chaztuna(at)adelphia.net
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 Posted: Thu Jul 06, 2006 5:27 am Post subject: Instrument Lighting was RE: Insrument |
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Jerry,
I scanned the first page of the assembly sheet for the MPJA pulse width controller. See
http://www.mpja.com/productview.asp?product=4057+MD
I've sent it to you off list. I can scan the 2nd page if this is not enough. I've built 3 of these units. I had very little experience with electronics kits previous to assembling these. The first unit took 1.5 hours to build. (reading and understanding the instructions) The third unit only took 0.5 hour to assemble. For a dimmer circuit, the slide switch shown on the diagram can be omitted. It's purpose is only to allow polarity reversal for reversing direction of a motor.
Charlie Kuss
| Quote: | Hey Guys,
Thanks to Eric, Ken , and Charlie for your suggestions. However, I don't have any idea on how to build a PWM or variable duty cycle circuit. Do you guys have any schematics or more info? Thanks Charlie for including the link.
I don't plan on disconnecting the instrument or covering it up. I have the g-meter lit for aesthetics and symmetry. I paid extra to have it lit, so I plan on using it.
I called the manufacturer of the g-meter. The reason the instrument is so bright is that it actually has three bulbs drawing a total of 0.56. The company verified my amp draw correct for a 12 v unit. So at least I know I have the right bulbs. I still need to dim it down a bit to make the brightness similar to the rest of the instruments. Bob K., do you have any ideas?
Thank you,
Jerry
--> AeroElectric-List message posted by: Ken < klehman(at)albedo.net ([email]klehman(at)albedo.net?subject=Re:%20Instrument%20lighting&replyto=44A5201A.9050100(at)albedo.net[/email])>
Hi Jerry
I'd say you did it all correctly but you are just surprised at how much
the 2.5 watts heats up a resistor. A 5 watt bulb is about what
incandescent children's night lights are and does seem ridiculous for an
instrument so I agree that is is the wrong bulb. I would leave it as is
with your resistor. Second choice would be change the bulb to one that
is really 12 volts. Third choice is a variable duty cycle circuit that
pulses short 12 volt pulses to the lamp so that it averages the same
power as running it on 5 volts. That avoids heating up a resistor and
dumping the 2.5 watts but I doubt it is worth the trouble. Fourth choice
is kind of silly but if you had another similar instrument you could
wire them is series...
Ken
Jerry & Ledy Esquenazi wrote:
> Hey Guys,
> I have a simple problem that most likely has a simple solution.
> The solution, however, eludes me. I have an internally lit g-meter on
> my instrument panel (Don't ask me why  ). The g-meter's lighting is
> internal to the instrument, i.e. it's not a lighting tray that is
> screwed to the top. The instrument would have to be opened up to get
> to the lamp.
>
> I purchased the g-meter from Chief a while back ago, and when I
> received it, I realized it was a 5v unit. Chief didn't have any other
> lighting option listed or available. I thought about dropping the
> voltage with a resistor. Before doing this I called the manufacturer
> Wultrad (aka Falcon Gauge). They said they had a 12v unit in stock
> that they could swap for mine. I thought... "great problem solved."
> They shipped me one back with a 12v label over the original label. In
> testing my instrument panel lighting, I discovered that the g-meter is
> considerably brighter than the other instruments to the point it would
> be annoying at night. The interesting thing is the light is nice and
> white and very uniform though out the face of the instrument. Anyway,
> I thought... "gee I'm back to putting in a resistor to dim the light!"
>
> "OK, no problem. I'll figure out what size resistor I need and
> move on," I thought. The problem is the resistance of any
> incandescent lamp changes whether it hot or cold. The lamp is drawing
> 0.56 A at 12.0 v. Seem's like a lot to me. From that I deduced the
> lamp's resistance at 21.3 ohm. The problem is that when I figure out
> what size resistor I need to get say a 7 or 8 v voltage drop across
> the lamp and add that resistor to the circuit, the current changes and
> so does the resistance of the lamp. So how does one figure this stuff
> out? I decided to purchase a potentiometer and install it in my
> circuit, adjust it 'till I got the brightness I wanted and then
> measure the resistance of the pot. In doing this I began to smell
> smoke, the little pot was beginning to glow red as I dimmed down the
> light in the instrument. I went back and purchased a heavier pot.
> This did the trick. I decided a 20 ohm resistor in series would be
> perfect. I purchased a 22 ohm, 2 W resistor, temporarily installed
> it, and the light was perfect. The problem was I was still smelling
> that familiar acrid smell. I had a 7.4 v drop across the resistor and
> a .34 A current giving me 2.5 W to dissipate. I later tried two 10
> ohm, 2 W resistors in series. Still, they got too hot. I purchased a
> ceramic 22 ohm, 5 W resistor and it still got too hot to touch. The
> small battery I was using for testing was putting out 12.2 v under the
> load. I had a 7.4 v drop across the resistor, So I deduced I must be
> getting a 4.8 v drop across the lamp. The light was just right. I
> wonder if I still have a 5v lamp inside the instrument! I measured
> 0.34 A in the circuit. The 5 W resistor ought to be handling the 2.5
> W I need to dissipate just fine. Something is wrong. Do any of you
> smart guys know what I'm doing wrong?
>
> Jerry
> "Smelling smoke in Georgia |
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