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05-07-2020, 12:06 AM
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Join Date: Dec 2013
Location: Moorpark, CA
Posts: 157
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Dimming GAD 27 PWM output
I powered up my panel after mounting it I the fuselage for the first time today. Everything works as advertised, but I can?t get the dimmer circuit in my switches to go dim enough (AML-24 switches with internal lights). I?ve played with the lighting curves in the G3X, but the lights turn off before they get dim enough.
I?m using the PWM function of the GAD27. 13v going into the lights, the GAD 27 pulses the ground side to dim.
Will a resistor in series with the 13v source allow them to dim further? I don?t know enough about LEDs to know how they?ll behave if I knock the supply voltage down while still using PWM dimming.
Chris
__________________
1976 C-185
1947 J3C-65
RV-8 project
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05-07-2020, 08:20 AM
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Join Date: Nov 2008
Location: Kansas City, MO
Posts: 120
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No, LED's dim by lowering the frequency, which is what Pulse Width Modulation (PWM) does. I had the same problem with AML series switches. The best thing I think was to switch to an incandescent bulb. The light is a bit yellow compared to the LED, but you can dim it down to nothing.
https://www.digikey.com/product-deta...1228-ND/414124
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05-07-2020, 01:31 PM
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Join Date: Dec 2013
Location: Moorpark, CA
Posts: 157
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Thanks.
Will incandescent bulbs work in series like LEDs, or will I have to rewire them in parallel? I?m probably not going to make the effort if I need to rewire the switch harness given how rarely I fly at night.
__________________
1976 C-185
1947 J3C-65
RV-8 project
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05-07-2020, 01:32 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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Almost. With PWM the frequency is constant but asymmetric, e.g., you vary the fraction of ?on? time vs ?off?. E.g., on for 9 msec, off for 1, would be near max. On for 1 ms and off for 9 ms would be dim. Either way the period is 10 msec. In principle PWM should be okay for LEDs (the frequency is high enough your eyes don?t see the flicker, they average). Sounds like your particular module doesn?t go dim enough. These interior lights draw little power, so a conventional rheostat or adjustable dc supply (with a series resistor) works, too.
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05-07-2020, 01:35 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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Quote:
Originally Posted by Cannon
Thanks.
Will incandescent bulbs work in series like LEDs, or will I have to rewire them in parallel? I?m probably not going to make the effort if I need to rewire the switch harness given how rarely I fly at night.
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I don?t think you want LEDs in series here. That?s why the dimmer isn?t working as it should.
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05-07-2020, 01:51 PM
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Join Date: Sep 2010
Location: Olathe, KS
Posts: 2,885
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Quote:
Originally Posted by Cannon
I powered up my panel after mounting it I the fuselage for the first time today. Everything works as advertised, but I can?t get the dimmer circuit in my switches to go dim enough (AML-24 switches with internal lights). I?ve played with the lighting curves in the G3X, but the lights turn off before they get dim enough.
I?m using the PWM function of the GAD27. 13v going into the lights, the GAD 27 pulses the ground side to dim.
Will a resistor in series with the 13v source allow them to dim further? I don?t know enough about LEDs to know how they?ll behave if I knock the supply voltage down while still using PWM dimming.
Chris
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Chris,
The simple answer is yes, a resistor would help dim. Do you have any more information on the light you are using, that may help us provide further insight.
Thanks,
Justin
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Garmin G3X Support
g3xpert@garmin.com
1-866-854-8433 - 7 to 7 Central Time M to F
Please email us for support instead of using Private Messaging due to the limitations of the latter.
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05-07-2020, 02:29 PM
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Join Date: Dec 2013
Location: Moorpark, CA
Posts: 157
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Thanks, Justin.
The switches originally came from Stein, so I assume these are the bulbs.
__________________
1976 C-185
1947 J3C-65
RV-8 project
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05-07-2020, 04:19 PM
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Join Date: Dec 2013
Location: Moorpark, CA
Posts: 157
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I was wrong.
I just looked at some pictures of my install. They’re installed in parallel (all positives daisy-chained, then all neg daisy-chained). I need to double check in person.
Does that change anything?
__________________
1976 C-185
1947 J3C-65
RV-8 project
Last edited by Cannon : 05-07-2020 at 04:24 PM.
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05-08-2020, 07:23 AM
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Join Date: Jun 2005
Location: Georgetown, TX
Posts: 331
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Chris,
You aren't alone!
The light output from those LED bulbs seems to "fall off" rather precipitously when dimming; video is easier to see the effect, but in essence it goes from OFF to 20% of max bright then scales to 100% fairly smoothly.
Unfortunately, 20% is "Crazy Bright" and IMHO not usable.
A resistor between the +13V rail and the LEDs might help this effect; for experimentation, try using a 10K pot instead - dial it down to something you like, measure the resistance and then use that value for a resistor. Keep in mind that the PWM inputs are limited to 500mA.
Cheers!
B
__________________
Brian Decker
Retired Firmware Guy and Airplane Builder
2020 RV-14 QB -- Under construction - Tailcone & Empennage Complete.
2018 RV-7 QB -- Built, Flying
2007 RV-7 QB -- Built, Drowned, Resurrected and flying
1998 RV-8 QB -- Started, Sold
1986 Mooney 205SE
1980 Mooney 231
Aided and abetted building a number of other RV's.
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05-08-2020, 08:52 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by bjdecker
Chris,
You aren't alone!
The light output from those LED bulbs seems to "fall off" rather precipitously when dimming; video is easier to see the effect, but in essence it goes from OFF to 20% of max bright then scales to 100% fairly smoothly.
B
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This has been my experience with LEDs as well. It is critical to get an LED with a low enough brightness level that 25% dim will be low enough to meet your requirements. Incandescents are different and dim very low, regardless of their full brightness.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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