VansAirForceForums  
Home > VansAirForceForums

-POSTING RULES
-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.

  #1  
Old 07-08-2021, 01:07 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,539
Default Question for the EE's about dimmers

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
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Reply With Quote
  #2  
Old 07-08-2021, 01:34 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,771
Default

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 regulatorís output. The regulator will keep the voltage constant regardless of the load. Note I assume your LEDs have internal dropping resistors. If they didnít you would have burned them out by now.
Reply With Quote
  #3  
Old 07-08-2021, 03:34 PM
RV6_flyer's Avatar
RV6_flyer RV6_flyer is offline
 
Join Date: Jan 2005
Location: NC25
Posts: 3,747
Default

Quote:
Originally Posted by BobTurner View Post
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 regulatorís output. The regulator will keep the voltage constant regardless of the load. Note I assume your LEDs have internal dropping resistors. If they didnít 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.
__________________
Gary A. Sobek
NC25 RV-6
Flying
3,400+ hours
Where is N157GS
Building RV-8 S/N: 80012

To most people, the sky is the limit.
To those who love aviation, the sky is home.
Reply With Quote
  #4  
Old 07-08-2021, 03:46 PM
rapid_ascent rapid_ascent is offline
 
Join Date: Mar 2008
Location: Dublin, CA
Posts: 1,571
Default

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 Tonks
2020 Donation Paid
Titan IOX-370, Dual PMAGs, 9.6:1 Pistons, FM-150
RV-7 Fuselage in progress
* Cabin Interior - In progress
RV-7 SB Wings
* Both Wings fully skinned
* Fuel Tanks Complete - No leaks finally
* Ailerons Complete
* Flaps Complete
RV-7 Empennage - Complete (a little fiberglass work left)
Vans Training Kit # 2 - Complete
RV-7 Preview Plans
Vans Training Kit #1 - Complete
EAA Sheet Metal Class - Complete
Reply With Quote
  #5  
Old 07-08-2021, 06:41 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,771
Default

Ray, you might note that he his dimmer on both the leds (current devices) and his G3X (voltage sensitive).
Reply With Quote
  #6  
Old 07-08-2021, 08:17 PM
rapid_ascent rapid_ascent is offline
 
Join Date: Mar 2008
Location: Dublin, CA
Posts: 1,571
Default

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.
__________________
Ray Tonks
2020 Donation Paid
Titan IOX-370, Dual PMAGs, 9.6:1 Pistons, FM-150
RV-7 Fuselage in progress
* Cabin Interior - In progress
RV-7 SB Wings
* Both Wings fully skinned
* Fuel Tanks Complete - No leaks finally
* Ailerons Complete
* Flaps Complete
RV-7 Empennage - Complete (a little fiberglass work left)
Vans Training Kit # 2 - Complete
RV-7 Preview Plans
Vans Training Kit #1 - Complete
EAA Sheet Metal Class - Complete
Reply With Quote
  #7  
Old 07-08-2021, 09:56 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,539
Default

Quote:
Originally Posted by BobTurner View Post
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
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 07-08-2021 at 10:01 PM.
Reply With Quote
  #8  
Old 07-08-2021, 09:59 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,539
Default

Quote:
Originally Posted by BobTurner View Post
Ray, you might note that he his dimmer on both the leds (current devices) and his G3X (voltage sensitive).
My leds have internal resistors and respond very nicely to dimming via the voltage divder/pot.
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Reply With Quote
  #9  
Old 07-09-2021, 08:05 PM
Paul from Flyleds's Avatar
Paul from Flyleds Paul from Flyleds is offline
 
Join Date: Apr 2016
Location: Melbourne, Australia
Posts: 218
Default

Quote:
Originally Posted by lr172 View Post
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

__________________
Paul McVitty
Flyleds.com
VAF advertiser
Reply With Quote
  #10  
Old 07-10-2021, 07:45 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,539
Default

Quote:
Originally Posted by Paul from Flyleds View Post
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
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 07-10-2021 at 07:49 AM.
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 12:18 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.