Guide Running Screens Off Lower Voltages

Discussion in 'Guide Submissions' started by jefflongo, Jul 9, 2018 at 5:07 AM.

  1. jefflongo Broke BitBuilt .

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    Introduction
    Most screens used in portables run off a standard ~12v out of the box. We would like to run these off a lower voltage because most people use a 7.4v battery pack. Luckily, the vast majority of screens can be easily modified to run on lower voltages. This guide is not a one size fits all; all screen drivers are designed differently. This guide exploits common design patterns to work in our favor.

    The easiest way is to run a screen on a lower voltage is to simply connect the output from your batteries directly to the screen's power input. Some screens will work just fine this way. Be sure to see if the screen continues to work when the batteries are at a low charge, because the voltage output of the batteries will decrease when they are at a low charge. If this works for you, then that's all! You're done!

    Running on 5v
    If the battery voltage is too low to run the screen or you wish to use a regulated voltage line, a few more steps must be taken. Most screens will regulate their input voltage to 5v. This means we can operate the screen on 5v if we can locate the voltage regulator, and bypass it—connecting 5v to its output.

    Documenting the Chips
    The first step is to keep track of the names of all the chips on the drive board. They are boxed in red.
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    Finding the Regulator
    Now we will want to find out what each chip's function is. Perform a Google search for each of the names on top of the chips to find their datasheets. Voltage regulators are pretty common, so they should show up in the Google search. If they don't, it's probably not a regulator. The other chips might be memory or chips containing firmware. I've located three voltage regulators on this board: AMS1117-3.3, AMS1117-ADJ, and FR9886. I know they are voltage regulators from the Google search

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    Generally, we want to work with the voltage regulator with the highest output voltage, as this one will likely only be dependent on the input voltage. For example, if we try to supplement the 3.3v regulator with a 3.3v supply, the board might not work because that 3.3v regulator may have been powered by the 5v regulator. In other words, work down the voltage hierarchy. Generally, VCC (12V) -> 5V -> 3.3V -> ...

    Measuring the Output
    Looking at the two AMS1117 regulators, it is clear from the datasheet that AMS1117-3.3 refers to a 3.3v regulator, and AMS1117-ADJ is an adjustable regulator. Looking at the FR9886 datasheet, it also appears to be a adjustable voltage regulator. Great! We have narrowed down our 5v regulator to either be the AMS1117-ADJ or the FR9886. We will need to measure the output voltage of both regulators to determine which is the 5v regulator. Refer to the datasheet to find which pin is the output.


    AMS1117
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    FR9886
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    Note that the small circle on the top of the chip on the board indicates pin 1.

    Now that we've located the output pins on the voltage regulator, we need to measure the output voltages. Let me emphasize here that this is where you need to be EXTREMELY careful. If you try to just measure the output with a multimeter lead you risk shorting pins on the voltage regulator. This is very likely to instantly and irreparably damage the drive board and/or the screen. I recommend soldering a small wire to the output pin (alternatively you can solder this wire to a component like a capacitor or inductor which is likely connected to the pin, you can verify this with your multimeter's continuity function) and soldering a small wire from the screen's GND. Use alligator clip leads on your multimeter to measure the voltage from the wires. This way you have no risk of shorting anything on the screen with your multimeter probes.

    The AMS1117-ADJ measures an output of 1.25v and the FR9886 measures an output of 5v. Great! we've located the 5v regulator! Now you can supply 5v to the output pin (or a connected component such as a capacitor or inductor) to power the screen. Be sure not to connect your previous input voltage to the 5v output, and be sure not to supply any other input voltage to the screen besides the new 5v input, or you risk damaging the screen. I also highly recommend removing the voltage regulator. Because we are forcing an output on the voltage regulator, it is possible this could have some negative long term side effects. Removing the regulator creates an open circuit to everything before the 5v output, making the procedure completely harmless.

    Running on 3.3v
    If 5v is not an option (which it usually is), it may also be possible to operate the screen off a lower voltage such as 3.3v, though this is not as guaranteed as running on 5v. I stated earlier that I located the 3.3v regulator on the board (AMS1117-3.3). After removing the regulator and supplying 3.3v to the output, the screen does not turn on. Because the screen works on 5v, this means there is a dependency on 5v somewhere. Use your multimeter's continuity function to determine if 5v is an input to any of the chips. As it turns out, the 1.25v regulator uses 5v as its input. It may be possible to supplement this input with 3.3v. Looking at the AMS1117 datasheet, the necessary condition is that Vin - Vout must be between 1.5v and 12v. Because the input (3.3v) minus the output (1.25v) is above 1.5v, 3.3v should suffice.
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    And there it is, the screen successfully operates on 3.3v. 20180708_192800.jpg

    Strategy for DN-D43AT Driver Board
    Because this driver board is common, the following is how you can run a screen with this drive board on 5v or 3.3v.

    5v: Remove the chip boxed in green, supply 5v at the pin boxed in red, and gnd at the pin boxed in black (or any gnd location).
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    3.3v: Remove the chips boxed in green, supply 3.3v at both locations boxed in red, and gnd at the pin boxed in black (or any gnd location).
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    Last edited: Jul 9, 2018 at 6:42 PM
    splain, Kitsen, MRKane and 8 others like this.

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