Hi Dave,
I think the op amp inputs are the correct way round. As the battery voltage rises the + input (Battery -) becomes more negative so the op amp output becomes more negative reducing the current into the base of Q2 which will reduce the charge current.

psoke0,
Have you measured the 5.1 volts input when it is under load ?

Les.

i just measured it its 5,1 v with no load and 4,97 v underload

 

Still waiting for those node measurements.

 

Still waiting for those node measurements.

okay im really new at electronics i dont understand how am i put the leads of the multimeter to all the points i mean u want like i will measure the voltage across 4 k resistor and across following 1 k resistor across 1 k of the non inverting input resistor across 100 ohm resistor across Q1 right ?

 

I think you need to increase the supply voltage to 9V or higher, this will give you more headroom to drop the current when the battery is fully charged,..

Do you have pictures of your made circuit on breadboard?

 

I think you need to increase the supply voltage to 9V or higher, this will give you more headroom to drop the current when the battery is fully charged,..

Do you have pictures of your made circuit on breadboard?

no but icant use higher voltages its have to be 5,1 v

 

yes but for this curcuit i needed doing this way for stop current flow to the battery when battery is at 4,10 v .

The early cells required 4.1V, most current types like 4.2V - I've seen a new type that needs 4.3V.

The tolerance is about 0.05V - so getting the types mixed up is something you don't need!

 

The early cells required 4.1V, most current types like 4.2V - I've seen a new type that needs 4.3V.

The tolerance is about 0.05V - so getting the types mixed up is something you don't need!

this cell im using is 4,2 v cell and i dont want to fully charge it anyway . and its better not to fully charge it its increases life time of the cell i just need to charge it with constant current with curcuit like this u can ignore the comparator section the collector of the Q1 is connected 5,1 v supply with 100 ohm resistor think of it like that because if i can fix the low constant current problem with help of you guys then i can think about cutting the voltage at some point

 

no but icant use higher voltages its have to be 5,1 v

The shunt regulator I mentioned earlier is the easiest way to work within that constraint.

There are undoubtedly off the shelf single chip solutions for a USB to lithium charger - but you'll probably be in fine pitch SMD chip territory.

 

The shunt regulator I mentioned earlier is the easiest way to work within that constraint.

There are undoubtedly off the shelf single chip solutions for a USB to lithium charger - but you'll probably be in fine pitch SMD chip territory.

im hobbyist so i dont want to use chips i want to do it with transistors because i love transsitors and i want it to see how its working while building it otherwise there is no pleasure of doing it

 

im hobbyist so i dont want to use chips i want to do it with transistors because i love transsitors and i want it to see how its working while building it otherwise there is no pleasure of doing it

Trying to help someone who keeps quibbling with the answers gets boring very quickly................

 

its been about 2 month or so when i was start to working on this project. i did everything i think of for make it work but couldnt so i realized that i have to ask this to other people that have knowledge so that why im here to learn from you guys how work this out

 

its been about 2 month or so when i was start to working on this project. i did everything i think of for make it work but couldnt so i realized that i have to ask this to other people that have knowledge so that why im here to learn from you guys how work this out

I told you how I've been doing it for about 3 or 4 years, the TL431 is just barely a chip - some data sheets call it a "programmable zener".

If your application needs to sink more than the 100mA limit for the TL431 - you can use a transistor or two to boost the current handling.

If you cant find a supplier for the 431 - quite a lot of consumer electronics SMPSUs have one driving the opto that isolates the regulation feedback. They're even more common in less consumer oriented stuff.

 

I told you how I've been doing it for about 3 or 4 years, the TL431 is just barely a chip - some data sheets call it a "programmable zener".

If your application needs to sink more than the 100mA limit for the TL431 - you can use a transistor or two to boost the current handling.

If you cant find a supplier for the 431 - quite a lot of consumer electronics SMPSUs have one driving the opto that isolates the regulation feedback. They're even more common in less consumer oriented stuff.

okay i have the tl 431 can you give me a schematic of how you do this for charging 1 cell with constant current of 700 mA or so with a pass transistor

 

okay im really new at electronics i dont understand how am i put the leads of the multimeter to all the points i mean u want like i will measure the voltage across 4 k resistor and across following 1 k resistor across 1 k of the non inverting input resistor across 100 ohm resistor across Q1 right ?

No.
I want all the voltages measured from each node to ground/common.

 

No.
I want all the voltages measured from each node to ground/common.

ohh okay its late here can i measure them tomorrow and write all of them on paper and take it to here later ?

 

ohh okay its late here can i measure them tomorrow and write all of them on paper and take it to here later ?

Sounds good.

 

In post #10 I suggested reducing R1 to 0.5 ohms. I was forgetting that you cannot change the voltage reference voltage (Which would also need to be reduced..) for the current source as it is defined by vbe of the transistor Q1.

Les.

 

Sounds good.

i did measured the nodes on breadboard i hope i measure the right places here and also you make me realize that i think the comparator shut off the base of the transistor accourding to this measurements here

 

i think the comparator shut off the base of the transistor accourding to this measurements here

You've got it, grasshopper.
Looking at all the voltages often make it clear what's happening in the circuit.
The op amps positive input voltage is slightly lower than the negative input so its output is low, and there is thus no voltage to turn Q2 on.

It would appear that your supply voltage is only 4.85V, not 5.1V as you earlier stated.
But how did you measure 4.85V, 4.83V, and 4.81V for the same supply voltage?
(Which brings up the point that all those nodes are at the same voltage so only one measurement would have been required.)
Is the source a battery that was nearly discharged?

So you should use a reference voltage source, (such as the cheap TL431) to provide the voltage for IC1a's (-) input so that any supply voltage change doesn't affect its trip point.

 

Hi crutschow,
As there is no load on the output it explains why it is working in voltage limiting mode. I am wondering how low the "5.1 volts" will drop if it is attempting to supply 0.7 amps. (The TSs constant current figure.)

psoke0,
Can you load the output of your 5.1 volts source with a 6.8 ohm 5 watt (or larger.) resistor to simulate the load that would be taken by the charging circuit and measure it's voltage under those conditions. (This will draw just over 0.7 amps)

Les.