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uc3842 smps battery charger

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kg300

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Hi, this is a SMPS charger based on uc3842 chip and deliver 12V 8A output. the problem is that the supply voltage of 3842 VCC are oscillating and i couldn't fix it at 16V and supply cannot start because 3842 ON volt are 18V. when I remove the chip from circuit it goes to 18V accurately.
I tried without transformer but Vcc are not ok.
Do it work only with the startup circuit (a resistor connected to the 300VDC)?
I change 3842 twice but the problem is same. anybody solve the problem.

View attachment bat-charger.bmp
 

Yes R29 and R28 are for startup only.
When transformer will start switching,D11 will provide vcc through R26 to pin7.

We can not increase volts at 7 pin higher than 18v.
A 18v zener diode is conected on pin,7.
As i see this charger have short circuit and reverse polarity protection.
That,s why we can not read volts at out put.


Because Q4 75n75 is in series of negtive bus bar.


Uc3842 have 5v regulator in it at pin8(ref volt).
You can adjust out volt with R24(5K).
 
are the R29 and R28 value correct? because come time charger dos't work when I apply 220v. and if I use UC3843 what changes in circuit doing?
 

Yes values of R28 and R29 are ok.(i see in attached datasheet a off line flyback circuit they use 56K only,but input volts are 117v)
You can use uc3843 without any change in schematic.
only difference is in under-voltage lockout.
 

Attachments

  • 3842,datasheet.pdf
    436 KB · Views: 601

Hi, kg300, is your charger works?
have you pcb design?
 

i try to do a 12v 10A smps using forward topology. The ic used is UC3842. instruments. the mosfet used is 8n80 (fairchild). The gate waveform is not correct. it has some spikes,the supply is steady. so the mosfet is getting hot.

Input voltage= 150vac-275Vac
Output voltage=12Vdc
Output current=10A
Frequency(switching)=115KHz
bobbin-etd34
Here i attached the circuit diagram

how can i solve it???????????????
 

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can you attach schematic diagram again?
 
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    kif123

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Hi i have a smps own desine.when i turn on it, the circut start slowly and when i put load on it the voltage go down to 0v.Please help me.
 

What is the capcitor magnetuide attached to pin 7 an pin 5 of ic.
 

kg300 said:
Hi, this is a SMPS charger based on uc3842 chip and deliver 12V 8A output. the problem is that the supply voltage of 3842 VCC are oscillating and i couldn't fix it at 16V and supply cannot start because 3842 ON volt are 18V. when I remove the chip from circuit it goes to 18V accurately.
I tried without transformer but Vcc are not ok.
Do it work only with the startup circuit (a resistor connected to the 300VDC)?
I change 3842 twice but the problem is same. anybody solve the problem.
Click to expand...

what is the use of IC4 in the schematics attached in post#1 ?
 

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