Ultimage GPS v3 Resets Raspberry Pi when powered by 5V Source

[davidb11]

I have the GPS pins connected properly to the Raspberry Pi using the 3.3V power source and I have verified the GPS output via CuteCom.
But when the GPS is powered by the Raspberry Pi's 5V source the Raspberry Pi resets immediately.

Why is this?
Thanks in advance!

[adafruit_support_bill]

Probably a sag in the supply voltage. What are you using to power it?

[davidb11]

The Raspberry Pi's 5V pin.
I also have the 'DS1307 Real Time Clock breakout board kit' (I love adafruit!) connected to the Raspberry Pi which is also using the Raspberry Pi's 5V source. Not sure if that matters.

[adafruit_support_bill]

The Raspberry Pi's 5V pin

But what is powering the Raspberry Pi?

[davidb11]

Oh, right..
A micro usb phone charger.

[adafruit_support_bill]

Most phone chargers are good for about 500mA - barely enough current to run the Pi alone. Adding more load will cause the voltage to sag and the processor to reset.

This is a good basic power supply for the Pi: https://www.adafruit.com/products/501

[davidb11]

My charger is listed at 5.1V and 850mA and the reset still occurs when the RTC isn't connected.
I measured the operating voltage of the Raspberry Pi to be 4.94V using the TP1 and TP2 test points.
My Digital Multimeter can only measure up to 200mA so that doesn't help too much for checking the Current.

If my DMM could measure a higher Current (like 1A max), would it be a valid test to use it to check the operating current at TP1 and TP2?
Or are we just concerned with the input Current from the power supply?

I'll purchase the power supply and reply later if that was the remedy or not.
Thanks for the quick replies!

[adafruit_support_bill]

I would expect 850mA to be sufficient. The RTC doesn't draw much, and the GPS will add probably 25-20mA. But the symptoms do sound like a power supply problem.

The actual current draw is not the most important thing to know (power supply ratings are sometimes conservative, but often optimistic). What matters is that it can continue to deliver the voltage continuously under load. Does your 4.94v at the test point hold steady through the reset? Some supplies will go into thermal shutdown (sometimes cycling in and out) when overloaded. Others will just start to sag.

[davidb11]

The operating Voltage actually goes higher to 5V-5.1V when resetting.

[davidb11]

I thought I tried this before, but I guess not. I powered the GPS Vin and Gnd with an external power supply (usb to ttl serial cable connected to my pc) and had the GPS Tx and Rx connected to the pi. This worked well so I think all roads lead to trying that power supply.

[davidb11]

The confusion continues.. I tried the power supply from adafruit (https://www.adafruit.com/products/501) AND I tried a 5V 2A power supply I had laying around..

With the 5V 2A power supply, I was able to successfully power the GPS with Raspberry Pi's 5V output ONCE without resetting the Pi. Each try after would reset the Pi.
Confusing..

[adafruit_support_bill]

The only thing I can think of is that the GPS startup current demand causes a brief dip in the supply voltage and that is enough to reset the Pi. If you have access to an oscilloscope you might be able to see it.