## How does this charger function ?

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Guyzz

I just got my hand on some chargers (0.6\$ pcs), witch are actually small switchers.
I couldn't buy the comps ot the little housing for that money. And the opamp seems nice , if i can unsolder it wo. damaging it.

There is a small DC-Jack , and it says 9v AC
There is also a USB Jack (The square one like on the back of printrs), and i think it might be used as an "Input" , instead of 9v AC.

There is a schematic , but a lot of component values are missing , so i have listed my findings/measurements below (values have been measured on components mounted on the pcb).

I have been told it's a charger , and that it could be used for LI-PO cells.

My guess (remember im an analog noob) , is that the Opamp is somehow measuring the current or voltage of the 4 1R's in parallel , and is "twiddeling" with the LM2575 FB (Feedback) , and maybe even adjusting the voltage or ??

Question 1: If i Lift the LM2575 FB (or unsolder the BAV70) , and connect FB directly to C104. Wouldnt that make a "normal 5v 1A" switcher out of it ?

Question 2: If someone could explain the "magic" in this circuit , i might even modify the resistors to so other usefull things. I promise ... I'll start reading about opamps :oops:

```Comp	Mark	Value		Measured
R103	182	1800
R104	182	1800

R102  01D   100K        101K		Think i can read (Links bwlow) that 01 = 100 and D = 1000 in Mult Factor
R105  01D   100K        23K

R106	01Y   1	      0,4 Ohm	01 = 100 & Y = 0.01 in mult factor,
R107	01Y   1
R108	01Y   1
R109	01Y   1	      0,3 Ohm

L101	331	330 uH      NA		I think 330uH , its a "Square SMD inductor in a Ferrite core, quite big"

L102	NA	??          NA		Grey , about twice the length & height of a 1206 Cap

L100	NA	??          NA		Grey , about twice the length & height of a 1206 Cap

D105	EC61	Quad-Transil		ESDA6V1SC5 (SOT23-5L) http://www.st.com/stonline/products/literature/ds/7056.pdf

U100  LM2575				http://www.national.com/ds/LM/LM1575.pdf
U101	LM8261M5				http://www.national.com/ds/LM/LM8261.pdf

http://www.talkingelectronics.com/ChipDataEbook-1d/html/SM-Resistors.html
```

Measured Vout across C10 5.67v , Vfb is 5.06v

TIA

/Bingo

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Nice find... It amazes me how it can be so cheap..
Care should be taken as it's not isolated...

Michael

Hi Bingo:

The Opamp is doing current limiting of the regulator output. This circuit is explained in a NSC appnote (I don't remember which, but remember is in the Simple switcher appnotes)

Quote:

Question 1: If i Lift the LM2575 FB (or unsolder the BAV70) , and connect FB directly to C104. Wouldnt that make a "normal 5v 1A" switcher out of it ?

Yes, providing the IC is the 5V version

Quote:

Question 2: If someone could explain the "magic" in this circuit , i might even modify the resistors to so other usefull things. I promise ... I'll start reading about opamps Embarassed

The magic here is that the opamp is amplifying the sense resistors voltage and therefore faking the IC regulator into believe that there is an output voltage going up (when in reality is the load who is loading current beyond the limit) and consecuentially making it to lower the output voltage and so mantaining a constant current. This will occur when reaching some limit determined by the sense resistors and the opamp gain. But most of the time the FB is regulating constant voltage (assuming you have a healthy SLA battery) The dual diode is a switch, which makes the change between CV or CC regulation

And Finally, I don't know if this is a LI-PO charger. It has constant voltage and current limiting, but I don't see any "mode switch", nor charge control in order to charge a LI-PO. Maybe the designer decided to rely in the battery protection mechanism to make a sort of charging (and let the battery protection cut by overtemperature and/or overvoltage) but this is very dangerous!!

For me, it appears to be a SLA battery charger.

Nachus

My first guess respect to your first question is yes, it would be 5V switcher.

Voltages posted also make sense. My first guess respect to your second questin is that OpAmp is a diferential amplifier. This, coupled with te double diode makes this thingie a current limited, voltage limited (to 5V6 approx) power supply. The workings are simple: when the current raises over acertain level the output of the OpAmp becomes more positive than at C104, thus the switcher reacts reducing its output voltage, that is effectively a reduction in output current at the load.

Guillem.
"Common sense is the least common of the senses" Anonymous.

Oooops, I'm late!

I agree that this is by no means a Li-Pol charger. An 4-5 cells Ni-MH charger perhaps? Li chemestry is quite sensitive to voltage limiting, and it is usually between 4.1 and 4.2 V, with a required precission of 50 mV. Definitively not the accuracy and voltages of the present schematic.

Guillem.
"Common sense is the least common of the senses" Anonymous.

nachus001 wrote:

And Finally, I don't know if this is a LI-PO charger. It has constant voltage and current limiting, but I don't see any "mode switch", nor charge control in order to charge a LI-PO. Maybe the designer decided to rely in the battery protection mechanism to make a sort of charging (and let the battery protection cut by overtemperature and/or overvoltage) but this is very dangerous!!
Nachus

@Nachus ... Edit and Guillem

Thankyou for the information :-)

I have measured (unloaded) : The opamp outout is 76.6 mV

It has been used for some kind of a PDA Battery charger , so it's prob not for a SLA. But i think i found the device , and it contains a Rechargeable Lithium Polymer battery 4.2 V 2000mAh.

Edit... It says a Li-Po , and it says it needs at least 8 hours to charge

/Bingo

Bingo600 wrote:

I have measured (unloaded) : The opamp outout is 76.6 mV

That's ok for no load (no CC regulation)

Quote:

It has been used for some kind of a PDA Battery charger , so it's prob not for a SLA. But i think i found the device , and it contains a Rechargeable Lithium Polymer battery 4.2 V 2000mAh.

Edit... It says a Li-Po , and it says it needs at least 8 hours to charge

/Bingo

Ok may be this box is a "preregulator" with current limiting and the PDA is actually doing the battery charge/measure work. Sounds more feasible to me

Nachus

I just lifted the BAV70 , and connected FB to C104.

Now i have 5.06v@Output.

Neat switcher for 0.6\$ :-)

I have some 24v DC apps ... bit C101 is just 25v.
I just have to change C101 to a 35 or 63v , then i'm running. I'll take the chance that the Ceramics are 50v. Ansd the opamp can stand 30v , not that i use it now.
But they smell so bad then they burn :-)

/Bingo

Well, everything then makes sense. This unit will power a full device that probably (I'm almost sure) will have a LiPol charger, and this IC will require a minimum of 5V. Since it also probably contains some internal diodes and so on, just to supply the remaining circuitry from the (external) charger instead of the circuitry (or internal charger), then a little bit more than 5V are required for the internal diodes. Then this switcher simply has a current limit for safety purposes. Or at least, is what I would do if I was the designer for a commercial product.

I can't buy a simple 2575 for less than one â‚¬...

Just for safety: ceramics in 0603 or smaller sizes, tend to have a lower voltage rating, especially with higher capacities.

Guillem.
"Common sense is the least common of the senses" Anonymous.