Turnigy 9X "Lightning the Display"

Originally posted by yulo99 Hi! I have just did backlight mod as described by "ex-trainer". But there is sth. strange: switching light on causes radio reset! is it normal?  (I took +5VCC and GND from the mainboard) and the second issue: is it ok that inverter generates some sound when activated?

sometimes the 5V regulator can't handle the backlight circuit and the tx reboots. and that is not normal. some people had the same problem. try to put a resistor in series with the backlight 5V line. try  25 to 40 ohm resistor. hope this helps..

well, if someone will just hook wires with On/Off switch to 5V rail inside Tx, and there is any capacitor in backlight unit, the voltage at the Tx board is simply shorted to discharged capacitor, at the very first moment. While capacitor is discharged, it is simply a short circuit - reboot is natural..
Resistor should help, if backlight circuit can work with it. Anyway, internal 5V regulator is not designed with additional load in mind, and result is not predictable without looking at it (both cooling and max current).

You've dug up a very old thread... but what "mod" are you talking about exactly? This particular thread has been about using an EL panel. That doesn't require a FET at all.

In general, for backlight there are two main options:

1) EL panel. This requires either an inverter (not recommended - too much audible and FM noise) like the original author used, or an inductor based EL driver IC, which is much more complicated but vastly superior. The only IC based one I know of is the SmartieParts board (which I sell - full disclosure). With custom firmware (the SmartieParts board is also a programmer for custom firmware), the backlight can be assigned to any switch and therefore controlled by the radio's CPU.

2) LED backlight that HobbyKing sells here. Its cheap, bright, and effective... but it can't be controlled by the CPU/custom firmware. At least not without modification.

So I figure by your question, you're asking about that particular modification... which I don't believe has anything to do with this thread. But if that is what you're asking about... sure, you can do it with an NPN. Why not? But which one? You have to carefully select it, while almost any (mos)FET will do just by the nature of what they are.

To answer your "how much current" question... we have to know what backlight you're referring to. And if its the LED, then how much current depends on how much voltage you're giving it and it varies as the battery goes from full charge to not. That's not really the proper way to drive an LED (they're supposed to be constant current devices) but it works.