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Old Nov 17, 2011, 04:24 PM
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Austria
Joined Jan 2010
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Turnigy 9X - transistor choice for backlight control

Greetings all!

I hope this is the right place for me to ask this question, but if not please let me know, I'm new here.

Short version:
I used a KN2222A instead of a BS170 for controlling an LCD backlight. It works but I'm nervous. Could this cause problems later on?

Long version:
I have a Turnigy 9X v2 radio running the th9x firmware with a HobbyKing LCD backlight kit installed. I wanted to make use of the backlight control feature in the th9x firmware and so followed the tutorial linked below. I did not cut the tracks as shown in the tutorial as I do not use this radio with a simulator. In digging around for parts I found I did not have a BS170 MOSFET as recommended but I did have a salvaged KN2222A, which while not a MOSFET like the BS170 did seem to be very similarly rated. This worked and I was happy with the result, but I'm an electronics noob and don't know enough to know if this could come back to haunt me.

What I'm looking for is someone who can tell me whether or not I've done this the right way, and most importantly for me, why. Any opinions will be most welcome, I'm here to learn!

Links:
Tutorial: http://ok-rimr.com/en/how-to-do-it/t...ceni-hk-skiner
BS170 Datasheet: http://www.fairchildsemi.com/ds/BS/BS170.pdf
KN2222A Datasheet: http://www.datasheetcatalog.org/data...c/KN2222_A.pdf
Radio: http://www.hobbyking.com/hobbyking/s...idproduct=8991
Backlight: http://hobbyking.com/hobbyking/store...dProduct=16720
Firmware: http://code.google.com/p/th9x/
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Old Nov 17, 2011, 10:06 PM
"Simplify, then add lightness"
Raleigh,NC
Joined Nov 2000
2,701 Posts
Would be fine as long as you have a series resistor in between the output of the ATMEGA and the base of the transistor to limit the current. Something around 1k would work. Without the resistor you would draw excessive from the ATMEGA and may eventually burn that pin out.
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Old Nov 18, 2011, 03:16 AM
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Austria
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Thanks jeffs! The transistor's base is currently connected directly to the ATMEGA. I will track down a suitable resistor to put between the two before my next flight. It's nice to know I wasn't just being paranoid here.
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Old Apr 10, 2012, 07:02 PM
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United States, HI, Kailua
Joined Jun 2010
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Hi, I'm having trouble finding a NTE490 or a BS170 transistor in Hawaii. Could someone recommend others I could use?

would this one possibly work. I saw it as a BS170 replacement somewhere else.

http://www.radioshack.com/product/in...ductId=2062618
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Old May 28, 2012, 02:36 PM
Newbie,3D Wannabe :)
United States, NJ, Somerset
Joined Feb 2011
93 Posts
Will this Unit work instead of BS107 or BS170?Its sold at radio shack under IRF510 thanx for any help
Model IRF510
Product Type Transistors
Enclosure Color Black/Silver
Recommended Use Low voltage, high speed switching applications
Built-in Devices Low on-state resistance and high switch speeds
Fits What
Model IRF510
Miscellaneous Features
Supported Languages English
Mounting Kit Case Type: TO-220
#per pack 1
Power Features
Power Device V(DS): 60 V(GS): +/-20 I(D): 3.0 Amps Power Dissipation: 20W
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Old May 28, 2012, 04:36 PM
"Simplify, then add lightness"
Raleigh,NC
Joined Nov 2000
2,701 Posts
No, the turn on voltage for the IRF510 is too high to be driven reliably by an ATMEGA. The original poster used a 2N2222A which is a bipolar transistor and is available at Radio Shack.
http://www.radioshack.com/product/in...ductId=2062608
With the bipolar transistor you would need at least a 1K resistor in between the base of the transistor and the output pin on the ATMEGA.
http://www.radioshack.com/product/in...ductId=2062343

PS with the bipolar transistor, you would connect the Collector where the Drain would go, the Emitter where the Source would go, and the Base(through a 1k resistor) where the Gate would go.
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Last edited by jeffs555; May 28, 2012 at 04:46 PM.
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Old Jun 02, 2012, 01:26 PM
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Joined Jun 2012
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Radio Shack have this

Does Radio Shack still carry the parts for this? Also what do you use to keep the wires in place, a click bond or a fastener of some sort?
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Old Jun 03, 2012, 06:29 PM
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The Ultra simplistic version:
When selecting a substitute device. you want to match the voltage, the power dissapation and the turn on voltage. Most of the Fets or transistors you use for a backlight switching application are going to be rather low power, source voltage under 25 Volts and turn on voltage of around 5 volts or less. there's a whole range of semi's you can use that will be triggered by the ATMega. The package sizes should be either TO-92 (the small plastic ones) or the TO-220's (slightly larger with the metal mounting tab).
With Fets you get 2 states; On and off. with Transistors, you get more of an analog result with off, mostly off, kinda on, mostly on, etc. For Switching on and off of a backlight, a FET would be my choice for design. You can get the BS170's all over the place on line.
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Old Jun 04, 2012, 02:01 PM
"Simplify, then add lightness"
Raleigh,NC
Joined Nov 2000
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Quote:
With Fets you get 2 states; On and off. with Transistors, you get more of an analog result with off, mostly off, kinda on, mostly on, etc.
You were doing great until you got to this statement. Both FETs and bipolar transistors are analog devices and varying the input will vary the output. The main difference is that FETs are controlled by voltage, and bipolar transistors are controlled by current. Since the output of the microcontroller is a voltage, it can drive an FET directly. To use a bipolar transistor in the same application you need a resistor in the base to limit the current(ie convert the voltage into an appropriate current).

In this application there would be no difference in the operation of the backlight, with the only difference being the need for a base resistor with the bipolar transistor. The advantage of the bipolar is that you can just go to Radio Shack and get the parts for a couple dollars. You can however order the BS170 off Ebay for just a few dollars including shipping, so not much difference one way or the other.
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Old Aug 01, 2013, 10:43 PM
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INNERKIP,ONTARIO,CANADA
Joined Feb 2006
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Turnigy 9X - transistor choice for backlight control

There must be lots of electronics shops or Ebay that will have a BS170 and ship it any place in the world. I got 2 in a pack from SAYAL Electronics here in Canada, and under $3.
HK instructions show the backlight in with wires on the other side, in the way of the 4 way direction switches.
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Old Jan 15, 2014, 11:33 PM
A little C.A. will fix it!
United States, TN, Pigeon Forge
Joined Dec 2013
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If you house a 222 transistor do as explained above and it works perfectly.
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