Author Topic: Constant current constant voltage power supply for low power application  (Read 1830 times)

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Offline Gyro

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That's because the cathode current was 1mA+ :) The load was not grounded. Here is the schematic I used (see attachment).

Ah, got you now.  :)


@Adhith:  Thanks for the clarifications. Is there any chance that you could tell us the intended application? (it sounds a bit like you're trying to mains power a battery clock mechanism or something). That might help us to suggest the best / most economical / most efficient / most optimal solution.
Chris

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Offline Adhith

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@Gyro Thank you so much for your patience and support :)

we are doing a small college project with Al-ion battery. We have build the cell and now looking for charging it.
We are also planning to build a crude version of BMS with over/under charge protection using one tl431 circuit (I do have few question regarding it as well, but planning to do that as a separate thread). The CC/CV supply is for the charging of this cell, where voltage is around 2-3 volts and current draw should be restricted to 0.1milliA to 0.5milliA.

This is just the first trial test where we are basically following a trial and error method for different cell chemistry and charging voltage and later figuring out the efficient values for each. Also, just want to let you know that we are taking all the precarious for a fire. However Al-ion being non explosive, we are pretty much safe in doing these experiments :) (or I would say at least so far :D)

this is the setup. hope this gives the complete picture, which I should have explained at the beginning. Sorry for the confusion
 
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Offline exe

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Are you sure there is no specialized IC for your application?


Regarding testing of lm317l, it didn't work as I expected :(. It seems there is a minimum load current, I forgot about that (spoiled by regulators that don't require minimum load). Compliance voltage is not great too, like min 5V input for 1mA current. May be I'm doing something wrong.

I wondered why another circuit worked with lm317l and no load. I think that's because the output resistor divider provides enough current for it to work.
 

Offline Gyro

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@Gyro Thank you so much for your patience and support :)

we are doing a small college project with Al-ion battery. We have build the cell and now looking for charging it.
We are also planning to build a crude version of BMS with over/under charge protection using one tl431 circuit (I do have few question regarding it as well, but planning to do that as a separate thread). The CC/CV supply is for the charging of this cell, where voltage is around 2-3 volts and current draw should be restricted to 0.1milliA to 0.5milliA.

This is just the first trial test where we are basically following a trial and error method for different cell chemistry and charging voltage and later figuring out the efficient values for each. Also, just want to let you know that we are taking all the precarious for a fire. However Al-ion being non explosive, we are pretty much safe in doing these experiments :) (or I would say at least so far :D)

this is the setup. hope this gives the complete picture, which I should have explained at the beginning. Sorry for the confusion


Ah, ok. That sounds interesting. :)

In that case, voltage accuracy sounds like the most important characteristic - with current accuracy not far behind.

If you are in the experimental cell chemistry stage, then something like an LM317 with decent resolution adjustment pot and a load resistor on it's output to shunt away most of the quiescent current might be an option. Followed by a protective resistor and current meter?

It sounds that you need good monitoring of the battery's charge characteristics so I assume that there will need to be a uA/mA meter in series with the charging supply. If so, it will be necessary to account it's voltage burden. A high impedance voltmeter too, but that won't involve significant loading.

Your BMS also sounds as if it might have leakage current issues which could  preventing accurate monitoring of the amount of charge going into the battery.

Sorry, the above isn't a solution (either), just some random thoughts related to the application.


EDIT: Sorry, from @exe's post, the LM317(l) might not be a suitable candidate (although a resistor might be used to shunt away the quiescent current as I mentioned above). You want to avoid this resistor draining the cell if the regulator voltage drops though - maybe a series diode, with voltage measurement taking place after the diode.
« Last Edit: April 08, 2021, 11:30:41 am by Gyro »
Chris

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Offline exe

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I just tried two ht7333 in series: the first one sets current, the second one sets voltage. This gives independent control of current and voltage.

The output current is set somewhat imprecise due to 20k trimpot loading the output [1]. Could be somewhat fixed with a higher resistance trimpot, but I was too lazy to bother. It seemed stable when I loaded the output with 2k resistor (no AC voltage detected, although my dmm seems only has 10kHz bandwidth). I used no capacitors because otherwise that may create large inrush current when connecting to the load which might be undesirable (idk if this is gonna destroy your cell or not). I attach picture of the test setup just for fun.

Since this IC is absolete (I bought mine from aliexpress, so can be fake, but seems to be working fine), I looked at other options.There is MCP1702, for example. The datasheet says it needs some minimal capacitance at the output. I'm curious how badly it will oscillate without it. May be it doesn't need much capacitance at output under light load. I'm gonna buy MCP1702-1202E/TO just for fun.

[1] I used a fixed-voltage regulator the same way we use adjustable regulators: I treated GND pin as an adjustment pin. Of course, voltage cannot go below the fixed voltage in costant-voltage mode, which is 3.3V in this case. But it's not a problem in constant-current mode.
 
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Offline Terry Bites

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OK
I went to town over the Easter bad weather weekend. I modded my original idea a bit to make a constant current /constant voltage low power PSU. Happy days.
You can program the Vout and Io. With external voltages or pots. Vo=Vin and Io =500uA/V. [attachimg=1]
Nothing smart or novel I'm afraid but it works well. I made a PCB for it, if anyone feels they want to play, I'll post the gerbs. None of that auto router rubbish here- note ground planes not shown.[attachimg=2]
 
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Offline exe

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I like the idea of using inamp. Did you check the circuit for stability and behavior when switching to CC mode and back? Why do you need a sziklai output?

PS remote sensing for 2mA output, that's a nice touch :)
 

Offline Jwillis

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Sziklai pair has about half the turn on voltage than a Darlington pair.
 

Offline Adhith

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I just tried two ht7333 in series: the first one sets current, the second one sets voltage. This gives independent control of current and voltage.

The output current is set somewhat imprecise due to 20k trimpot loading the output [1]. Could be somewhat fixed with a higher resistance trimpot, but I was too lazy to bother. It seemed stable when I loaded the output with 2k resistor (no AC voltage detected, although my dmm seems only has 10kHz bandwidth). I used no capacitors because otherwise that may create large inrush current when connecting to the load which might be undesirable (idk if this is gonna destroy your cell or not). I attach picture of the test setup just for fun.

Since this IC is absolete (I bought mine from aliexpress, so can be fake, but seems to be working fine), I looked at other options.There is MCP1702, for example. The datasheet says it needs some minimal capacitance at the output. I'm curious how badly it will oscillate without it. May be it doesn't need much capacitance at output under light load. I'm gonna buy MCP1702-1202E/TO just for fun.

[1] I used a fixed-voltage regulator the same way we use adjustable regulators: I treated GND pin as an adjustment pin. Of course, voltage cannot go below the fixed voltage in costant-voltage mode, which is 3.3V in this case. But it's not a problem in constant-current mode.
Great! is there any way that I could get a rough circuit diagram? Thnx
 

Offline Adhith

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OK
I went to town over the Easter bad weather weekend. I modded my original idea a bit to make a constant current /constant voltage low power PSU. Happy days.
You can program the Vout and Io. With external voltages or pots. Vo=Vin and Io =500uA/V. [attachimg=1]
Nothing smart or novel I'm afraid but it works well. I made a PCB for it, if anyone feels they want to play, I'll post the gerbs. None of that auto router rubbish here- note ground planes not shown.[attachimg=2]
cool! I'll check this out. will go out for component purchase and update soon
 

Offline Adhith

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Could someone also enlighten me regarding few of my thoughts.
Suppose I have a source of 5V, 1A wall adapter. I need to reduce the voltage to say 3.3V and limit the current to 0.1milli A. I believe, the voltage could be reduced to 3.3V by using a resistor divider whereas, limiting the current is where all the precision work should be done. Could that be done using transistors as well? what is the advantage of using opams or IC and its supporting components than a transistor?

or the concept of CC/CV is not just about restricting voltage & current but something else?

If someone could me an overall idea regarding this topic, it will be a huge help.

 

Offline exe

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My advice is to team-up with a EE person on this subject (it can be another student, for example). There is simply too much to learn and too many ways to screw it. Blindly copying someones' design rarely gives anything working (speaking from experience). Imagine the cost of, say, misbehaving circuit that is, say, marginally stable that sometimes doesn't work as expected. Like, oscillating between CV/CC modes, that can destroy your battery.

PS don't use bread-boards for final version. A loose connection can ruin your day.
 

Offline srb1954

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Could someone also enlighten me regarding few of my thoughts.
Suppose I have a source of 5V, 1A wall adapter. I need to reduce the voltage to say 3.3V and limit the current to 0.1milli A. I believe, the voltage could be reduced to 3.3V by using a resistor divider whereas, limiting the current is where all the precision work should be done. Could that be done using transistors as well? what is the advantage of using opams or IC and its supporting components than a transistor?

or the concept of CC/CV is not just about restricting voltage & current but something else?

If someone could me an overall idea regarding this topic, it will be a huge help.
You need to be more specific with your requirements - what is the purpose of the 0.1mA current limit? Is it just limited for circuit protection? Or is it for a particular circuit function e.g. charging a super capacitor or battery?

Using a resistive divider to generate the 3.3V is only feasible if the load current is accurately fixed or is very low compared to the standing current through the resistive divider. However, if you have a variable load current the 3.3V requirement is more easily met with a simple 3.3V low-dropout voltage regulator chip.

The current limit function is a little more difficult to implement depending on the accuracy required of the current limit. If only modest accuracy is required then you could install a current source or a current mirror prior to the LDO regulator input to regulate the maximum available current. The choice of a suitable current regulator is severely restricted by the available working voltage between the 5V input and the minimum input voltage to the regulator, maybe 3.5V, giving only 1.5V across the current regulator to work with. An LM334 current source IC would work here but its accuracy is limited by temperature drift. The temperature drift can be compensated by external circuitry (refer to the LM334 datasheet) but with this compensation circuitry it might be difficult to get it to work within the 1.5V available to the current regulator.

Since some of the input current into the LDO regulator goes to ground rather than the load the achievable accuracy is further limited by the variation of this ground current. This effect can be minimised by selecting an LDO regulator with very low ground current.
 

Offline Jay_Diddy_B

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Hi group,
This is a pretty unusual requirement, such low power. Let me throw an unusual solution at you.

[attachimg=1]


This a shunt regulator. The voltage-controlled current source determines the maximum output current. The ideal clamps the voltage to regulate the output.

This can be implemented like this:

[attachimg=2]


I am using LT1013 op-amps in the model. In this application LM324 would work equally well. The LT1634-5 can be replaced with a 6.2V Zener diode with a few minor modifications.


I have attached the model for those playing along at home.

Regards,
Jay_Diddy_B
[attachurl=3]
 

Online magic

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By the way, I don't recall this being spelled out explicitly. Maybe I missed it ::)

Does the voltage/current limits need to be adjustable or is it OK if they are fixed and just within the right ballpark (~3V, ~500µA)?
What are the acceptable means of adjustment? Is swapping different values of some current limiting resistor OK? Is a pot OK? Does it need to be a control voltage referenced to ground, maybe from some DAC? What reaction time and bandwidth is desired, if so?

What exact accuracy is required? 1%? 10%? 0.001%?

I have a suspicion that concrete answers to these questions may dramatically cut down the complexity of some proposed solutions.
« Last Edit: April 16, 2021, 06:38:47 am by magic »
 

Offline Jay_Diddy_B

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Hi,

If the requirements are not precise, here is another idea:

[attachimg=1]



This uses the low voltage version of the TL431. The current requirements of the TL431 is too high to clamp the low current directly. That is why the diodes are used.

This can be modeled like this:

[attachimg=2]



The 'box' represents the TLV431.

The results are:

[attachimg=3]


This shows the current limit behavior, it is not true constant current, but it may be good enough.

I have attached the model.

Regards,
Jay_Diddy_B

[attachurl=4]
 


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