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-   -   Dimming GAD 27 PWM output (https://vansairforce.net/community/showthread.php?t=182173)

Cannon 05-07-2020 12:06 AM

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

N54SG 05-07-2020 08:20 AM

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

Cannon 05-07-2020 01:31 PM

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.

BobTurner 05-07-2020 01:32 PM

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.

BobTurner 05-07-2020 01:35 PM

Quote:

Originally Posted by Cannon (Post 1428334)
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.

I don?t think you want LEDs in series here. That?s why the dimmer isn?t working as it should.

g3xpert 05-07-2020 01:51 PM

Quote:

Originally Posted by Cannon (Post 1428172)
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

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

Cannon 05-07-2020 02:29 PM

Thanks, Justin.

The switches originally came from Stein, so I assume these are the bulbs.

Cannon 05-07-2020 04:19 PM

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?

bjdecker 05-08-2020 07:23 AM

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

lr172 05-08-2020 08:52 AM

Quote:

Originally Posted by bjdecker (Post 1428509)
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

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


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