Voltage regulator... proportional?

[Kikerumbo]

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!

 

[ronsimpson]

I see the 10.2 to 12.6 battery. Then a step down converter to about 3.8v.
I do not see how you know the voltage. R1, R2 ???
Something measures the voltage. Is that the camera?

 

[Kikerumbo]

Thanks for the answer and excuse me for my lack of info.

When I wrote about R1 and R2 I was referring to the components of a voltage divider. But since this isn't going to work... never mind.

The camera has an OSD (On Screen Display) and part of the info it shows is the voltage at what it is powered. Since I need to power this camera and VTx (video transmitter) from the 3S battery to not drain just one cell, I need something that gives me a voltage that goes down at the same time as the cells in the battery go down. Let's say an average voltage of the three cells.

 

[Nigel Goodwin]

Your post makes little or no sense - I'm 'presuming' that you have a switch-mode converter converting the 12.6V down to 4.2V?, and you want to monitor the voltage across the three cells? (which would be the obvious requirement).

There's no problem doing this, all you need is a simple resistive attenuator - BUT you need individual access to the input that measures it's battery voltage, presumably that is simply connected to the 4.2V battery input, you would need to separate it.

Far easier solution, put the cells in parallel instead of series, everything then works just as it was originally made to.

 

[Kikerumbo]

Your post makes little or no sense - I'm 'presuming' that you have a switch-mode converter converting the 12.6V down to 4.2V?, and you want to monitor the voltage across the three cells? (which would be the obvious requirement).

There's no problem doing this, all you need is a simple resistive attenuator - BUT you need individual access to the input that measures it's battery voltage, presumably that is simply connected to the 4.2V battery input, you would need to separate it.

Far easier solution, put the cells in parallel instead of series, everything then works just as it was originally made to.

Excuse me for my lack of explanation.

I do not have a switch converter now. I just use the balance connector of the battery and plug directly to just one cell. This is and issue, because the 200mW video transmitter drains the cells that it's connected to much more than the others. This will shorten the LiPo battery life.

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 cell more than the others, gives me the average voltage of the three cells to power the camera/VTx. I can't use a fixed buck converter for this reason.

I cannot put the cells in parallel because the motor needs to run at 3S.

 

[ronsimpson]

I think the camera's power supply is what you are monitoring. If the camera had a separate pin to measure the voltage then the divider will work. But because the camera supply will load down any divider we need a different idea.

Most power supplies have a single job. To hold the output voltage constant. I think you need a supply that at 10.2V makes 3.3V and at 12.6V makes 4.2. Or cuts the voltage to 1/3. I think this can be done with some thinking. Do you know how much power the video transmitter and a camera use?
Data on these products will help.
Do you have the power supply yet?

 

[Kikerumbo]

I think the camera's power supply is what you are monitoring. If the camera had a separate pin to measure the voltage then the divider will work. But because the camera supply will load down any divider we need a different idea.

Most power supplies have a single job. To hold the output voltage constant. I think you need a supply that at 10.2V makes 3.3V and at 12.6V makes 4.2. Or cuts the voltage to 1/3. I think this can be done with some thinking. Do you know how much power the video transmitter and a camera use?
Data on these products will help.
Do you have the power supply yet?

Yes sir, that's exactly what it does. It shows the voltage at what it's powered. Sorry for not explaining it myself.

They should use around 500mA (data sheet). 200mW is the power that, supposedly, the VTx gives.

Cut the LiPo voltage was what I tried with the voltage divider, but I didn't realise that the load will add resistance to the divider... I'm an electronics fool.

 

[ronsimpson]

Maybe we can find a Chinese buck power supply that can be modified.
Vin 10 to 13 volts.
Vout (3 to 5V)
Current 700mA
I need to find a supply that uses a IC where I can get to both inputs of the error amplifier. Too much technical. Normally there is an amplifier that looks to see if the output voltage is good. Normally one input goes to a good reference voltage. (1.25v or 2.5V or 5V something good and stable usually built in side the IC) The other input of the amplifier looks at the output voltage through a resistor divider. Your 3.3V output is divided down to match the reference voltage.
The 1/3 ides is to remove the reference voltage and replace it with (1/3 Vin). So the IC will be working to make Vout = Vin/3. (not normally what a pwmIC does)
Maybe Nigel can thing of the right IC.

 

[Kikerumbo]

I don't want to take much of your time, but it'll be much appreciated.

It seems is a common issue that no one could solve in RCGroups. So, some more people than me will benefits from this solution.

 

[ronsimpson]

Here is a typical buck power supply. Look at the error amplifier. It compares the output voltage to 1.23 volts. Some parts allow the use of an external voltage reference. We need to use 1/3 of vin as the reference.

 

[Kikerumbo]

Here is a typical buck power supply. Look at the error amplifier. It compares the output voltage to 1.23 volts. Some parts allow the use of an external voltage reference. We need to use 1/3 of vin as the reference.
View attachment 120232

Thanks!

 

[ronsimpson]

Incase some one is following this. The Chinese buck supplies use many different ICs. Some are the MC34063. The older IC uA78S40 is the same thing but in a bigger package. The 78S40 will do what we want but the Chinese do not use it. I was hoping to fine a $3.00 supply that could be easy modified. Will keep looking.

 

[rjenkinsgb]

This is purely to prove the idea and not a final design, but it seems to work - add an opamp to compare a fraction of the supply to the output & feed that to the sense pin on the regulator, with appropriately low gain to prevent oscillation.

I've not included the .asc as the LM2576-5 is not the ideal part & the model I found seems a bit odd, but it's nearest I could find to the LM2577 example. It really needs a .adj version. The opamp is just picked pretty much at random from the ltspice library.

Green = input, blue = output and red = feedback.

 

[ronsimpson]

add an opamp to compare a fraction of the supply to the output

exactly!
I was looking at old parts like the 78S40 and the 494 that have FB+ and FB- inputs. Then you connect the two FB pins like your U2 amp. But no one make a small board any more. We would have to make it. The mc34063 is the same part as the 78S40 but without the extra op-amp and with only one FB pin.

Then I tried taking a low cost "GangGood,com" board and adding two transistors in a dif-amp but could not get it to work. Then I tried a one transistor inverter to invert the input signal and added it back in to FB along with the output voltage. That almost works.

I think what you did will work with almost any Chinese DC-DC board.

 

[gophert]

Kikerumbo
In which city/state/country do you live?

 

[Kikerumbo]

Kikerumbo
In which city/state/country do you live?

I'm from Spain, sir.

 

[alec_t]

Here's a sim of an alternative adaptation of the ubiquitous Chinese DC/DC module shown in post #14, which doesn't need the module's pot to be disconnected and doesn't require an opamp.

The left-hand dotted rectangle contents emulate the DC/DC module.
The output voltage is the lower of (1) the voltage set by the SetV pot in the module and (2) Vbe + the desired input voltage fraction set by the SetProp pot in the add-on.
The add-on connects to the wiper of the SetV pot to provide feedback.
R1 and R2 are current limiters in the event that either pot gets cranked to its zero Ohms end.

 

[Kikerumbo]

Thanks sir.

I'll study your design.