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IBL Panel Dimming with LM338T?
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diegov



Joined: 30 Dec 2007
Posts: 47
Location: New Zealand

PostPosted: Mon Oct 13, 2008 4:06 pm    Post subject: IBL Panel Dimming with LM338T? Reply with quote

Hello Fellow Builders,

I am wondering if anyone has been able to succefully dim the IBL panels from their full brightness at 5V to 1.25V or 0V with the supplied 80K pots in the Main Hardware kit.

Unfortunately I am not much of an electronics wiz and am hoping someone can assist me.

My experience thus far has been wiring the panel lights in parallel with the 5V power supply produces very nicely lit panels. As soon as I add the supplied POT as a voltage divider, the panel lights dim slightly, even with no resistance applied by the POT and then dim to full darkness when resistance is applied with only a quarter turn. And when I take a Panel off the circuit the the remaining panel become brighter as if the panels are connected in series, which they are not. Remove the POT from the circuit and all Panels light up brightly with no effect when adding or removing additonal panels

Then I tried a LM388T voltage regulator with the supplied 800K pot and have found the dimming to be a lot linear and smoother, but can only get the panels to light up at 80% of their brightness. Measuring Vout on the LM388T gives me about 3V. Does is use the other 2V for regulation/buffering?

Would be very greatful for any advise.

Again my electronics skills has been limited to what i learnt in highschool Smile

Thanks

Diego
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Jetcos



Joined: 30 Sep 2005
Posts: 1245
Location: Newmarket,Ontario

PostPosted: Mon Oct 13, 2008 6:16 pm    Post subject: Reply with quote

Hi Diego,

The pots supplied are only there to hold the knob and allow you to turn the knob. These pots are not able to handle the resistance and or load.

To control IBL lighting you would need a dimmer that can handle the load of the bulbs (amps). There are dimmers on the market that can handle up to about 6 amps. You would need to measure the load with a Digital Volt Meter (DVM) and see how many amps the bulbs draw. The overhead with all its panels draws almost 28 amps. There are some dimmers on the market that are modules that can handle the amps. This setup would require 4-5 modules that can be controlled by a single pot.

We have been experimenting with different dimmers and I would be interested in hearing how others have been dimming their panels. In most cases there really is no need to dim the panels as they are bright enough in the dark and you are able to see them in daylight.


Steve
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diegov



Joined: 30 Dec 2007
Posts: 47
Location: New Zealand

PostPosted: Tue Oct 14, 2008 3:02 am    Post subject: Reply with quote

Hi Steve,

Thanks for the reply. I too would like to see what other people are doing. I thought i could use the 80K, actually 50K as i look at it now, pot as a voltage divider. Didnt work.

This is why i went down the voltage regulator route to let the IC take care of the voltage and to use the POT as the reference. Its just to dim as i only get about 3V out with no resistance.

You are right about the Panels looking great day or night, but i feel being able to dim them adds that extra level of realism, and can't imagine it would be to difficult to implement it.

Cheers

Diego
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A320East



Joined: 03 Nov 2006
Posts: 88
Location: Halifax, NS, Canada

PostPosted: Tue Oct 14, 2008 8:36 am    Post subject: Reply with quote

Diego,

How are you powering your panels? If you are using ATX type computer power supplies it is possible to modify some models of them to supply a variable output voltage. Some are easier to hack than others - some will only adjust down to about 2 volts out before shutting down due to their own protection circuitry - in some this can be easily bypassed.

If you are using a conventional power supply and wish to use the LM338T to adjust the voltage it is possible to use a "pass transistor" on the output of the regulator to buffer the full amount of current required to run a whole overhead. This method does require some parts and a good heat sink as the current required is pretty high as mentioned by Steve.
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Christopher
"A320East"
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Alexander



Joined: 11 Sep 2004
Posts: 44
Location: France

PostPosted: Sat Oct 18, 2008 7:53 am    Post subject: Backlighting on the IBL panels Reply with quote

Hello guys,

I did not bother to look further in the specifications of the LM338. It requires a serious heatsink (upto 15 insq) if you want to dim the lights from bright (8A) down to where they go 'off' (where these 8 Amps will have to be dissipated into heat...(*,)

For Back lighting I am using super bright LED's. LED's are so much easier to dim by using Pulse Width Modulated dimmers. The idea is that they switch on and of at rate of about 200 times per second, where the amount of 'on' time varies, so the human eye will interpretes it as a different lighting level.
for more info on PWM dimming (cheap solutions as well...) take a look at
http://www.reuk.co.uk/LED-Dimmer-Circuit.htm

Action video: http://www.youtube.com/watch?v=wnuircEL8Oo

or search in google for PWM dimming of LEDS

Good luck ]
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Alexander Laut

The A160 Project
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pcos
Site Admin


Joined: 03 Nov 2003
Posts: 2282

PostPosted: Sat Oct 18, 2008 9:54 am    Post subject: LEDS Reply with quote

Alex,
For the IBL series panels we are using real aircraft Bulbs.
Not sure you are connecting your post with the actual panels/method.
IF you have the older panels and you are adding your own backlighting then LEDs are an option. For this post I think discussing LEDs is somewhat confusing the actual post/question.
Not sure you were connecting our IBL panels into your thoughts.
PC
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Ivar Hestnes



Joined: 28 Aug 2006
Posts: 319
Location: Flemma, Norway

PostPosted: Sat Oct 18, 2008 2:56 pm    Post subject: Reply with quote

What should be a good idea for dimming the IBL-panels is using a DC motor speed controller which works via "pulse width modulation" concept.

I am going to use this. If I can find one that can handle enough amps.

A very big advantage with this is that it does not build up heat when dimming (slowing down the motor).

It works like this: It cuts the power to the motor (IBL) in an electronic controlled cycle, the more you turn the lighting down, the bigger "gaps" when the controller cut the power to the engine (IBL).

Important is the hz frequency on the power cutting cycle, so that the eye wont catch it.

Smile
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diegov



Joined: 30 Dec 2007
Posts: 47
Location: New Zealand

PostPosted: Sat Oct 18, 2008 9:51 pm    Post subject: Reply with quote

Hi Steve, Christopher

Just working on the MIP at the moment. So not drawing 28 amps as yet.

I am using a standard PC ATX power supply, but the first phase for me is to power then dim the Captain side panels.. so this is all the panels up to the landing gear (i think thats as far as the captains dimmer regulates) and then set up the same circuit for the FO side.

Ill discuss a pass through transistor with my friend who is an electronics guru to see if he can help me implement it in the circuit.

@Alexander: So the draw so far is about 1.5 AMPs for captain side contolled panels.. and sure its a little warm but is a huge heat sink really required for this?

@Ivar, your idea seems interesting, but probably out of the realm of a simpleton like me..

If you do get it working, i would be greatful if you can share your discovery.

Many Thanks

Diego
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Ivar Hestnes



Joined: 28 Aug 2006
Posts: 319
Location: Flemma, Norway

PostPosted: Sun Oct 19, 2008 3:05 am    Post subject: Reply with quote

Actually the pulse width modulation idea is not advanced. I provide a link here for an example. Thats a DIY kit. it has a pot where you turn to adjust the light (speed of DC motor).

http://www.electronickits.com/kit/complete/motor/ck1400.htm

But this example can not control enough amps for the IBL. I am going to buy two of these kit to control the map-lights where I am going to use 12V car bulbs.

Guess there are lots of different controllers around that can be useful. Researching necessary I think Smile
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A320East



Joined: 03 Nov 2006
Posts: 88
Location: Halifax, NS, Canada

PostPosted: Sun Oct 19, 2008 2:03 pm    Post subject: Reply with quote

Hey guys - Ivar is right -

Using PWM to dim your IBL panels is totally simple and easily do-able. In fact using a PWM circuit almost identical to a motor control circuit will work for either LED or real a/c bulbs such as in the IBL.

If you are using an ATX supply to get your 5 volts you can use one of many simple 555 timer based circuits and a small handful of parts to make a PWM control circuit and dim your lighting. A Google search for "PWM using 555" will yield plenty of circuits that can do this....

The only difference in the two applications is the different current draw which
is still easily handled by a FET in the output section with a small heatsink. The nice thing about PWM is that the efficiency characteristics are much better so there is much less heat waste and therefore no massive heatsink is required to handle the load of a full set of overhead or pedistal panels.
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Christopher
"A320East"
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diegov



Joined: 30 Dec 2007
Posts: 47
Location: New Zealand

PostPosted: Sun Oct 19, 2008 8:05 pm    Post subject: Reply with quote

Cool. So i kind of get this and reckon i can give it a crack. From what i can see it is duty cycle base. So aren't we essentially turning power on and off quickly? Will this effect the bulbs at all? Will it decrease its life? Or have i missed the mark.

I found these in Aus http://www.oceancontrols.com.au/motor_controller/dc_motor_controllers.html

Im game to try.
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A320East



Joined: 03 Nov 2006
Posts: 88
Location: Halifax, NS, Canada

PostPosted: Mon Oct 20, 2008 6:23 am    Post subject: Reply with quote

Diego,

That controller will do fine. Read the pdf data sheet linked with it, it gives a really good explanation of how it works including waveform pictures. Another cool thing is that they tell you that you can increase it's current capability by beefing up the traces on the PCB - should be about perfect for an IBL set of panels - at worse case you might have to move the output FET to a larger heatsink.

The switching frequency is fast enough that the bulbs don't really care and since they are an inductive load they tend to naturally smooth the edges of the switched waveform anyway.
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Christopher
"A320East"
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Ivar Hestnes



Joined: 28 Aug 2006
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Location: Flemma, Norway

PostPosted: Mon Oct 20, 2008 10:32 am    Post subject: Reply with quote

YEP, that controller from ocean seems to cover our needs Smile .

So if we replace the traces on the PCB with wires, we should be OK I guess. Bit I read that it operates voltages between 8 and 26 Volts. What can possibly happen then if we use it with 5 volts? Will the circuit still work?

Other link here also to a hardcore controller regarding to amps Shocked
http://www.myskunkworks.net/servlet/the-22/DC-MOTOR-POWERFUL-PWM/Detail
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A320East



Joined: 03 Nov 2006
Posts: 88
Location: Halifax, NS, Canada

PostPosted: Mon Oct 20, 2008 4:35 pm    Post subject: Reply with quote

Ivar,

I believe they mean that you need 8-26 volts for the control circuitry - it does say in the documentation that you can use any voltage for the motor using the card with the motor jumper removed. You could easily use the +12 volt portion of the ATX supply to power the control circuit and the +5 volt supply to run the IBL.
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Christopher
"A320East"
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Ivar Hestnes



Joined: 28 Aug 2006
Posts: 319
Location: Flemma, Norway

PostPosted: Mon Oct 20, 2008 4:54 pm    Post subject: Reply with quote

Ahhh, then I get it Smile

Thanks for the input Christopher.

Someone going to buy this and test? I am very curious. It is not my first priority to dim the IBL-panels, so it will take some time before I am ready to put this on my shopping list.

Still need to order and pay my pedestal Wink
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