Voltage regulator... proportional?

Thread Starter

Enrique Rumbo

Joined Aug 31, 2019
15
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!
 

MrChips

Joined Oct 2, 2009
19,270
You have guessed correctly.

10K + 5K voltage divider will divide the input voltage by 5K / ( 10K + 5K) which is equal to one-third.
This is true only if you take zero current from the divider. Once you connect a load to the divider, the load has resistance in parallel with the 5K resistance and you have to recalculate the voltage drop.

A voltage divider is not a good way to reduce the voltage when trying to power another device, as you have discovered. It also wastes a lot of energy.

Your best solution is a buck converter.
 

marcf

Joined Dec 29, 2014
256
Here is a possibility:

KNACRO DC-DC 12V to 3.3V 3A 10W Step-Down Power Supply Module Car Power Converter
 

Thread Starter

Enrique Rumbo

Joined Aug 31, 2019
15
You have guessed correctly.

10K + 5K voltage divider will divide the input voltage by 5K / ( 10K + 5K) which is equal to one-third.
This is true only if you take zero current from the divider. Once you connect a load to the divider, the load has resistance in parallel with the 5K resistance and you have to recalculate the voltage drop.

A voltage divider is not a good way to reduce the voltage when trying to power another device, as you have discovered. It also wastes a lot of energy.

Your best solution is a buck converter.
Thanks for your answer.

Buck converters are fixed voltage output if I'm right, and this isn't going to work for my case.
 

MrChips

Joined Oct 2, 2009
19,270
What are you trying to do?
Are you trying the measure the voltage on the cell?
 

Thread Starter

Enrique Rumbo

Joined Aug 31, 2019
15
What are you trying to do?
Are you trying the measure the voltage on the cell?
Yes, sir.

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).

That's why 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.
 

jpanhalt

Joined Jan 18, 2008
7,871
Thanks for your answer.

Buck converters are fixed voltage output if I'm right, and this isn't going to work for my case.
Do you realize that using a voltage divider (or similar) on the full output voltage of the 3S pack will give no accurate indication of the voltage of each cell? That is why for charging and balancing, the pack has a connector to each cell.
 

Thread Starter

Enrique Rumbo

Joined Aug 31, 2019
15
I recommend a buck-boost convert (in buck configuration) for that. Here's one example of many: https://www.sparkfun.com/products/15208

Why do you want the supply to the video to be proportional?
Do you realize that using a voltage divider (or similar) on the full output voltage of the 3S pack will give no accurate indication of the voltage of each cell? That is why for charging and balancing, the pack has a connector to each cell.
Thanks.

I understand, but I just need to know the voltage "ball park", no need to know it exactly. The use for this is to not over discharge it and I have no other means to know this. Time flying is not accurate since it depends on many factors.
 

MrChips

Joined Oct 2, 2009
19,270
If you wish to measure the battery voltage, you can still use the 10K + 5K voltage divider.
The current drain will be approximately 12V / 15K = 0.8mA.
The rule of thumb is that the load to be connected to the voltage divider should have a resistance greater than ten times R2, i.e. 50K in this case.

The setup you are going to use to measure the voltage should present a resistance greater than 50KΩ. Most ADC input circuits meet this requirement.
 

jpanhalt

Joined Jan 18, 2008
7,871
So why do you need to monitor the "average" (e.g, sum) of the cells? Doesn't the battery pack have a protective circuit?

This thread reminds me of "20 Questions" :

You have yet to tell us how you will know the voltage whatever it is. And given that, why not use a buck converter -- much more efficient than a voltage divider -- then monitor the cells individually and report the lowest?
 

Thread Starter

Enrique Rumbo

Joined Aug 31, 2019
15
If you wish to measure the battery voltage, you can still use the 10K + 5K voltage divider.
The current drain will be approximately 12V / 15K = 0.8mA.
The rule of thumb is that the load to be connected to the voltage divider should have a resistance greater than ten times R2, i.e. 50K in this case.

The setup you are going to use to measure the voltage should present a resistance greater than 50KΩ. Most ADC input circuits meet this requirement.
This particular camera/VTx drain around 500mA, and have around 3800 Ohm.
 

Thread Starter

Enrique Rumbo

Joined Aug 31, 2019
15
So why do you need to monitor the "average" (e.g, sum) of the cells? Doesn't the battery pack have a protective circuit?

This thread reminds me of "20 Questions" :

You have yet to tell us how you will know the voltage whatever it is. And given that, why not use a buck converter -- much more efficient than a voltage divider -- then monitor the cells individually and report the lowest?
Excuse me. I don't explain myself well in english (maybe, in spanish neither).

RC LiPo batteries don't have protective circuits.

The camera/VTx (video transmitter) 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 what I'm asking for.
 

Tonyr1084

Joined Sep 24, 2015
3,577
So you want to monitor the battery voltage of the plane using the video camera? Is there another solution out there for remote battery voltage monitoring? My drone does. When the drone battery gets low my remote starts beeping. Unless you want to shoot video at the same time.
 

MrChips

Joined Oct 2, 2009
19,270
This particular camera/VTx drain around 500mA, and have around 3800 Ohm.
You don't power the camera from the voltage divider. You need to power the camera from a step down switching converter.

If you need to monitor the battery voltage, you need another way to measure the voltage using a system that has a high impedance input.
 
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