Current regulation circuit?

[danrogers]

Hi all. I would like to be able to regulate current (around 4A @ 17.5V). I have a constant voltage supply that seems pretty stable but I want to be able to regulate the current. Google searches only seem to return low current circuits, or suggest to use small current regulation devices.

Which is the best direction to turn guys?

Thanks in advance

 

[bountyhunter]

The best way is to build in a constant current loop into the bench supply's control loop so you can choose either constant voltage or current operation. It's hard to hang it on the outside, easy to build it in.

 

[danrogers]

Hi, I should have explained a bit better. The power supply I have is a iGo, it runs from 12V dc or 240V AC. I want to use it to charge my quad copter batteries, I works when the batteries are nearing full with a reduced current but obviously if the battery pulls too much charge current then the supplies over current protection kicks in and it turns off, I need to work out a way around this somehow :/

 

[ronsimpson]

I would like to be able to regulate current

So are you wanting to build a power supply that has current limit?
OR
A constant current source that might be used for driving LEDs?

 

[danrogers]

So are you wanting to build a power supply that has current limit?
OR
A constant current source that might be used for driving LEDs?

Neither as above, I have a power source but I need to be able to limit its current output with a external circuit

 

[bountyhunter]

Neither as above, I have a power source but I need to be able to limit its current output with a external circuit

As I was pointing out: doing it externally is a major deal compared to just building it into the supply. You basically have a supply putting out 17.5V @ 4A (around 70 Watts). To be able to control the current you would have to build in some kind of linear dropping element like power FETs which could be used as a giant variable resistance element. And it will be burning a lot of power so it will need a very large heatsink.

 

[ronsimpson]

What ever circuit you put on the outside of you power supply will drop some voltage.

 

[audioguru]

If your battery is Lithium than it needs a Lithium battery charger circuit designed or programmed for it. The battery charger circuit should limit the current until each cell reaches 4.20V then it regulates the voltage of each cell at 4.20V (a balanced charger) until their current drops low then it shuts off. If you limit the current near the end then the cell with the lowest capacity is not yet fully charged like the other cells then its voltage keeps rising until the Lithium ignites.

 

[AnalogKid]

As above, lithium batteries are temperamental and dangerous. But beyond that, what do you want to limit the output current to? 3 posts, no numbers. An external, series current limiter can be as simple as four parts. Note that with such a circuit, the maximum output voltage will be about 2 volts less.

ak

 

[danrogers]

The packs are so called intelligent, all the management is done internally. To charge them one just needs to provide 17.4V at a constant current, as I stated around 4A would be perfect. I guess then it is not possible to throttle the current without dropping some voltage and wasting energy as you guys say?

 

[audioguru]

Please post the datasheet for your Lithium battery pack to see if it has a simple "protection" circuit (common) or if it includes a proper charger circuit (I have never seen a battery with it).
4A is a high current to charge a lithium battery. Is it a huge battery?

 

[danrogers]

Hi Guru. They are pretty advanced packs that also balance the cells for u. https://store.dji.com/product/phantom-3-intelligent-flight-battery

It is a 4.48Ah so 4A is still under 1C and only 70ish watts i guess. The stock chargers are 57 and 100w.

 

[audioguru]

Your flight batteries cost 19 times as much as my Li-Po 2-cells flight batteries. Most of my radio controlled airplanes came with a balancing charger separate from the battery.

 

[danrogers]

The DJI packs are crazy money! But they make the best quadcopters on the market so I'm stuck with them

Indeed, I have lots of other lithium packs for RC cars and aircraft, plus I work for a company in the UK that make large (very large) lithium packs for vehicles

So is it possible to throttle the current output of my PSU

 

[AnalogKid]

Yes, but as you mentioned above the output voltage will sag.

ak

 

[danrogers]

hmmm I can increase the output voltage? What are the options, a huge resistor?

 

[AnalogKid]

No matter how complex a load on a power supply is in real life, in a first-order analysis a load is a fixed or variable resistor. If you attach a constant current source to a fixed resistor, a constant voltage is developed across it. As the resistance varies, so will the voltage appearing at the output of the constant current source. A charging battery can be thought of as a weirdly-varying resistor. So no matter what you do to the constant-voltage output of the supply, the voltage at the output of the following current limiter will vary as the load changes. You can adjust the output of the supply to compensate for the minimum voltage drop across the current limiter, but that will not place a constant voltage with constant current across a varying load.

ak

 

[bountyhunter]

hmmm I can increase the output voltage? What are the options, a huge resistor?

You could. You have a supply that puts out 17.5V at a large current? If you put a very large 5 Ohm resistor in series with the battery, that would limit max current to about 3A (in reality, a lot less since the battery voltage will reduce the drop across the resistor). Using a limit resistor will mean it takes longer to charge the battery. Do NOT increase the final voltage you charge the battery up to which I assume is 4.200V/cell.

 

[gary350]

Hi all. I would like to be able to regulate current (around 4A @ 17.5V). I have a constant voltage supply that seems pretty stable but I want to be able to regulate the current. Google searches only seem to return low current circuits, or suggest to use small current regulation devices.

Which is the best direction to turn guys?

Thanks in advance

AC or DC?

 

[spec]

Hi all. I would like to be able to regulate current (around 4A @ 17.5V). I have a constant voltage supply that seems pretty stable but I want to be able to regulate the current. Google searches only seem to return low current circuits, or suggest to use small current regulation devices.

Which is the best direction to turn guys?

Thanks in advance

Hy Dan,

Care to tell us your county.

I think many of us have been holding off suggesting a circuit to meet the requirements of your request due to a concern about safety. Now that you have told us a bit more about your application I fell a bit easier.

To answer your question, yes there are circuits that will provide a constant current with the following general characteristics, all within reason that is (reasonable limits in brackets):
(1) any value of constant current (10uA to 20A)
(2) any accuracy of constant current (+- 0.5%)
(3) any input voltage range (6V to 50V)
(4) any output voltage range (0V to 50V)
(5) arbitrarily low dropout voltage (100mV)
(6) modest cost (£2 UK to £20 UK depending on specification)

Obviously, the characteristics are interdependent. For example, without using switch mode techniques, it would be difficult to provide a constant current of 20A with an input range of 6 to 50V and an output range of 0V to 50V because of the resulting dissipation in the series pass element of 1KW.

I am about halfway through a design which should meet your requirements and will post a schematic soon for your consideration.

spec