Low SMPS current output problem

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mido_ameer

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I have 12VDC switching power supply adapter of my ADSL router
when I connect it to the mains the router doesn't come on, it has to stay 3 days until the router switches on and operate.
when I tested the power supply's output volt it is 12V (11.88V), but when I connect a load (12VDC-150mA computer fan) the voltage drops to 2.5V, where even the fan cannot rotate.
I tried to change the opto-coupler and its switching transistor but no result.
Can anybody got the knowledge help me, please.
 

Hi Mido
I think your adapter is kind of flyback converter . ( and perhaps a current mode one ! ) hence try to find current limiter section ( a high wattage resistor ) try to measure voltage across it under the load , and then try to measure fed voltage ( in transistor of your opto coupler ) how much is , that when you have putted a load in out put ? try to check your out put capacitor too . perhaps it has been destructed .
If so you'd better to change it .
Do you have access to the oscilloscope ? if yes try to see PWM signal when you've full load . and tell me , how much is it's duty cycle . is that around 50 percent ? how much is it's frequency ?
Best Wishes
Goldsmith
 

I don't know weather it is AC or DC (I guess AC), but I checked both types and it is absolute zero
What should I do ?
 

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