How to decrease the output current and voltage from this DC to DC converter?

[MrAl]

Hello,

Ok great. I'll see what i can throw together for you using those parts.

Dont give up if your interview fails, they may call you later, or you may get another interview at another place and then get hired there instead. In any case though good luck with it.

 

[Willen]

Hello,

Ok great. I'll see what i can throw together for you using those parts.

Dont give up if your interview fails, they may call you later, or you may get another interview at another place and then get hired there instead. In any case though good luck with it.

Thank you for your wishes and encouragious words. If I felt depressed or totally sad then I use to busy with electronics (discussion forum, repairing or creating new device myself), it make me fresh. So I think I never will give up electronics.

One thing- I talked about before I have a Li-ion charger from China (220V IN and 4.2V OUT tiny supply) Its output current is VERY low than normal (50mA to 100mA almost) but it is amazing--

It just has LM358 and only few transistors (two or three) as a special components. Main amazing thing is its LEDs-
1st turns ON if I connected battery there. It also turns ON if I connected the charger to 220V to power it. But It turns of if it is connected to 220V but bad battery (totally discharged) or opposite polarity or short occurred on charging terminal.

1st LED- it starts to glow if battery terminal voltage is higher than 1.5V or more. So it can work as a opposite terminal detector (LED turns OFF) or damaged battery detector (LED turns off because damaged battery has zero or just only few mV).

I turned ON the charger and connected damaged cell to charge. When turned ON the charger, the 1st LED was glowing fully, but when I connected damaged battery there I got it turned OFF instantly. I measured the charging voltage and got just 500mV. (I charged this fully damaged cell few hour carefully But charger didn't get hot, charging current & volts was 50mA, 0.5V ). I don't know how 4.2 charging voltage decreased to 0.5V automatically, I guessed charging voltage determined by battery voltage level.

Yesterday mistakenly I connected opposite polarity and charged on same situation almost half an hour, but luckily happened nothing. Charger and battery both did't get hot. It means it has opposite polarity protection too, wow.

2nd LED- it starts to blinks frequently while charging. I don't know how it it blinks. I didn't see pair of transistor to blink it, but there is a single transistor near of it (I didn't see other in detail)
It turns OFF (with dim) if charging is full
3rd LED- It glows (starts from dim) if charging is full.

Nice indication! Can we have few too in your upcoming circuit too?

Sorry for my long post, actually I did two mistakes with Chinese charger- charged fully discharged Li-ion and charged with Opposite polarity but there is nothing wrong happened! So I was just excited with it.

 

[audioguru]

A Lithium charger has a Maximum output of 4.20V. It also has a Maximum output current, you said yours is only 50mA. Then if you try to charge a defective or completely discharged battery its output current will be limited to 50mA but the load (the battery) determines the output voltage that can even be zero volts if the battery is shorted.

 

[Willen]

A Lithium charger has a Maximum output of 4.20V.

Oh yes! Actually it has 4.30V output. But how it won't overcharge (more than 4.2V) if supply is more than 4.2V? Why we cannot set output to 4.2V?

It also has a Maximum output current, you said yours is only 50mA. Then if you try to charge a defective or completely discharged battery its output current will be limited to 50mA but the load (the battery) determines the output voltage that can even be zero volts if the battery is shorted.

But I don't know why it has such low charging current. I am charging now Li-ion cells rejected from cell phone. Will these rejected cell take such less current like 50 or 100mA while charging? (I think it take more current than this.)

 

[audioguru]

Oh yes! Actually it has 4.30V output. But how it won't overcharge (more than 4.2V) if supply is more than 4.2V? Why we cannot set output to 4.2V?

4.3V is too high. Either your charger or your meter is WRONG. Is the charger a Chinese one? Enough said.

But I don't know why it has such low charging current. I am charging now Li-ion cells rejected from cell phone. Will these rejected cell take such less current like 50 or 100mA while charging? (I think it take more current than this.)

A new lithium battery cell will try to take a very high charging current which will destroy it. Therefore a charger limits the current.
An old battery might have such a high internal resistance that it will not take more than 50mA of charging current.

 

[Willen]

4.3V is too high. Either your charger or your meter is WRONG. Is the charger a Chinese one? Enough said.

Yes. Is it good to set almost 4.25V (with safe charging system)?

A new lithium battery cell will try to take a very high charging current which will destroy it. Therefore a charger limits the current.
An old battery might have such a high internal resistance that it will not take more than 50mA of charging current.

I found good Li-ion never discharge lower than 3V (approx.) or something like that in ideal operation. Then how 'battery level indicator' can sense the level of battery? (on cell phone or laptop or on analog devices). May be due to its tiny decreasing voltage...right?

 

[audioguru]

Do not charge a Lithium battery higher than 4.20V.
There are complicated ICs called "battery fuel gauge" available. They are an accurate battery level indicator.
Simply measuring the voltage then offsetting it a few volts is terribly inaccurate.

 

[MrAl]

Thank you for your wishes and encouragious words. If I felt depressed or totally sad then I use to busy with electronics (discussion forum, repairing or creating new device myself), it make me fresh. So I think I never will give up electronics.

One thing- I talked about before I have a Li-ion charger from China (220V IN and 4.2V OUT tiny supply) Its output current is VERY low than normal (50mA to 100mA almost) but it is amazing--

It just has LM358 and only few transistors (two or three) as a special components. Main amazing thing is its LEDs-
1st turns ON if I connected battery there. It also turns ON if I connected the charger to 220V to power it. But It turns of if it is connected to 220V but bad battery (totally discharged) or opposite polarity or short occurred on charging terminal.

1st LED- it starts to glow if battery terminal voltage is higher than 1.5V or more. So it can work as a opposite terminal detector (LED turns OFF) or damaged battery detector (LED turns off because damaged battery has zero or just only few mV).

I turned ON the charger and connected damaged cell to charge. When turned ON the charger, the 1st LED was glowing fully, but when I connected damaged battery there I got it turned OFF instantly. I measured the charging voltage and got just 500mV. (I charged this fully damaged cell few hour carefully But charger didn't get hot, charging current & volts was 50mA, 0.5V ). I don't know how 4.2 charging voltage decreased to 0.5V automatically, I guessed charging voltage determined by battery voltage level.

Yesterday mistakenly I connected opposite polarity and charged on same situation almost half an hour, but luckily happened nothing. Charger and battery both did't get hot. It means it has opposite polarity protection too, wow.

2nd LED- it starts to blinks frequently while charging. I don't know how it it blinks. I didn't see pair of transistor to blink it, but there is a single transistor near of it (I didn't see other in detail)
It turns OFF (with dim) if charging is full
3rd LED- It glows (starts from dim) if charging is full.

Nice indication! Can we have few too in your upcoming circuit too?

Sorry for my long post, actually I did two mistakes with Chinese charger- charged fully discharged Li-ion and charged with Opposite polarity but there is nothing wrong happened! So I was just excited with it.

Hi,

Very interesting charger there. We could add more to the main charger but it will probably take more circuitry. Not complicated stuff however, but simpler circuits to detect faults and prevent charging.

First a quick note about chargers that appear to charge over 4.20 volts...
It is possible to build a charger that puts out more than 4.20 volts, because for one thing the cell max voltage is really 4.25 volts but that is to be considered as an ABSOLUTE max not a design target point. So your meter could be reading that as slightly higher for example.
Also, another reason is that the charger may have a somewhat sophisticated cell internal resistance measurement compensation, which could make the voltage actually go up higher such as to 4.30 volts. It is possible to build such a charger, but i cant not recommend it highly because the output cell would have to be only a certain make and model unless we used a quite complex circuit to actually make the required measurements and that's hard to recommend too.
If the circuit is simple though then it would be hard to believe they made this charger for ANY cell, but probably for only one make and model cell so care has to be used when using with other make and model cells as it could overcharge.
This of course means im not sure we want to use that charger with other cells except the one that came with the charger or as recommended by the manufacturer.

As to building our own fault circuits, the simplest one is an LED connected in reverse to the output with current limiting series resistor. This is simple but the charger must be used correctly or it does not work at all.
The idea is that if the charger is NOT turned on (no power applied) and the cell is connected backwards, the LED turns on to tell the user that the cells is connected backwards. The user then reverses the cell and then turns the charger on.
I think this can be done using an LED, resistor, and a few careful diode placements.

Automatic turn off is a little more difficult because we have to have the monitor circuit turn off the output and that requires either a MOSFET or relay.

So think about these things a little and i'll try to get back here with a circuit for the current monitor today some time.

 

[NorthGuy]

Charging at 4.25V might be Ok. On the other hand, if you only charge at 4V (or even 3.9V), the useful cycle life of the battery will be greatly extended.

 

[MrAl]

Hello again,

Ok with the parts mentioned previously we can make a current monitor that monitors the current down to something around 50ma or maybe a little lower.
This will require at least one power resistor, i'll make a list here.

1 ohm, 2 watts or better, probably the minimum value we should use, should work ok though.
2 ohm, 4 watts or better, a little better choice although we loose a little more overhead voltage.
Anything higher than that better but we loose more overhead voltage and waste more power. 5 ohms rated 10 watts would loose around 5 watts and 5v for example, so that should be the max.
So we are looking at a value of 1 to 5 ohms, rated 2 to 10 watts, and 2 or 3 ohms is probably a good choice.

So the question is, do you have any of these or can you get any of these?
Keep in mind that two 5 ohm 5 watt resistors in parallel make a 2.5 ohm 10 watt resistor, and two 1 ohm 2 watt resistors in series make a 2 ohm 4 watt resistor, just for a couple examples of parallel and series combinations. You can make a value you need this way just remember to calculate the total power as well as the total resistance before using.

 

[Willen]

Thanks NorthGuy for life extendable trick, it is nice and safe too.

Hi MrAl,
The Chinese charger has no complicated circuits for its 3 nice indicators. It was designed for very small Li-ion cell so its max output is around 150mA poorly. I didn't used it anymore. Onec it took two days to charge a cell, charging current was just 21mA he he he. Now I am just using its monitors.

And I can get almost any value's power resistors. I can find parts which can be used on general TVs, and radio. Power resistors can be used on TVs so I simply can find it on any electronics shop. Don't worry about parallel and series I will use Online resistor calculators found on internet.

It would be great if current is around 10mA (decreasing) or battery voltage is around 4.1V (increasing) then current indicator start 'ON', can't we do? But it's ok if you design yourself what you think good. Your experience and knowledge is thousand times more than mine.

At last, you used 2N4401 and 2N4403 as NPN and PNP, I just have 5 pieces of 2N4401 and 2N4403 (had sent by Charles from Texas) and I want to preserve it for RF purpose because these have good RF characteristics than BC547 . So I want to use BC547B and BC557B instead of 2N4401 and 2N4403.

I tested their gain-
BC547B NPN has average hFE- 355
BC557B PNP has average hFE- 380

Please take care about it (simulate with it during design). I have almost 40 pieces of them because it is easily available here and very cheap too- $1 (one US dollar)= almost 40 pieces I bought once. But other general purposes like 2N4401 or 2N3904 are not available. Available at capital city but more than five times expensive.

Thank you again.

 

[MrAl]

Hello again,

Ok here is a circuit so you can see what this will look like. This shows the basic charger and the current monitor.
The second LM317 can be the LM317L type, and there is a chance we can use the other one for that too but i think it will be better with two separate ones.

Remember again that the cell does not connect to ground so when adjusting the voltage the voltage has to be measured right across the cell not to ground. Just a kind reminder.

 

[Willen]

*Double post removed!

 

[Willen]

Yes, I got relief!

I think I can set output current same as before by adjusting 4 watts power resistor as before- I= 0.6V/R or what? (it would be good if there are any ohms law formula about 4 watts and 0.2 watts resistors for desired current output.)

I think I have to set 10k pot to select a desired current level to turn OFF the LED in correct level.

I was unable to find LM317L and LM317M. There I was found just LM317T, so bought them few pieces. I'll use two of them here. Thank you!

 

[MrAl]

Hi again,

Well actually there are now two current settings as well as one voltage setting.

The first current setting is the same as before, it's the max current output and that is set with the 0.6 ohm resistor.
0.6 ohms gives 1 amp max output, 1.2 ohms gives 0.5 amp max output, etc. The power is P=I*I*R but the resistor rating should be twice that P=2*I*I*R.

The second current setting is the low level min current. That's like around 20ma to 50ma or around there. That is set with the 10k pot so no need to change the 2 ohm resistor. It's better not to change the 2 ohm resistor anyway because we need a certain level of voltage for the LM358 to work properly given the various imperfect spec's for that chip.

Adjustment would go as follows..
1. Adjust output voltage to 4.15 volts (that's across the cell with no cell connected).
2. Adjust max current via resistor value like 0.6 to 1.2 ohms and check when cell is connected.
3. Adjust min current set pot to turn the LED "ON" when the current gets below the min level like 20ma.
4. During the very first charge monitor the cell voltage to make sure it never goes above 4.15 volts.

For most cells 30ma would be a good min. You can even go higher for larger cells. If you want to try 20ma that is probably the lowest you should adjust this for.

If you want the LED to turn "OFF" instead of "ON" when cell is done being charged, then you can connect the LED and series resistor from the output of the LM358 to ground instead of to +10 volts, observing polarity on the LED.

You can most likely use an input voltage of less than 10 volts, like 7 or 8 volts but may require up to 9 volts.

The charger i use is slightly different but the max current is set in the same way, with the low value resistor. I have a larger value resistor soldered in place and that is for the low current setting for smaller cells. Then i have a switch that connects another resistor in parallel to that one. When the switch is closed, the resistance is lower so the charge current is higher. That way i can use it for two different cells.
The important point here though is that the higher current setting MUST be in effect when the switch is OPEN, just in case the switch fails with a lower rated cell. Of course the settings have to be clearly marked as well and care when setting the switch for the lower rated cells.

 

[Willen]

Hi again,

The first current setting is the same as before, it's the max current output and that is set with the 0.6 ohm resistor.
0.6 ohms gives 1 amp max output, 1.2 ohms gives 0.5 amp max output, etc.

But there are two resistor in series with battery from Gnd- 0.6 ohms and 2 ohms. This '2 ohms resistor' is confusing me. So I calculated output current like this way as before- 0.6V/(2R+0.6R)=230mA I don't know what I understood.

The second current setting is the low level min current. That's like around 20ma to 50ma or around there. That is set with the 10k pot so no need to change the 2 ohm resistor. It's better not to change the 2 ohm resistor anyway because we need a certain level of voltage for the LM358 to work properly given the various imperfect spec's for that chip.

I thinking about my own idea, may be it is fool- I guessed it would be better if we could set voltage comparator (4.15V) instead of current comparator. Isn't it? Because battery is critical to voltage than current. If LED turns ON at 4.1V or 4.15V, i think it would be more easy and safe, wouldn't it?

 

[MrAl]

Hi,

Well actually the new 2 ohm resistor does NOT affect the max current setting (with proper input voltage of course). Only the 0.6 ohm resistor changes that. So the output current limit is set ONLY with the 0.6 ohm resistor as before. Again, 0.6 ohm will give about 1 amp max, 1.2 ohm will give about 0.5 amp max. For 750ma then it would take about 0.9 ohm, and check the wattage making the rating in watts equal to P=2*I*I*R.

I assumed you wanted a minimum current indication but if you want a max voltage indication that's every simpler. I'll draw that next and add that to the circuit so you can have an LED come on when the voltage reaches say 4.15v or whatever you want to set it for. Of course when you set the output voltage itself it has to be a little higher than that though so the adjustment might get a little tricky. You might set the voltage to 4.16v and then set the LED to come on at 4.15v or something similar to that.

I'll try to get the new schematic drawn up tomorrow sometime, probably afternoon. It's just going to be a comparator, voltage reference, pot, couple resistors.

Do you still want the min current indicator LED too?

 

[Willen]

Hi,

Well actually the new 2 ohm resistor does NOT affect the max current setting (with proper input voltage of course). Only the 0.6 ohm resistor changes that. So the output current limit is set ONLY with the 0.6 ohm resistor as before. Again, 0.6 ohm will give about 1 amp max, 1.2 ohm will give about 0.5 amp max. For 750ma then it would take about 0.9 ohm, and check the wattage making the rating in watts equal to P=2*I*I*R.

I assumed you wanted a minimum current indication but if you want a max voltage indication that's every simpler. I'll draw that next and add that to the circuit so you can have an LED come on when the voltage reaches say 4.15v or whatever you want to set it for. Of course when you set the output voltage itself it has to be a little higher than that though so the adjustment might get a little tricky. You might set the voltage to 4.16v and then set the LED to come on at 4.15v or something similar to that.

I'll try to get the new schematic drawn up tomorrow sometime, probably afternoon. It's just going to be a comparator, voltage reference, pot, couple resistors.

Do you still want the min current indicator LED too?

Oh yes! I will do as you said.

There is another channel of LM358 not used in the circuit. Can't we use this not used channel of the OpAmp as a voltage monitor (without removing previous current monitor)? Excited lot more!!

I like to keep lots of indicators, that is safe and attractive and looks advanced!

 

[MrAl]

Hi again,

Ok, if you want to use the other half then here is the circuit for the Voltage Monitor. Note this one monitors the voltage while the previous one monitors the current.

I left the 2 ohm resistor in this circuit so you can see that it stays right there if you still want the Current Monitor too. If you do not want the Current Monitor (with the other LED) then you can short out that 2 ohm resistor. If you still want it then just use the other previous circuit as well as this one.

This may be a little tricky setting the voltage trip point (the 1k resistor). Normally we dont do this because what we have to do is sense a voltage that is JUST below the normal max charge voltage, so it gets close in setting.

Note the LED resistor is again around 1k or near that. (value is not shown on the drawing yet).

 

[Willen]

Hi,

OK I learnt more actually!
- I have no LM317L, only have bulky 'T' version. I feel little uneasy to use this Bulky LM317T as a 2nd voltage regulator. I have TL431 shunt regulators (and few months ago you were taught me almost everything about the TL431). So I tried to modify the circuit designed by you. Please check it once (its bias and resistors values).

- There I marked 'A' in red color. I am confused how measure 4.15V there. Positive point is A but as a negative point........is it Gnd or after the 0.6 ohms resistor?

- And at last, won't the regulator chip (LM317T) get hot due to 10V input and average 0.75A output? Won't it dissipates around more than 2 watts? I guesses 2 or more watts produces horrible heat on the IC.

Than you again.