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

  • Thread starter Thread starter benjman
  • Start date Start date
  • Tags Tags
    Usb
Click For Summary

Discussion Overview

The discussion centers around the challenges faced when attempting to power a 2x AAA battery-powered headset using a USB connection. Participants explore the methods used, including the implementation of diodes, and the resulting issues with power delivery and device functionality.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes their method of connecting three diodes in series to step down the voltage from USB to power the headset, but reports failure to power the device.
  • Another participant suggests that USB ports may not provide power without a connected device that responds, and mentions the importance of signaling device connection to the USB port.
  • Concerns are raised about the maximum current draw from USB, with a suggestion to test the voltage output under load conditions to determine if the device is drawing too much current.
  • A participant notes a discrepancy in voltage readings, observing a 4V output with diodes in series and questions whether this voltage is too high for the headset.
  • Another participant questions the configuration of the batteries, suggesting they may be in parallel rather than series, which could affect the power requirements.
  • There is uncertainty about the current requirements of the headset and whether 100mA is sufficient for its operation.

Areas of Agreement / Disagreement

Participants express various hypotheses and concerns, but no consensus is reached regarding the cause of the issues or the correct approach to resolve them. Multiple competing views on the power requirements and configurations remain unresolved.

Contextual Notes

Participants mention potential limitations in their testing methods, such as measuring voltage without a load and the need for proper device enumeration to draw more current from USB.

benjman
Messages
2
Reaction score
0
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
 
Engineering news on Phys.org
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.
 
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?
 
Last edited:
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.
 

Similar threads

Do Movies Teach Us Realistic Life Skills?
  • · Replies 1 ·
Replies
1
Views
10K