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Dimming GAD 27 PWM output

Cannon

Well Known Member
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
 
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.
 
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.
 
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.
 
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
 
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?
 
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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
 
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
 
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-detail/en/jkl-components-corp/73/289-1228-ND/414124

Another thought about going with incandescent. Those bulbs are 80ma each. 12 at full bright is almost 1 amp. Would limiting the GAD 27 output to 50% keep it safe, or does that not work because the PWM circuit uses system voltage and would potentially generate 1A pulses?

Chris
 
12 of those little CM73(??) bulbs would draw (or sink) 960mA by my math -- which would exceed the 500mA max of the GAD 27.

In the FWIW category, I originally used incandescent bulbs, but they proved to be susceptible to vibration induced failures. Plus they were yellow-ish -- or as my wife likes to call it "A Warm Light" (yuck).

I am going to try the dropping resistor this weekend and see how well it works.

Cheers Y'all

B
 
My installation used incandescent bulbs (warm light) in the switches that were much too bright. I put a 10k ohm potentiometer in the circuit (set for about 1k IIRC) to get the range closer. I subsequently switched to LED bulbs (much cooler/whiter light) and could tweak the pot to get a good range. Fine tuning is of course easily done on the GAD27 with the lighting curves, but I was seeking a range where at the bottom end (last 10%) the lights would extinguish.
 
I settled on a 3k ohm resistor in series with the switch lights (LEDs), and the light levels are *much* better. They get way dimmer, though max brightness is lower as well. No biggie, as they could get much brighter than I?d ever use without the resistor.

I played with a 10k pot to figure out what resistance worked best. Even at 600 ohms the brightness was roughly cut in half (subjectively, anyway).

Chris
 
I left the 10k potentiometer in the circuit, but mounted it on the subpanel rather than the panel and with no knob - accessible on the ground for adjusting but no danger of accidentally adjusting.
 
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