Powering RFM69 from Uno, and 3.3V Uno output

Hi all,

I am trying to interface an RFM69HW radio (3.3V device) with a Uno.
The Uno is powered by a 5V supply on its 5V (Vcc) input.
I am trying to power the RFM69 (GND & 3.3V pins) from the Arduino's 3.3V output.

When I put my DMM between GND and the RFM's 3.3V input, the voltage goes up to 3.95V and stays there for several seconds before eventually coming back down to 3.3V; and I suspect I have blown up a couple radios because of this.

I would prefer not to add an additional step-down converter (the final project is already pretty crowded), can't a 3.3V device be safely powered from the Uno's 3.3V output?
I am using the RFM69 at its lowest power setting, so max current should be ok.

The complete circuit is pretty simple, I am using 3 voltage dividers to feed the 3.3V RFM inputs:

supply      UNO                RFM69HW

+5V         5V
GND         GND                GND
            3.3V               3.3V
            MOSI    divider    MOSI
            MISO       -       MISO
            pin 2      -       DIO0
            SCK     divider    SCK
            NSS     divider    NSS


A DMM cannot show reliable DC values with a distorted DC source (spikes, ripple...). In the following a verified undistorted DC voltage is assumed. Add sufficient smoothing caps at the radio module supply lines.

I'd check for a defunct voltage regulator, by measuring the 3.3V output at the Arduino board.
If the regulator maintains the right voltage, a higher voltage at the radio requires substancial current flow on the radio supply wires between the radio and the Uno/voltage regulator - check your circuit for such unwanted current flow; a star topology for all Gnd connections, with the origin at the Uno, should work well.

A minor chance exists that the voltage increase results from some other radio module connection (antenna, signal lines...). The voltage regulator can not reduce an excess voltage fed into its output - check the current flow direction. Remove all connections except the power lines from the radio and measure again.

If the radio contains some oscillator, or powerful RF sources (wall warts...) exist near it, a voltage overshoot can result from rectified oscillations.

Thanks, good to know about DMMs. I am replacing the divider with an HC4050 buffer, I’ll see if that helps.


Everything works right out of the box after replacing the resistor voltage divider with an HC4050... lesson learned, I’ll make sure to always have a few of those around.