?

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In summary, the goal of powering a 2x AAA battery powered headset with a USB port was attempted using 3x diodes in series but failed. Different methods were tried, including connecting the diodes and wires in various ways, but the device still did not power on when plugged into a computer. It was suggested to try signaling device connection to the USB port and to be cautious of drawing too much current, as the maximum load for USB is 44 ohms. The use of a multimeter showed a 4V output, but it was unclear if this was due to a parallel battery setup or a switching converter. Further troubleshooting and testing were recommended to determine the current requirements and power system of the headset.
  • #1
GOAL: Power a 2x AAA battery powered headset with a USB port

METHOD: 3x diode in series

RESULT: Failure

Detailed Methods:

Connected three diodes head to foot. Connected USB 2.0 (side with USB to computer) red wire to 1st diode @ the opposite side of the band. Connected an empty red wire to 4th diode (also tried 3 diodes with no change in success). Connected black ground wires. Connected wires to headset (first red to positive battery terminal wire, then switched). None of these allowed the device to power on when USB was plugged into computer. Using a voltameter, I found there was a circuit when USB plugged into computer. For whatever reason, in DC mV I saw a 200 mV signal. Please help, Thanks
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  • #2
USB may not stay powered up if there is no device connected. And, some may not stay connected if the device does not respond. One can signal device connection to USB (as a full speed device) by connecting a 1.5K resistor to 3.3V and connecting it to the DP pin.

Also, drawing too much current can cause shutdown. The max USB load is 44 ohms in parallel with 10uf (1 unit load) which is 100ma at 4.4 volts (the lowest V+ can be). I suggest you play a bit with a constant load and look at what the V+ line does over time. Does it initially produce 5V and then shut down? Or, are you just drawing > 100ma and it is shutting down.

There are some tricks you can play with the USB dead battery provision to get a modern USB port to give you power for a while. But the only way to get reliable power or more that 100ma is to enumerate.

I've built lots of devices that charge or run from USB, but I've always had fully implemented devices that communicate with the host and could request 500ma. I haven't played around with what Host ports do with "illegal" devices.
  • #3
hmm so with the multimeter connected before i plug in the usb, i get a 4 V output. I have 4x 1n4001 diodes in series so i don't know why its only a 1V decrease? anyway, Ill try using this with the headset power held down before plugging in the usb to see if that works. maybe 4V is too much for the 3V headset??

UPDATE: hmm didnt work. I am stuck now. would decreasing the 4V to 3V fix the problem?
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  • #4
No idea.

Are you sure the two batteries are in series? Sometimes they put batteries in parallel and have a switching converter.

The 1N4001 will drop what you expect if there is enough current being drawn. I don't know whether you are reading the 4V with no load (just the multimeter). Have the diodes drive a 47 ohm load and measure across that, with no headphones. That at least let's you determine what the USB will do for well behaved loads.

I have no idea of the current requirements of your headphones or what kind of power system they implement. It could be that 100ma isn't enough.
  • #5

It seems that the issue may lie in the method used to connect the USB to the headset. The use of diodes in series may not be the most effective way to power the headset. Diodes are typically used to regulate voltage and prevent reverse flow of current, but they may not be suitable for providing enough power to the headset. Additionally, the use of only 3 diodes may not be enough to provide the necessary voltage and current for the headset.

I recommend trying a different method, such as using a voltage regulator or a USB power adapter designed for charging devices. These methods will ensure a stable and appropriate power supply for the headset. It may also be helpful to consult the specifications of the headset to determine the required voltage and current for proper functioning.

In addition, it is important to ensure that the connections are made correctly and that there are no loose or faulty wires. Using a voltmeter to test the circuit is a good idea, but it is important to make sure that the readings are accurate and that there are no other factors affecting the results.

Overall, it is important to carefully consider the method and materials used when attempting to power a device with a USB port. It may also be helpful to seek guidance from a more experienced individual or consult the device's manufacturer for assistance.

1. How do I convert USB to 3v 100mA?

To convert USB to 3v 100mA, you will need a voltage regulator, which can step down the 5v output of a USB port to 3v. Make sure to choose a voltage regulator with a maximum output of 100mA, as exceeding this limit can damage your device or USB port.

2. Why is my device not working after using a USB to 3v 100mA converter?

There could be several reasons why your device is not working after using a USB to 3v 100mA converter. The converter may be faulty, the voltage regulator may not be suitable for your device, or there could be a problem with the USB port. Double-check all connections and try using a different USB port or converter to troubleshoot the issue.

3. Can I use any type of USB to 3v 100mA converter?

No, not all USB to 3v 100mA converters are suitable for all devices. Make sure to check the specifications of your device and choose a converter with the appropriate voltage and current output. Using the wrong converter can damage your device or USB port.

4. How do I know if my USB to 3v 100mA converter is working correctly?

You can use a multimeter to test the output voltage and current of your converter. Set the multimeter to measure DC voltage and connect the positive lead to the output pin of the converter and the negative lead to the ground pin. The reading should be close to 3v. You can also test the current output by setting the multimeter to measure DC current and connecting it in series with the device. The reading should be close to 100mA.

5. What should I do if my device requires more than 100mA?

If your device requires more than 100mA, you will need to use a different converter or power supply that can provide the required current. Using a converter with a lower current output can damage your device or cause it to malfunction. Make sure to always check the specifications of your device before using a converter.

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