It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

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jph
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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#11 Post by jph »

Hi Jacques,
Have you actually measured the current of a single LED using your meter ? - I presume you are powering the LED via the original wiring connector(s) on the panel ?
Can I presume you are using 12V from the PC PSU rail, and also that one darlington pair drives a single led block ? - I believe you said there were 2 leds per illuminated legend block ?. what is the current per darlington driver pin on the ULN ? when just that pin is on, and also when all pins are on ?

Also, are all the leds the same colour ? - presuming white ? .. and the colour of the legend comes from the colour of the cap under the etching or a filter of some kind ?.

Also, please try powering both illuminated parts of just one single switch at the same time ? the heat in the IC's tend to indicate a high current situation, but the dimming seems to be more of a voltage limitation when fully illuminated. Only the measurements will tell really.
Do you have an adjustable bench PSU - that would be a fantastic tool if it is voltage and current adjustable using it to power a single switch and adjust voltage and current limit could give a stack of info. For example, you could power a single switch with no current limit and increase the voltage until it is correctly illuminated. Then adjust the current limit until it starts to dim. If you do this on both illuminated parts of a single switch - one at a time and then both - then you would see if the readings are sensible and logical and also the correct voltage and current required which in theory should be a single voltage and current for one legend and the same voltage and double the current for both legends at the same time.
Joe. CISSP, MSc.

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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#12 Post by jph »

@JackZ
Jacques, just a quick question. Where is the 'COM' pin wired to on the 2803s ? - also, can you confirm that the power definitely goes TO the leds first, and THEN to the 2803 ?. so the 2803s are grounding the leds . The 2803 is a current SINK only, it cannot supply current, it can only sink it. Sorry if it sounds like I am 'teaching my granny to suck eggs' - certainly not the case. the 2803 has caused people many sleepless nights.
Just a thought.
Anything strange there would cause a multitude of issues. For now, the 'com' pin can be left disconnected in this application
Joe. CISSP, MSc.

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Sling
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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#13 Post by Sling »

If they have a part number on them you can look them up. Could be Korry, Eaton, Vivisun, Staco or perhaps someone else but the information should be out there. The new ones are all led but depending on the age of this they could be filament lamps. The current should be consistent per indicator as the colour comes from the lens. I think the ones I have are 24mA x 4 for each indicator but those are not led.

Tony

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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#14 Post by jph »

I think a lot depends on the way the panel is wired and where the inputs and outputs are taken from. It would be interesting to see the switch details though as to if they have a current limiting resistor internally, There must be one somewhere - but it could be external. Also if the leds are wired in series or parallel and what voltage the current limiting to the leds was designed to be driven from etc
- a bit of 'thinking out loud' .. (pondering....)
The 2803 getting hot is a bit worrying, well, depending how hot, as virtually the only heat dissipation should really be from the darlington output transistor(s) collector / emitter junction Voltage drop = approximately 0.6V multiplied by the current which should be absolutely naff all really. At 65mA - (for example 2 high brightness SMD 30mA leds in parallel) - would equal a maximum dissipation per channel of the ULN2803 of a mere 40mW only. All 8 channels on would still only be 320mW (0.3W). - or double if each pin drives 2 leds @ 65mA each - which would be an unusual value and incredibly bright. Around 80mW and 640mW per device. That can still feel rather hot if running all channels constantly, but perfectly normal. Just looking at the datasheet it is possible to calculate - more or less - the thermal performance as the datasheet quotes a rise in Temp over ambient of 55 deg C per W. so if we were actually running 640mW per device then the temp of the device would be 55 x 0.64 = 35 degrees C + ambient temperature so that can feel pretty warm to hot to say the least. If we were running 65mA per channel then the temp would be 55 x 0.32 = 17 C above ambient which is just warm to the touch.
Datasheet - https://pdf1.alldatasheet.com/datasheet ... 2803A.html - (notation - RθJA = 55°C/W under MAXIMUM RATINGS)

As a wattage comparison, a normal bog standard LED that we would plug into a breadboard would have a forward V drop of around 2.5V and a current of 20mA hence have a power dissipation of 50mW when on - which is more than a single pin of the 2803 at 65mA which as said above is only 40mW.

The 2803 can, in theory handle 500mA per channel and the transistors are rated up to 50v safely, giving a theoretical max power dissipation of 0.5A x 0.6V = 300mW per channel or 2.4W per device. Now that would be damn damn bloody bugger hot. ;-)
Strange one indeed.
Joe
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JackZ
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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#15 Post by JackZ »

I use only the 5V ramp from the PSU. And yes, I am using the original connectors of the panel. The +5V goes directly to the connector, then the others are going back to the ULN.
So I have no control over a single led, more to a legend, via each pins.
Just noticed that a few of the pins of the connector are internally wired to thicker wires. That could be the problem?
I noticed that having the COM of the ULN2803A set to GND illuminates all the corresponding digits.
If I go like this, the dimming of all the digits is less noticeable, even though it’s still present.

@Sling These are Eaton switches.
6B87E8E5-B726-43E9-9CCC-1E97A9AAD6F9.jpeg
Last edited by JackZ on Sun Aug 29, 2021 9:19 pm, edited 1 time in total.
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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#16 Post by JackZ »

Hmm, I think I found the problem, thanks to @jph
I was using only One wire for all the 5V pins (5 different Vsup pins, guess for redundancy). When I split the load from the 5V PSU by using three wires the white ON switch lights up much more than it was previously!
Both ULN2803A are still very hot to the touch, probably around 60C not to the point I burn myself, but hot enough to have to remove quickly the finger :D
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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#17 Post by jph »

Excellent stuff @JackZ
Glad you got it sorted.

re the 'com' connection, yes, if you put it to ground it simply bypasses all the output transistors of the ULN2803 and grounds the LEDs / Loads via the flyback diodes and can actually be useful for that purpose in some cases. The 'com' connection is very badly named in my opinion but I suppose it is 'common' to all the flyback diodes. They are really only there for use with inductive loads such as relays etc and act in the same way as wiring a reverse connected diode across the relay coil but they give you this internally which is rather neat. In non inductive use it can be left o/c, used as an output transistor bypass test by grounding it or more usually connected to the power supply line that feeds the input to the loads - in this case your 5V supply. With any inductive load on any pin it should always be connected to the main power feed.
Temps on the ICs seem fine at what you have and in line with the figures you gave and also relative to the datasheet.
Just for info for anyone interested - A ULN circuit is usually designed so that under absolute maximum load the temperature of the IC doesn't go above around 90 / 100 C max. If the theoretical max loads that each pin is capable of (500mA) were applied all at once on each pin then the chip would be pushing up daisies as the junctions would fail at 150C (tJ max). This would mean that under a worst case scenario the max total design current passing through the combined pins should be around 2 amps as opposed to 4. This would give around 1.2W hence a temp rise of 1.2 x 55 = 66C. with an ambient of 25 we should then be running at 91C. Actual power dissipation of 1W total - maximum - would be an even better and more usual target though. to give around 80C at 25 ambient so If, for example, all 8 pins are used simultaneously with the same load values on each pin then the max would be around 200mA per pin for a 'safe' design

One thing I would consider - that you may already be doing - is to wire the switch loads to opposite ULNs in certain circumstances. ie - the top half of one switch display to one ULN and the bottom half of the same switch display to the other ULN depending on the switch display usage frequency. what I am thinking is that if, for example, the switch has a 'normal' display in the bottom half that is commonly used, and a far less frequent top half display then it would be good, in the case of 8 switches, to wire the frequently used part of the switches to be split across the 2 ULNs so 4 to each and the infrequent usage display parts to be wired to the other 4 pins on each, That way, if all 'normal' displays are on then the actual load is split across 2 ULNs so only half the current is passing through each ULN giving much lower temps and spreading the load - just a thought.

Nice work though. You certainly cannot beat the real thing. That panel is excellent. Can you post another vid of the panel in all it's glory now Jacques ? it would be good to see.
Joe
Joe. CISSP, MSc.

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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#18 Post by jph »

@JackZ
Jacques,
There is a very interesting document here about the Eaton switch module. (it would appear to be of the same type as your units)
It would seem that your units appear to be fitted with coloured incandescent lamps as you have a white unit.
On the datasheet here - presuming the same bulb / led options are for your switch type as well - then the LEDS are coloured but with no blue or white option. where as the incandescent 5V units are available in all colours including blue and ​white.
Also, the incandescent lamp is shown as having a 5V 60mA option (type F8) - which is looking correct for your readings.
Very useful as well is the font and font size options that are used. Interesting reading.
The font information is great for AM panels.

https://www.google.com/url?sa=i&url=htt ... AdAAAAABAD

Joe
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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#19 Post by JackZ »

Thnaks Joe for the information.

After some test, I figured that the main culprit was in fact the ULN2803A. Apparently, the circuit is not able to withstand more than 3 or 4 pins draining (read closed) simultaneously with these kind of bulbs that seem to draw quite a lot of current.
If I use the 8 output pins, the brightness of the legends goes down when all are used.
If on the other hand I split the 15 led pins from the connector onto 4 or 5 ULN2803 with only 3 to 4 pins used for each, when all the bulbs are illuminated, the brightness lowers a bit but the whole thing is still bright.

So my guess is to use at least 5 ULN for this panel with 15 annunciator legends (3 pins used out 8), and as you suggested make sure to split the load over diffrent ULN for legends that are likely to appear simultaneously. Each ULN is less than 1€, so I can just increase their number per panel.
1793B67E-65C6-43ED-8FF5-E460B9B33FE7.jpeg
7A164587-E650-46BF-8A77-9AE8D5F7585D.jpeg
Thanks all for your help and advices.

Jacques
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Re: It’s alive! A real A320 Brake panel interfaced with Air Manager and FFA320

#20 Post by jph »

Jacques,
Would you be kind enough to measure the following for my own information and interest. I am concerned as to why the problems are occurring with the 2803 units.
Best way to get an accurate reference -
Can you power the lamp for one legend from your 5V supply and then connect the other side of the lamp to PSU GND via your multimeter on current range ?. (nothing else at all in circuit) - no 2803 etc. Can you let me know the result ?. If possible, also try various other switch legend lamp units to see if the current used is more or less identical.
Also, if possible, if you have the tool to remove the cap and a lamp can you tell me the markings on the lamp ?
Thanks, Joe
Joe. CISSP, MSc.

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