Why Didn?t I Think Of That?!

[Dave Cole]

Paul,

Your post was perfectly timed with the only remaining panel work on my -7; wiring the annunciator lights. I have seen reference in one of your previous posts to your use of the MPJ panel mount LED indicators for annunciators. What resistor value are you using for the "dim" light seting of the MPJ indicators?

[Ironflight]

Originally Posted by Dave Cole

Paul,

Your post was perfectly timed with the only remaining panel work on my -7; wiring the annunciator lights. I have seen reference in one of your previous posts to your use of the MPJ panel mount LED indicators for annunciators. What resistor value are you using for the "dim" light seting of the MPJ indicators?

Through trial and error, we settled on about 30KOhms.

[jeffk]

Hi Paul, thanks for the detailed posting and accompanying schematic. Which software do you use to draw your schematics?

[n82rb]

If you are using a EFIS to provide a ground, just make sure the EFIS can sink the current that the led will draw. most led circuits should be designed to run about 25 ma. (check the data sheet on the LED) if the EFIS cannot sink that much current ( i hope they are designed better than that, but who knows?) you could fry some expensive parts.

bob burns

[Ironflight]

Originally Posted by n82rb

If you are using a EFIS to provide a ground, just make sure the EFIS can sink the current that the led will draw. most led circuits should be designed to run about 25 ma. (check the data sheet on the LED) if the EFIS cannot sink that much current ( i hope they are designed better than that, but who knows?) you could fry some expensive parts.

bob burns

Good advice Bob - something folks would need to check. In my case, I had the idea for doing it this way came from one of the design team members for my EFIS.....it BETTER work!

Oh, and I am using ExpressSCH for a drawing program these days.

Paul

[MattL]

Paul,
I'm a long time lurker on this forum, but since I am an EE, I might be able to help.

I'd be surprised if you got satisfactory result with your circuit. I would expect the brightness of the indicators to vary significantly when dimmed as indicators turn on and off.

Assumptions:
1) Indicators have a built in current limiting resistor sized for full-brightness operation at 12 Volts (1KOhm is reasonable for a red LED at 12V)
2) LED forward voltage drop (Vf) is constant (it actually varies with current)
3) All indicators are the same color (Vf varies by color)
4) Vf=1.2V

For one indicator lit:

Itotal = V / R = (12 - 1.2)/(1K + 30K) = 10.8/31000 = 348uA

For two indicators lit:

Itotal = V / R = (12 - 1.2)/(500 + 30K) = 10.8/30500 = 354uA

But only half that current flows through each LED, so:

Iled = Itotal/2 = 177uA

For N indicators lit:

Iled = (12-1.2)/(N*(1000/N + 30000)) =

N Iled (uA)
1 348
2 177
3 119
4 89

It won't be quite this bad because Vf is exponentially proportional with current.

The solution is to use a small voltage regulator in place of the 30K dimming resistor. The venerable LM317A would work well for the ground-switched indicators: http://www.national.com/ds/LM/LM117.pdf

The line-switched indicators would need a negative voltage regulator: http://www.national.com/ds/LM/LM137.pdf

I'd keep the switch, that way you could bypass the regulators if there is a fault. At these current levels, I doubt there would be a problem with power dissipation in the regulator(s), but if you want adjustability all the way up to max brightness, it would be a concern.

N.B.: I'm a digital designer, a real analog engineer could probably come up with a better solution.

[Ironflight]

Originally Posted by MattL

I'd be surprised if you got satisfactory result with your circuit. I would expect the brightness of the indicators to vary significantly when dimmed as indicators turn on and off.

I certainly rspect your design expertise, and I've run similar numbers to what you did in your post, but the truth is, I have run a very similar panel for years in my RV-8 (but with a couple of the LEDs driven by relays to invert them) and it has worked just fine. There is no doubt that doing it with some active components and regulation would make it BETTER, this is true, and I agree with you. But experience shows that this does actually work if you are willing to accept some variation in brightness, and want to keep it simple.

Paul

[Dave Cole]

Originally Posted by MattL

...I'd be surprised if you got satisfactory result with your circuit. I would expect the brightness of the indicators to vary significantly when dimmed as indicators turn on and off.

I recognized that different brightness levels will result with additional indicators turned on or off, but I expect that to be an infrequent event. In general, annunciators will be on one at a time.

[MattL]

Originally Posted by Ironflight

I certainly rspect your design expertise, and I've run similar numbers to what you did in your post, but the truth is, I have run a very similar panel for years in my RV-8 (but with a couple of the LEDs driven by relays to invert them) and it has worked just fine. There is no doubt that doing it with some active components and regulation would make it BETTER, this is true, and I agree with you. But experience shows that this does actually work if you are willing to accept some variation in brightness, and want to keep it simple.

Paul

Clearly I've succumbed to the curse of the engineer: "But if I just do this, I can make it better!"

In addition to relying on others to tell us when we can make something simpler, sometimes we need others to tell us when it's good enough.

Let's not get started on what it would take if you wanted consistent brightness across different colors of indicators

[tonyjohnson]

Paul,

Thank you for the excellent idea. I am wiring up my warning lights according to your system.

I notice that your diagram does not show fuses on the lines that carry power through the switch and run to ground. The plane power wiring diagram shows a 1 amp fuse in the warning light circuit. I also previously wired an inline fuse in the circuit from the starter contactor to the warning light.

The other circuits will be boost pump, canopy, landing light and auto pilot. Would you put a fuse in those lines?

Thanks again for the great idea.

Tony