LiPo RX Regulator causing Interference

[Peter Hawtin]

Guys I am going to be a real meany as I have discused this with a manufacturer and they want to make this.  You may say this is a cop out but they like the design and are happy to pay for it.

 Peter

[Tim Mackey]

So you have designed a device that a manufacturer has taken on board and will be making and selling commercially - I thought you said it was a circuit that was used in the automotive industry to reduce 12V down to 6V.

I am even more puzzled.

[Peter Hawtin]

Hi Gents

Yes tyhe circuit was designed many years ago to allow 12v car phones and radios to be used on 24v vehicles. I slightly modified the circuit to work with our voltage.

In the mean time I have been playing with various forms of low cost but reliable voltage droping circuits that can be easily manufacture by mr average for use with 2S Lipos on the RX.

 I have spoken to a few scale guys who build very large scale models and they both looked at me as if I had gone mad when I asked what regulator they used.  They simply use 3 x , 6A, 50V; Voltage, Vrrm:  P600A  standard diodes in series, and these drop the voltage down by 0.7v per diode, so 2.1v in all.

 I have made a couple of these and run them in paralel into two unused channels on my RX. The Diode stops and back feed if one cell short circuits and although they are rated at 6a, they can easly handle 20a if a servo stalls. Total cost £1.50 including heat shrink.

 I have tested a single unit running 6 servos (4 x digital 9kg  + 2 x analog 3kg) with each servo loaded with a weight half of their maximum rating.

The supply was a 2S 2100 RX pack from Flight Power into a Futaba 9 channel RX connected to the TX using a DSC lead rather than the Radio. 

The TX was set using servo test function to continually move ALL channels and was left to run for just over 2 hours.  The result was a 1 - 1,5 amp drain on the battery pack and when the analog servos were deliberatly stalled the load rasised to just over 2 amps.

 During the test the diodes did get hot, and using and infrared thermometre they peaked at 50deg C.

The cell was fully charged at the start of the test and fully recharged after the test and over the two hours of the test the cells used just about 1700Ma from the packs and the voltage of the 2s cell at the end of the test was 6.9 (4.8v at the RX) under load 7.1 (5v at the rx) off load.

I carried out the same using two parralel diode sets and two separate batteries and the temperature of the diode packs was just under 30deg C.

 This does not give you a constant voltage to the RX, but even with the LIPO under load for over two hours the cells still coped with the test rig.

I used a 5 cell 2500 AA NiMh flight pack on the same test rig, and the voltage dropped to less than 4v after only one hour.

 The servo test rig is something like I use in my 50cc models, but it NEVER has full loads all the time and two hours would give me about ten flights in a day.

The most interesting part of the test was the way the NiHi cells die quickly when loaded at the sort of loads we often use. Seems to me the LiPo is by far the best way to supply power to large models as they can give substatial power for most of their cappacity...the LiPo cells I was using were capable of supplying over 40 amps and there is no way the NiMh cells could do that.

From these results it seems that a far smaller LiPo could be used as long as you limit the flights to 3-4 per day without charging.

 Thoughts ?

Peter

[Former Member]

[This posting has been removed]

[Peter Hawtin]

I am now using this simple voltage dropper on my 68" cap and after 5 flights of 10 minuets it only drained 850 ma from the cells.

This model has 5 digital servos (14kg) on the rudder and although I do fly smoothly, a fair percentage was full on tumbles, Knife edge spins and servo intensive carzyness.

I discovered that SM services also use this method, so cant be that far off the mark.

Peter