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Pls help anybody knows the burnt resistor (R1) value of the Vcc of UC3844?

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#1supertech

New Member
What is your application of the UC3844, and where is the 560VDC (typo ?) coming from?

560VDC?

What are you using this on?

Frank
 

Blueteeth

Well-Known Member
The UC38** SMPS controllers are almost always powered by an auxiliary winding on a SMPS transformer, not by the high voltage line it is switching. There may be resistors connecting the chip VCC to the HV line, but its likely to be a high value.

If the resistor is burnt so you can't see its value, then at least take a photo graph of it, as its size/shape will indicate whether its a power resistor, or simply a small signal bleed resistor.

As Frank said....560V? are you sure? Sounds like rectified three-phase power....
 

mvs sarma

Well-Known Member
For an input of 110V 60Hz AC and using bridge rectifier and conventional single cap filer, ON semi recommends 56K series resistor for the starting power supply.
in this case the rectified DC voltage can be 160V approx.
as such for 500V DC expected, it would be better to have 180K and perhaps 1 watt is better, though used for a short while. even 1/2 watt MFR might work with some risk
as the voltage is said to be 500V DC , why not use two numbers of 91K 1/2 watt in series to make up for near 180K.
after start, the aux power supply will have to take over to reduce burden on these resistors.
 
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