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Question for the EE's about dimmers

lr172

Well Known Member
I setup a 5W potentiometer to cover dimming of my LED annunciators (12V LEDs that dim nicely) and my G3X (non-touch that doesn't have light sensors). I set it up as a voltage divider and it works well dimming everything. However, when I turn on the boost pump or pitot heat, the G3X dims quite a bit more (lower voltage=dimmer screen). I am thinking that the extra load from those LEDs (around 15 mA) is somehow dropping the voltage a bit and the G3X the dims further due to that. My curve goes from around 3 volts to around 10 volts in a linear fashion.

Should a Pot set up as a voltage divider work like this or is it possible I messed something up with the relays that are used on the boost pump and pitot heat circuits to activate the LEDs? The ground is shared by both the relay coil and the and the termination of the LED circuit - variable positive voltage is sent to the LED via the dimmer circuit.

I would like to avoid adding another dimmer just for the G3X, as I don't have the space for it near the other knobs.

Thanks for any insight or guidance in resolving this.

Larry
 
Yes, this is how a simple voltage divider reacts to an increasing load. Solution: Buy an LM317 adjustable voltage regulator (cost, about 50 cents). Mount it wherever convenient behind the panel. Use the pot to control the regulator, and attach all the things to be dimmed to the regulator’s output. The regulator will keep the voltage constant regardless of the load. Note I assume your LEDs have internal dropping resistors. If they didn’t you would have burned them out by now.
 
Yes, this is how a simple voltage divider reacts to an increasing load. Solution: Buy an LM317 adjustable voltage regulator (cost, about 50 cents). Mount it wherever convenient behind the panel. Use the pot to control the regulator, and attach all the things to be dimmed to the regulator’s output. The regulator will keep the voltage constant regardless of the load. Note I assume your LEDs have internal dropping resistors. If they didn’t you would have burned them out by now.

I have been using an LM317 on a home made dimmer in my airplane since first flight in 1997. Works great on old incandescent lamps.
 
Larry,

I'm not sure exactly how you have your circuit wired but let me provide some more info for you.

LEDs are really current driven devices. That is why a current limiting resistor is required to prevent them from burning out. If you use a pot to control the brightness then you need to think about it in terms of current not voltage. When you use the pot as a voltage divider you have some effective series resistance from the pot. As Bob suggested you probably also have series current limit resistors for the LEDs. Normally if LEDs are "12V LEDs" that infers that they have built in series resistors and the resistor has been selected to limit the LED current to the proper range for a 12V supply. When you adjust your voltage divider you are actually adjusting the series resistance that the LED sees.

It really is better to use the pot as an adjustable resistor in series with the other series resistor. So pick the range of operating currents that you want. Figure out the maximum current by using the current that the LED is spec'd at with the built in series resistor. With the pot set to 0 ohms that will be your max current. Then figure out the max resistance (minimum current) to determine the pot value.

At 15mA as a load current for these LEDs that circuit should not be effecting anything else unless you have it hooked up improperly. Now if the voltage into your dimmer circuit changes then yes the brightness will change because of Ohms Law >> Voltage = Current x Resistance or Current = Voltage / Resistance.
 
Ray, you might note that he his dimmer on both the leds (current devices) and his G3X (voltage sensitive).
 
Bob,

Oh that is what I wasn't getting. BTW, I wasn't saying that the regulator suggestion was a bad idea. I was just trying to provide some background on how these things work.
 
Yes, this is how a simple voltage divider reacts to an increasing load. Solution: Buy an LM317 adjustable voltage regulator (cost, about 50 cents). Mount it wherever convenient behind the panel. Use the pot to control the regulator, and attach all the things to be dimmed to the regulator’s output. The regulator will keep the voltage constant regardless of the load. Note I assume your LEDs have internal dropping resistors. If they didn’t you would have burned them out by now.

Thanks! I have a coiple 317’s in the spare parts box. Also, today i widened the voltage range on the G3X dimmer configuration and that really minimized the effect.

Larry
 
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when I turn on the boost pump or pitot heat, the G3X dims quite a bit more (lower voltage=dimmer screen).
Hi Larry
When you're observing the screen dimming are you on the ground using battery or an external power supply? If so it might just be the heavy load is dropping your system volts there.

Also curious to know what resistance value your 5W pot is.

Regards,
Paul

 
Hi Larry
When you're observing the screen dimming are you on the ground using battery or an external power supply? If so it might just be the heavy load is dropping your system volts there.

Also curious to know what resistance value your 5W pot is.

Regards,
Paul


I initially made the error of testing on the ground and realized it. Unfortunately, I also saw the dimming at cruise. I tested the other day with a charger and it appears that the wider voltage range in the G3X dimming config seemed to minimize the effect but need another test flight. if it doesn't help enough, I'll make a LM317 based regulator.

The pot is 0-5K rated at 5W

I may also just move to manual dimming, as I am still occassionally getting an oscillation of dim/bright on the G3X. It only happens occassionally but is very annoying. Garmin hasn't been able to help and I cannot come up with a logical reason for it based upon the elec arrangement. I have tried to go into config mode to observe the voltage, but ever time I go into config mode it stops oscillating. Oddly, after entering and leaving config mode, it won't oscillate any more; at least during that flight. Very weird issue.

Larry
 
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Your pot will do the job as expected for the G3X, as the G3X itself has a high impedance and just looks at the voltage presented to it by the pot. You're doing it the way Garmin manual recommends.

A 5k pot between battery and ground will have a constant ~2.4mA of current flowing through it by itself. However when a LED on the annunciator panel is turned on the extra current drawn by the LED effectively changes the value of the voltage divider (ie "moves the knob"), and so the G3X is responding to that change.

You could use a dual gang pot instead, one for the LEDs and one for the G3X, or a simple bright/dim switch for the LEDs.

 
Ok, seems I can't find a double gang pot over 2W in stock, so I'll go the 317 route. However, I have a .1 uF cap for the input side, but no 1-10uF cap for the output. I have a 47uF, but it is only rated for 6.5V. I have some caps that are 1000uF, but assuming that is too much. Can I get away with that? I can't seem to find a local store and wanted to avoid the 3 day shipping delay.

I am assuming that I don't want to skip the output side cap. I could skip the input side and use the .1 on the output (it is ceramic), but guessing that may not be enough.

Larry
 
You're only dimming a couple of LEDs. No need for a large pot.
If you do go the LM317 route, use the 0.1uF on the input. The data sheet recommends this one but it doesn't need an output cap. You do have a ~240 ohm resistor on hand as well?


 
You're only dimming a couple of LEDs. No need for a large pot.
If you do go the LM317 route, use the 0.1uF on the input. The data sheet recommends this one but it doesn't need an output cap. You do have a ~240 ohm resistor on hand as well?



I have 10 LEDs at 20mA max. So when driving all of them at full brightness (probably not realistic), I am at around 2.7 watts. Largest dual gang in stock at digikey was 2W.

I have a 240 resistor. I will try it without the output cap. I am guessing any ripple won't really bother the LEDs. Do you think the g3X might be sensitive to ripple on the dimming input? Or is the ripple pretty minor with the 317?

Larry
 
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