USB Power vs. External Power Supply

Well, here is a strange issue that I can’t seem to figure out:

I have a bluetooth module attached to the EXT header pins. I am using the 3.3V power on the header to power the bluetooth device. I have until now always had power coming from the USB device port. Bluetooth module works great.

Today I removed the USB and attached a 7v supply to the external power jack for the first time. Now my Bluetooth module will not power on. I can measure 3.3v at the header so I know there is power to it, but it simply will not work. Bad module you say? Well, I reattached the USB power and it works fine.

One theory is that there is not enough current to satisfy the bluetooth module with the external power jack. How could that be? My supply is a programmable power supply set to 7v and has plenty of current. The bluetooth module only needs 100ma @ 3.3v to initialize.

Could I have a bad LM1117MP-5? That is only difference I can see in the path power takes between USB vs. External Power.

The board itself appears to function fine regardless of power supply.

Any other theories? Its got to be a simple (humbling) explaination.


Can you probe the 5v pin (pin 3 on JP4) on the Domino to verify the 5v regulator. Please disconnect the bluetooth module and any other external circuit prior to this.

You can attempt this to check if the regulator is faulty or the presence of a short etc.

Hmm … thinking again if you are seeing the 3.3v the 5v regulator must be functioning proper ???

If that is the case then voltage would drop. You are seeing 3.3V then this is not the case


Here is what I am assuming, the bluetooth module is always working fine but when you use power pack you are not receiving any data from COM. I am also assuming you are using receive events from COM, right? Now the receive events always work if you are debugging over USB but if you are not debugging then receive events will ONLY work if you install the event handler before (or after) you open the port. I can’t remember which one is the correct one.

So all you have to do, is move the one line of code that install the event handler and all will work just fine :wink:

If this fixed the problem then you owe me lunch…just tell me where and I will meet with you :smiley:

The way I am determining that the bluetooth is not working is that I can’t see the radio signal using PC software…I think the issue is separate from code. I remember having to play with the placement of the event handler. That I actually figured out on my own. It’s after the port open.

Let me break out the oscilloscope and see if I can trap a voltage drop. I can’t see one with a meter and perhaps even a very short duration will cause this module to not initialize. I guess I could also test this expensive power supply :confused:

Don’t worry about lunch this time…you’ll earn it soon enough!


I would look at how clean the voltage coming out of your 7V supply is. I picked through my old box of wall-warts and the first 6V one I found was so noisy that it caused a noticeable flicker in the LCD shield (no need for the scope to check the ripple on that one.) The second wall-wart I dug out works great.

Hi Jeff

Andrew states that the PSU is a programmable bench power supply. I would expect these units to work within spec.

–but one never knows :slight_smile:

I still can’t explain it, but I need to be approx 7.5V or higher at the FEZ main power input for the bluetooth module to want to initialize. The bluetooth module is 3.3V of course and is designed to use a max of 100ma.

I could not see any noise to speak of from my PSU nor did I notice a voltage drop.

When connecting directly to 5V pin on FEZ I have no problem.

Strange. Does not really affect me as I plan to use the 5V pin anyway, but its a curious thing to say the least.


The bluetooth module maybe causing power surges when it works and so you need a good size capacitor really close to the power on the bluetooth module, 100uF or more.

I concur with Gus, I had the same issue using external power with a servo.

But wouldn’t you put the capacitor on the battery end? That way when power is not on the capacitor is charged, then when the surge occurs little voltage drop is experienced. The capacitor then charges up again when things stabilize.



Can you point me at the bluetooth module you are using ?

I am using Sena’s OEM modules. I have had great luck with all of them. Its one of the few that support DTR/DSR and RTS/CTS.


Hi Andrew

I looked at the Sena modules, 100ma is more than enough.

Did u try with another PSU ?