UK Heli Log

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/

The convenience of a lipo battery in your transmitter speaks for itself. The problem is many modern transmitters aren’t designed to take the higher voltage that lipos supply. Most transmitters are designed for use with NiCad or NiMh batterys which are 1.45v per cell fully charged, where as lipo batteries are 4.2v per cell fully charged. Stock NiMh TX batteries are 8 cell which equates to 11.6v fully charged by comparison aftermarker TX Lipo batteries are 3 cell which is 12.6v fully charged, that’s a 1.6v difference. Also lipos have a tendency to sit at relatively high voltage for longer unlike NiMh batteries which drop from peak voltage very quickly.

Currently there are plenty of people are using 3 cell TX lipos in there DX7’s and DSX9’s and have had no problems and maybe they never will? I’m guilty of doing this with my DX7 for months now with no problems to date. It’s no secret that the internal circuits of most transmitters is around 3v, which means the TX is using a voltage regulator to stepdown the voltage. Hence the higher the input voltage (from the battery) the more stress you are putting on the TX voltage regulator. So maybe your TX wont go pop immediately but the lifespan of the voltage regulator is probably shortened, either way it doesn’t fill you with confidence.

No one wants to open up their TX’s and mess about with unsoldering and re-soldering PCB components and in the process invalidate the warranty and/or destroy a perfectly functional transmitter. So how about stepping down the lipo battery voltage before it enters the TX…sounds good to me.

The Modification

Diodes have a property call a forward voltage drop, in normal use it’s a side effect rather than something that is specifically capitalized on, however that’s exactly what I’m going to do. By using two 1N5404 rectifier diodes in series (see photo above) on the positive side of the TX battery cable you will effectively have approximately (2 x 0.75v) = 1.5v stepdown. This will bring your lipo down from 12.6 to 11.1 which is a nice healthy voltage for your transmitter and close to what it would expect from a NiMh battery. The 1N5404 is a rectifier diode and designed for power supply/rectification application and is rated for 3A hence it should be fine in this application as the TX is only drawing approximately 300mA (this varies from TX to TX, normally it’s stated in the manual).

One thing to note is the diode stepdown can not be part of your charging circuit otherwise your lipo charger will complain about the voltage or in the worst case you might have a “little” fire on your hands The battery I used was a Outrage XPTX 11.1v 2500mAh (see photo) which has a TX plug, a standard rc connector and a balancer plug (Thunder Power) all pre-wired which turns out to be very handy.

Below you can see the redundant rc connector which is used to charge the lipo hence isolating the stepdown.

I urge anyone who is considering attempting this to do there own research in order to better understand what they are getting into. Here are a couple of links that may help:

http://www.kpsec.freeuk.com/components/diode.htm

http://www.fairchildsemi.com/ds/1N/1N5406.pdf

Summary

Don’t know yet too early to tell…seems fine so far