Interesting Spektrum DX3r Experiment

[BrianG]

A little bored and no projects I can work on (or should I say, have no money to work on), I started thinking of the "ideal" lipo solution for the DX3r. But before I can do anything, I need to get some data first. So, I hooked up my home-made adjustable power supply to the DX3r's battery terminals and fired her up.

I found that the radio will power up at 2.6v, but does not transmit. Current draw during this time is ~70mA. Once the voltage gets to 3.1v, current jumps to 127mA and starts transmitting. What I thought was interesting is that the current stays at 127mA all the way up to 6.5v, at which time I did not want to go any higher.

What does this mean? Well, I suspect any voltage above 3.1v does not increase transmitter range. You may say "But as the voltage goes up, the power goes up even though current stays the same, so the range has to be better with higher voltage". The first part is correct, but not so sure about the range assumption. Why? Because I highly doubt there is circuitry in there that is designed to draw X current no matter the voltage. Just doesn't make sense.

There isn't a switching regulator because as voltage goes up, current would go down.

The only logical explanation (to me) is that there is a linear voltage regulator inside set to around 3v. So no matter how high the voltage gets, the extra is being burned off as heat, but since current is only 127mA, that heat is minimal. So, I really think anything above 4 AA cells is just a waste. Kinda makes my 5-cell mod a little pointless. Unfortunately, testing the range is a bit cumbersome since I'd have to have someone hold a vehicle at various distances to test. Maybe someday.

Another interesting thing I found is that the volt meter on the radio isn't accurate. The display read a constant 3.3v when I applied 2.6v up to 3.4v. After that, the display reads 0.2v lower than reality. Hmm, I wonder if that's the v drop of a germanium diode?

Anyway, just thought I'd share my findings...