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  • Backlight supply voltage?

    Ok, I am venturing down the road to build a photocell based controller for the backlight of the Xenarc 7TSV. As of now the plan is to build a simple circuit composed of a photocell, a potentiometer (or a few), a 555 timer and a FET - not to mention various resistors, caps, etc. I've got an electrical guy leading me in the right direction and he is more than capable of this.

    It's late and I was hoping to find out if anyone out there has started down this road. I need to know the CCFL (cold cathode) wattage and voltage input. I would assume it was either 12V or 5V and I know the wattage must be below 9watts, but I just need something to start with... I would like to be headed in the right direction but I will want to wait as long as possible to tear open the dash (again and again)... any info will be appreciated.

    I will document the crap out of this and write a quick webpage to help those who might want to follow in my footsteps... thoughts, rants, etc....??

    as of now the plan is to use some level of caps on the photocells to help level out the lighting and the 555 will be used to serve as a PWM setup to effective reduce the duty cycle into the 60% range for night time viewing... I'm a mechanical engineer, he's the electrical... so we'll move slow enough for me to learn what the hell this is all about...

    fingers crossed...
    EPIA SP13000, 512 DDR400, Seagate 300GB, Belkin PCI 802.11g w/external antenna, Holux GM-210 GPS, XM Direct w/ serial, Dlink FM, Opus 120 p\s, Rockford P4004 amp, Xenarc 7" VGA touchscreen custom mounted in double din spot -- see pics and write up here!!

  • #2
    I don't know the exact voltage, but when I was playing with my inverter it burned my skin enough so you could smell it, and I didn't even touch it, apparently the voltage is large enough to penetrate 1mm of air.
    Have you tried using the voltmeter? I would imagine the analog ones wouldn't have the correct number on their gauge, but there should be digital ones that'll give you a large enough reading. Just an idea, I'm sure as an engeneer you have probably already thought of that, but anyway
    00 Saturn SL2, boosted @ 8 psi - CarDomain
    Newest install - my quickest ever
    My 2nd carputer - b4 I broke the touchscreen
    My very first carputer - voted the most ghetto

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    • #3
      oh, I totally am planning on controlling the voltage INTO the inverter... the output would be in the 7 to 12 THOUSAND volts... very small amperage (this is why you are alive)... volts give electricity the ability to jump, this is why you barely get a spark with 12v, but with the output of the inverter you can easily zap your skin... :-)

      I could easily use a voltmeter, but that means I have to take my screen out of my car and get to the "innards"... I was hoping that maybe someone had started down this road and could give me some info so that I could go ahead and order parts and build the circuit, then the screen could be removed... I am not lazy - IT IS FREAKIN' COLD!!! it is 2 degrees F right now... it will be UP to 15 degrees by the time I get off work... and it is all downhill after that... so screen removal is a task for only the brave... :-) when I remove the dash panel in my car the heater controls come with it, which means no heat until the dash is back together... so NO CAN DO!!
      EPIA SP13000, 512 DDR400, Seagate 300GB, Belkin PCI 802.11g w/external antenna, Holux GM-210 GPS, XM Direct w/ serial, Dlink FM, Opus 120 p\s, Rockford P4004 amp, Xenarc 7" VGA touchscreen custom mounted in double din spot -- see pics and write up here!!

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      • #4
        I've done this with the Lilliput, but just lowered the voltage a little with a resistor and connected it to the dimmer wire that went to the old HU. It just drops it to a range of about 7-10V which is much dimmer than full, but nowhere low enough to flicker/drop out which I think is what shortens the life of the tube. It's been working fine for at least 6 months now.

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        • #5
          Originally posted by Curiosity
          I've done this with the Lilliput, but just lowered the voltage a little with a resistor and connected it to the dimmer wire that went to the old HU. It just drops it to a range of about 7-10V which is much dimmer than full, but nowhere low enough to flicker/drop out which I think is what shortens the life of the tube. It's been working fine for at least 6 months now.
          where'd you find a resistor that could handle 5W?? it must be pretty big... you might have six months from this setup, but the longevity will be short, the reason to build a circuit is to keep all devices in the design range... for every 10 degrees you raise a piece of electronics equipment you cut its life in half... so I don't want to risk a fire by using a 1/4watt resistor inline with my $400 screen...
          EPIA SP13000, 512 DDR400, Seagate 300GB, Belkin PCI 802.11g w/external antenna, Holux GM-210 GPS, XM Direct w/ serial, Dlink FM, Opus 120 p\s, Rockford P4004 amp, Xenarc 7" VGA touchscreen custom mounted in double din spot -- see pics and write up here!!

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          • #6
            It's just a 1/4 watt resistor tapped into the point where a 1 ohm 1/4 watt resistor was powering the inverter originally. Maybe the Lilliput is a little different? As far as I could tell, the inverter cicruit is completely isolated from the rest of the board except for that line and ground so that has to be the only power.

            Edit: Sorry, both are 1/2 watt.

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            • #7
              well, the other factor to keep me from doing it this way is that today, for instance, it is snowing.... the sun is bright, it is REALLY bright outside, but I would drive with my lights on... so there goes my screen if I setup like you have it...

              but my point is that you are running 12volts and originally 1 ohm... = 12watts?? let's just say it was really 5volts... then just 5watts... so you are still putting a much larger load into that pot than it was designed for...
              EPIA SP13000, 512 DDR400, Seagate 300GB, Belkin PCI 802.11g w/external antenna, Holux GM-210 GPS, XM Direct w/ serial, Dlink FM, Opus 120 p\s, Rockford P4004 amp, Xenarc 7" VGA touchscreen custom mounted in double din spot -- see pics and write up here!!

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              • #8
                True. Although I have daytime running lights in the front, I turn at least the parking lights on during rain or snow and that dims the screen. It's acceptable if the I turn the dimmer all the way up though. It's maybe 70% brightness.

                I forgot. I did test the current when I was doing it. It draws 190mA, which is 2.3W? Well, that doesn't make sense, but it wouldn't be only component in the lilli that's too small to handle the power. LOL I am starting to get worried about the dashboard dimmer now, thanks! :-)

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                • #9
                  I think you guys have your Power computation a little mixed up. The power dissipated by the resistor is I*V = V*V/R = I*I*R where V is the voltage drop across the resistor and I is the current flowing through it. So in Curiosity's case where he used a 1 Ohm resistor and measured 190mA flowing through it, the resistor would dissipate 0.19*0.19*1.0 = ~36mW -- well within the operating margin of a 1/4W resistor. Also knowing that 190mA is flowing, you know the voltage drop across your resistor was 190mV. If the original input was 12V you effectively dropped it to around 11.8V.

                  I'm not sure I understand how you hooked your dimmer into the circuit. To use that presumably you'd need some sort of circuit that let's control the voltage to the high voltage transformer based on your dimmer voltage. Maybe I missed something in your explanation.
                  2004 4runner

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                  • #10
                    Exacly 11.8V rando. You're damn good! I used just a relay and resistor. The relay is powered by the dimmer line, so when it's on it takes the dimmer (something like 8-12V) through the 33 ohm to the backlight, otherwise it's just 12V to 1 ohm to backlight.

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                    • #11
                      Originally posted by rando
                      I think you guys have your Power computation a little mixed up. The power dissipated by the resistor is I*V = V*V/R = I*I*R where V is the voltage drop across the resistor and I is the current flowing through it. So in Curiosity's case where he used a 1 Ohm resistor and measured 190mA flowing through it, the resistor would dissipate 0.19*0.19*1.0 = ~36mW -- well within the operating margin of a 1/4W resistor. Also knowing that 190mA is flowing, you know the voltage drop across your resistor was 190mV. If the original input was 12V you effectively dropped it to around 11.8V.

                      I'm not sure I understand how you hooked your dimmer into the circuit. To use that presumably you'd need some sort of circuit that let's control the voltage to the high voltage transformer based on your dimmer voltage. Maybe I missed something in your explanation.
                      but 1ohm is what he replaced with a rheostat... so he is upping the resistance at this point to get the dimmer to function... so if he drops the 12v down to 70%, we'll just say 8V then he is dropping 4volts... which works to about 3/4 watts... this is my point...
                      EPIA SP13000, 512 DDR400, Seagate 300GB, Belkin PCI 802.11g w/external antenna, Holux GM-210 GPS, XM Direct w/ serial, Dlink FM, Opus 120 p\s, Rockford P4004 amp, Xenarc 7" VGA touchscreen custom mounted in double din spot -- see pics and write up here!!

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                      • #12
                        Originally posted by Curiosity
                        Exacly 11.8V rando. You're damn good! I used just a relay and resistor. The relay is powered by the dimmer line, so when it's on it takes the dimmer (something like 8-12V) through the 33 ohm to the backlight, otherwise it's just 12V to 1 ohm to backlight.
                        Oh I get it now, simple hi/low control. I thought you meant one of those dimmer knobs on the dash that lets you adjust your dash light brightness. I thought you had somehow hooked that up so that you had variable control of your backlight brightness instead of just two levels.
                        2004 4runner

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                        • #13
                          Originally posted by mgithens
                          but 1ohm is what he replaced with a rheostat... so he is upping the resistance at this point to get the dimmer to function... so if he drops the 12v down to 70%, we'll just say 8V then he is dropping 4volts... which works to about 3/4 watts... this is my point...
                          I thought your point was that the resistor was dissipating 5W which is actually twice what the entire circuit consumes (12V @ 190mA = 2.3W). I was only trying to explain how what I think is really going on.

                          I'm still not sure I understand the circuit exactly. I think what you're saying is that the original circuit uses a 1 Ohm resistor in series with the HV coil as a simple voltage divider. If that's true, and based on the numbers Curiosity provided above, we can approximate the input impedence of the coil primary winding to be Z = V/I = 11.8 / .19 = ~62 Ohms. If a 33 Ohm resistor is substituted for the 1 Ohm, then the entire impedence becomes 95 Ohms and the current through the circuit reduced to ~130mA. This create a voltage drop across the 33 Ohm resistor of ~4.2V. Power dissipation will then be ~530mW, right above the tolerance of a 1/2W resistor.
                          2004 4runner

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                          • #14
                            mgithens I understand what you're saying now. Lower volts/higher watts. What if I just use two 1/2 watt 66 ohm in parallel? That would give me a full watt.
                            rando Here's a pic of it. The 33 ohm is in series with the HV coil and, I guess some kind of reostat, in the dash as well when switched, so I can control brightness but not at a very wide range. I was afraid of going direct in case it goes up to 14.4+. I first used a 100 ohm pot but kept burning it out at different points. It was just there to figure out what I liked first.

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                            • #15
                              yep, I totally agree that the parallel/split load will work better (longer)

                              and the pot was burning out because as long as you kept it down to 1/4 watt dissapation then it was fine, but when you cranked it up to 25 or 30 ohms the heat burned it up... I bet the 33ohm still gets pretty warm...
                              EPIA SP13000, 512 DDR400, Seagate 300GB, Belkin PCI 802.11g w/external antenna, Holux GM-210 GPS, XM Direct w/ serial, Dlink FM, Opus 120 p\s, Rockford P4004 amp, Xenarc 7" VGA touchscreen custom mounted in double din spot -- see pics and write up here!!

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