This demonstrates both Spektrum and Futaba 2.4 spread spectrum systems work well in low voltage situations… it also shows Spektrum have the edge in terms of re-connection time after a brown-out or voltage drop
How safe is it to use lipo transmitter batteries in either the DSX9 or DX7 and if it’s not safe why is it not safe?
The JR DSX9 and Spektrum DX7 use the LM2937ES 3.3v Linear Voltage Regulator, basically the more voltage you supply it the hotter it gets. As it turns out the maximum supply voltage for the LM2397ES is 26v. This is quite interesting given we know we are drawing about 300mA either with a supply voltage of 11.6v via NiMh or 12.6v via Lipo, both of which are well with in the stated acceptable limits of the regulator according to this document: LM2937ES 3.3v Voltage Regulator Datasheet
This only leaves heat as the number one suspect for failure. Interestingly enough there is no heat sink anywhere to be seen on the regulator in either the DX7 or DSX9. Both installations of the voltage regulator are also clearly inside a sealed plastic TX case which doesn’t help matters. Throw into the mix an above average ambient air temperature and you have conditions for the voltage regulator which are not so favorable. But… if you use the NiMh battery you’ll be fine according to the powers that be?
Just for the record a fully charged 8s NiMh battery is 11.6v (maybe slightly higher if it’s old) and a 3s Lipo is 12.6v, so there is a difference of 1.6v. So by implication, offically if you change your supply voltage by 1.6v in less than ideal circumstances (described before) you will toast your transmitter. I’m not desputing that some people have been unlucky and toasted their TX voltage regulators by using lipo batteries. The issue I have is that the margin for error in terms of supply voltage is incredibly fine for a hobby which has such an emphasis on safety.
Is it safe to use lipo TX batteries in either the DSX9 or DX7? Clearly it’s not safe, however some people are getting away with it, the regulator will easily take the voltage but the heat generated is the weak link. Whether you get away with it is likely to be on a case by case bases largely dependent on your use of the transmitter and the environment it’s operated in.