Electrical question.....(??)

[Mach7]

Hello,

I am building a BAe146 simulator and am in the process of connecting up the backlights for the overhead panels. These panels are aircraft specific and operate at a max DC voltage of 5 volts.

The backlit lamps are of the incandescent type.

Although the panels are wired up for sim operation, I have a separate supply to power the back lighting in the panels...

I have an Omron 5 volt power supply (rated at 10 amps) which outputs to a positive and a negative terminal block giving me 10 outputs to feed the panels. Each panel has two leads running to these  terminal block(s) for connection.

I have a single push on/off switch located on the instructors panel to illuminate the overhead lights.

Here is my predicament....

The more panels I connect, the more the voltage across the terminal block drops....for example. If only one of the panels is connected, I read approx 4.83 volts, with two panels connected it drops to 4.66, and then 3.53 with all 4 connected. Also I am reading this voltage at the terminal block, not at the panel.

Should the power supply not maintain close to 5 volts at least to the terminal block? giving me something less at the end of the wire?

I have attached some pictures. Only the power supply is connected to the overhead panels, the MCP and engine instruments come off another source.

[KyleH]

If the terminal block only has 2 wires running to it from the power supply it will not maintain 5V at the terminal block. As you draw more current through those 2 wires, they act like resistors and will develop a voltage drop across them, which the control loop in the power supply will not detect. It will regulate to 5V wherever the sense point is. For a power supply to regulate properly like that, it needs to be a high quality one with voltage sense wires on the output side as well. Look at a PC power supply, on the connectors you'll see 2 thick wires and 2 thin wires to the same pins. The 2 thin wires are the voltage sense, while the 2 thicker wires deliver the actual power. This lets the power supply regulate the voltage directly at the connector and not suffer losses through the power wire.

The more likely scenario in your case is that your using a 50W power supply to light incandescent panels. You are possibly drawing more than 10A at some point, which the power supply cannot supply any more current and so starts to drop the output voltage.

[mickc]

Exactly. 

You will find that the OEM panels will draw quite a bit of current.  I have built Arduino based PWM dimmers, and most smaller 737-sized panels draw over 1amp each, and they are much smaller than your panels.

[Mach7]

ok...I am starting to understand what is going on here.

So  is the solution to my problem? Should I look at converting to LED bulbs, or adding additional power supplies, say one power supply for every 3 panel(s).

If the current is getting towards the 10 amp limit, I now have another concern with respect to my wiring....I am currently using 22 gauge wires to each panel, so an increase in amperage is not a viable solution at this point.

Thanks again for the information ....keep it coming!

[RayS]

Not only that, but if you don't have the metal backers attached to a metal frame, heat will quickly become an issue without something to pull that heat away.

I think 22 gauge is going to be too thin, even for a single panel, depending on the bulb count. Best to err on the side of caution here because if insulation melts somewhere and wires start shorting out, then you've got a REAL problem.

   

[Mach7]

Hello Guys,

Thanks for all the input,

@RayS, metal backers are not the problem...these panels came directly off the real aircraft, so I believe they can withstand just about anything.

In looking at this problem further, the solution for me is to look at changing everything over to LED's....however for now  I will place everything on hold until I figure out a conversion method.

I am going to contact Leo Bodnar as well....he might have an option that he can develop for me.

[fordgt40]

James

I am with Ray here, 22 gauge is too small. Try the simple things first:-

Only connect one panel to the power supply, measure the current draw and voltage. Replace the wire feed with  say 14 gauge wire and repeat the measurements. You may be surprised.

David

[Mach7]

@fordgt40,

Thanks for that information, I will give that a try.

Regardless, the thicker wire will transfer more power, but i will still be limited in the power supply because each bulb draws .15 amps, once I get over 60 bulbs (which is approx 4 panels, i hit that wall.

Has anyone successfully modified real aircraft boards to accept LED's? at least I would get away from the high amperage of incandescent bulbs and the heat they create.

[jackpilot]

I use a PC power supply (1200W) I got on EBay for my OVH
The main feed to the OVH is 10G wire which goes to a FDS   distrib board
http://www.flightdecksolutions.com/components/hardware/fds-ibl-dist-dim/

Each panel ( With several 5v bulbs) is fed from the male white connectors (femaleboard connector +20G wire supplied by FDS).
The board is offered with or without dimmer and can be daisy chained.

Works fine and much easier than gutting the bulbs for leds.

[Mach7]

Just to confirm my math is correct here....

it appears that these lamp boards are wired in parellel as the failure of one bulb does not effect the rest.

Having said this, if each incandescent bulb has a draw of .15 amps,then am I correct in assuming that this amperage is cumulative with respect to the power supply only but not the wires going to the individual boards?...let me clarify my statement....

Right now I have the hydraulic panel, electrical panel, apu panel, and engines panel connected to the terminal block that is supplying the 5 volt power source.

Each of these panels have separate wire leads (22 gauge) for the positive and negative connections to the terminal block, meaning all of these panels are separate from one another and can therefore be looked at from an individual amperage draw point of view, (with respect to the wires going to the panel....not the power supply)....for example;

The hydraulic panel has 10 incandescent bulbs therefore the amp draw through these wires from the terminal block would equate to 1,5 amps,

The electrical panel has 8 bulbs so the draw here would be 1.2 amps

The APU panel has 3 bulbs so the draw through the apu wires would be .45 amps

etc etc....

My view is that the 22 gauge wire should be satisfactory to deliver the power to each board as it is well below the recommended max amp limit for that type of wire, and essentially is only carrying the load for that associated panel.

However...... the wires going from the power supply to the terminal block should be of a lower gauge to handle the higher amperage rating as it will be cumulative from all the connected boards.

With the above in mind, I believe that if my wires were 14 gauge going from the power supply to the terminal block, this would allow me to keep my 22 gauge wires to carry a higher voltage to the respective panels and still remain below the maximum amp rating for those wires.

As I noted in my previous post, when all 4 panels are connected, the power supply voltage begins to drop below 5 volts at source, as reflected in he terminal block due to (IMO) the cumulative nature of all the amperage draw from all 4 boards..

If I am correct in this above statement...then my solution would be to get a 5 volt power supply with a higher amp rating and larger wires from said supply to the terminal block(?)

[Mach7]

I have included a picture of what I am trying to explain.

[fordgt40]

James

It is not just the wire gauge and current draw, but also the voltage drop which will depend on the length of your 22 gauge power feed wire to each panel. How long is your 22 gauge power feed wire and what do you consider to be the maximum power rating for the wire?

David

[Mach7]

Hello Dave,

From what I am reading, 22 gauge wire should handle a max amp limit of 3 amps, however 2 amps seems to be the recommended rating according to the internet.

The length of wire will vary from 3 feet to 6 feet, however I am not overly concerned about the voltage drop as long as I can get above 3 volts at the end of the wire for each panel using one power source.

My problem right now is the more panels i add, the more the source voltage drops off...so if I can place lower gauge wires from source to the terminal block,  i can maintain the 5 volts with sufficient available amperage headroom to feed the panels from there individual 22 gauge wires with negligible drop off, whilst still maintaining well below 2 amps per panel. (assuming my previous understanding of electrical principles is correct).

-Jim

[fordgt40]

Jim

I have just done the calcs ( http://www.rapidtables.com/calc/wire/voltage-drop-calculator.htm) based on your info in the picture and cannot duplicate your voltage drop! Perhaps I am missing something, or the power supply is not up to it or the bulbs are drawing more power than 150 millamps each.

David

[Mach7]

Hello David,

Just did some additional checking re the 22 gauge wires. Seems there are two values, one for power transmission and the other for chassis wiring'.

I think I am limited to approx 1 amp for this type of wire in a 4 or 5 foot length...but not 100 percent sure.

I think chassis wiring is the wire standing and stripped alone with no insulation(?)...this has a 7 amp rating for the same 22 gauge wire.

I will take another look at the power drop...could be I have a short somewhere as well(?)