Review: JR DSX9 and Spektrum DX7 Voltage Regulator
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.
Summary
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.
Also see: http://heli.brixtonjunkies.com/2009/07/02/transmitter-lipo-3s-11-1v-stepdown/
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If you install a heatsink on the regulator, will it still toast the TX or will it be as safe as the stock pack?
It would certainly help, I haven’t measured but you might find there’s not much room for the heatsink fins once the case is closed? Ideally you want the regulator mounted in such a way so the heatsink is on the outside of the case.
Hi alchemist, half year ago I have exchanged in my DX7 this regulator: DC-DC switching voltage regulator 3,3 V (http://www.dimensionengineering.com/DE-SW033.htm) instead original linear regulator LM 2937. It is the best way how to decrease heating problem and “toasting” in DX7 unit. After a few months I discovered that my DX7 has small power output like be push “binding” switch. Of course, after exchanging I have start to use LiPo Tx battery. And resault is the same as with LM regulator….in my mind regulator is not critical point. And two my friend have a same trouble with same configuration of DX7. Could you explain me why ?
P.S. Sorry for my English…I am Czech.
I think what you’re saying is (correct me if I’m wrong), since you have replaced the voltage regulator, the radio power output has decrease, like for example when you hold down the bind/range test button.
As long as the the internal circuitry is getting the correct voltage and the battery can handle the current draw or power requirements in theory all should be well.
It is possible you may have damaged some of the IC’s on the circuit board when soldering due to excessive heat from the soldering iron? This is the main reason I opted for the two diode approach http://heli.brixtonjunkies.com/2009/07/02/transmitter-lipo-3s-11-1v-stepdown/
@alchemist
No,the radio power output decrease a few month after replacement of regulator. After replacement regulator I have used original NiMH battery pack and ewerythinks was OK. Later I have used LiPol TX pack OUTRANGE and after time (a few weeks) I discovered range problem. It is not soldering fault.
I must say that does seem unusual, I’ve been using 2 diodes in series with no problems at all.