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    Voltage regulator... proportional?

    Hello everyone!

    I have a pretty basic knowledge of electronics, so I apologize in advance.

    I'm with a RC project and I want to use a 3S LiPo battery (10.2v to 12.60v) to power a video transmitter and a camera that only work from 3.3v to 4.2v. I could simply use a step down converter, but the tricky part is that I need to know the average voltage at what each of the cells is.

    I tried a voltage divider, but obviously it didn't work. I used 10K as R1 and 5K as R2 (because this combination gives me 4.2v when the LiPo is full and 3.4v when I have to land), but as soon as I connected both the camera and the video transmitter, the voltage dropped to 1.3v. I assume that is because they have a resistance that adds to the 5K resistor.

    Is there something more adequate to do this?

    Thanks in advance!

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    Re: Voltage regulator... proportional?

    A resistive voltage divider won't work for the reason you already worked out, the load (the RC project) is effectively across the bottom resistor making it's value appear to be much lower.
    The simplest solution here is to use a linear 3.3V regulator. They cost almost nothing and take in anything from about 4V to 30V on their input pins and give a stable 3.3V on the output pin. The downside is they are inefficient, particularly when dropping a higher voltage. The more they drop the hotter they get.

    A better option is a regulated SMPS supply. They cost a bit more but run cool. If you search for regulated SMPS you can find many that give a fixed 3.3V (it's an industry standard voltage) or have an adjustable output you can set to anything lower than the input voltage. Example: https://www.ebay.co.uk/itm/DC-4V-38V...EAAOSwVpVbIBvw

    Brian.
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    Re: Voltage regulator... proportional?

    Quote Originally Posted by betwixt View Post
    A resistive voltage divider won't work for the reason you already worked out, the load (the RC project) is effectively across the bottom resistor making it's value appear to be much lower.
    The simplest solution here is to use a linear 3.3V regulator. They cost almost nothing and take in anything from about 4V to 30V on their input pins and give a stable 3.3V on the output pin. The downside is they are inefficient, particularly when dropping a higher voltage. The more they drop the hotter they get.

    A better option is a regulated SMPS supply. They cost a bit more but run cool. If you search for regulated SMPS you can find many that give a fixed 3.3V (it's an industry standard voltage) or have an adjustable output you can set to anything lower than the input voltage. Example: https://www.ebay.co.uk/itm/DC-4V-38V...EAAOSwVpVbIBvw

    Brian.
    Thanks for the answer.

    A step-down or buck converter won't do in this case. Sorry for not explaining it myself.

    The camera shows the voltage at what it's powered. This is what I use to monitor the voltage, but since it works at 1S voltage I need something that connected to the full LiPo voltage (to not drain a cells more than the others), gives me the average voltage of the three cells to power the camera/VTx.

    I need to know the average voltage of the three cells, since I can't discharge them under a certain level (3.5V per cells to be safe).



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    Re: Voltage regulator... proportional?

    Hi,

    The SMPS input is the full 3S voltage, not a single cell, thus all cells see the identical current.

    Klaus
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    Re: Voltage regulator... proportional?

    Quote Originally Posted by KlausST View Post
    Hi,

    The SMPS input is the full 3S voltage, not a single cell, thus all cells see the identical current.

    Klaus
    Thanks. I understand. Excuse me because I don't explain myself well in english (maybe, in spanish neither).

    RC LiPo batteries don't have protective circuits, so I need to monitor the voltage, if not of each cell, an average of the three.

    The camera/VTx (video transmitter) that I'm using, has an OSD (On Screen Display) where it shows the voltage at what it is powered. Since this camera/VTx only works at 1S voltage and I don't want to connect it to just one cell, because it would discharge that cell only and leave the other too unbalanced (also shortening its life), I need something that powers from the 3S LiPo but gives 4.2V when the LiPo is at 12.60V (fully charged) and 3.4V when the battery is at 10.2V.

    This way, I can get an average voltage of each cell. I don't expect it show the lowest, just average voltage.



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    Re: Voltage regulator... proportional?

    The adjustable SMPS input is the whole 3S, as noted above
    The output is your desired voltage, 4.2V to 3.4V, based on LiPo at 12.6 to 10.2V

    you should be able to adjust the feedback voltage sample or the reference voltage to achieve this
    a voltage divider across the LiPo and an op amp should do



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    Re: Voltage regulator... proportional?

    Hi,

    I still don't understand.
    We never decided to connect "one cell only".
    But you always talk about connecting only one cell.

    Please show your circuit, hand drawn is OK.

    Klaus
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    Re: Voltage regulator... proportional?

    Hi,

    Just show a little sketch of your connection and we can help you improve it.

    I am having a feeling that the video transmitter being in parallel with your R2 in the voltage divider has an input impedance that is low enough as to significantly reduce the effective R2 resistance in the voltage divider: Req = R2//Rin_voltage_transmitter (Req highly depends on Rin_voltage_transmitter). If this is the case, one thing you can do to improve the situation is to insert an opamp voltage follower between the voltage divider and the voltage transmitter to provide a high-enough input impedance across R2.
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    Re: Voltage regulator... proportional?

    I am also unclear at your intention. If the camera displays the voltage it is fed from and that has to be regulated 3.3V, it will show 3.3V whatever the individual cells hold.
    If the camera has an independent voltage monitoring input, power the camera from the regulated 3.3V (regulated down from all cells in series) and use a resistive divider to drop only the measurement input. The voltage monitoring input will have a high impedance so two resistors to take a proportion of the cells voltage will work in that situation.


    Brian.
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    Re: Voltage regulator... proportional?

    Quote Originally Posted by betwixt View Post
    I am also unclear at your intention. If the camera displays the voltage it is fed from and that has to be regulated 3.3V, it will show 3.3V whatever the individual cells hold.
    If the camera has an independent voltage monitoring input, power the camera from the regulated 3.3V (regulated down from all cells in series) and use a resistive divider to drop only the measurement input. The voltage monitoring input will have a high impedance so two resistors to take a proportion of the cells voltage will work in that situation.


    Brian.
    I never said the camera had to be powered by a fixed 3.3V. I said from 3.3V to 4.2V so it can measure a cell voltage. I also said it measures the voltage that is powered from, so no independent voltage monitoring.

    Thanks for your time, but it's just not worth it. It's clear that I don't know how to convey what I need.

    This had to be a simple device, since there is no room in the plane, nor it can take too much weight.

    As said, thanks very much for your time. You can close this thread.



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    Re: Voltage regulator... proportional?

    The truth here is just that we are just guessing to help you out. Like they say, a picture is worth a thousand words, a simple drawing can convey the entire message.

    I am thinking that the voltage measurement input to your device does not have a sufficiently high input resistance and that the low input resistance combining with R2 results in an effective resistance thst is very low such that your voltage divider has resistances R1 in series with R_effective with R_effective being too low and hence dropping 1.3V instead of 4.2V that you expected.

    Insert a voltage follower like I suggested in my previous post and let us know the result.
    Last edited by Akanimo; 1st September 2019 at 10:49.
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    Re: Voltage regulator... proportional?

    Hi,

    I'm not sure of how the camera voltage monitor works in relation to anything here, but anyway... What about a cheap ADC and sampling the three battery voltages periodically? You're spoilt for choice with ADC types, and MCUs or simpler but bulky analog-style solutions.



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