Calculate UC3842 Frequency

[baby_1]

Hello
as you see this schematic the frequency is 40Khz

but i calculate the frequency with two formula and find the equvalent capacitor with Thevenin.could you help me on this?whats formula and the way to calculate.

 

[goldsmith]

Hi
See the datasheet . please . it contain some tables and some formulas too . you can use tables or formulas . simply
Best Wishes
Goldsmith

 

[baby_1]

Hello
i found and calculate with these formula that both of them return me the same result but it is different from the circuit schematic frequency that i show at the first post

could you calculate for me?

 

[goldsmith]

You have to select one of them ( R or C ) randomly and the equation will give you , the other one as well .

 

[baby_1]

Thanks Dear goldsmith
as you see this schematic the frequency is 40Khz


could you define the equation and how they create 40khz frequency?

Ceq=.0018 microfarad (Thevenin)
R=10K
so frequency should equal to 95Khz not 40Khz

if we assume on of the Capacitor instead of Equal Capacitor we have

C1=.01 microfarad
R=10K

F=17.2Khz (not 40Khz)


C1=.0022 microfarad
R=10K

F=78.18khz(not 40Khz)

how did they find 40Khz frequency?

 

[goldsmith]

I see that what is your problem . a simple way for you , is : add a potentiometer ( multiturn ) instead of RT , and then calibrate it for your desired frequency .

 

[baby_1]

Thanks goldsmith
but i want to calculate instead of experimental exercise.

 

[goldsmith]

The other way is following the table and curve . did you tested it ?

 

[baby_1]

Dear goldsmith
i want only calculation of the RC because maybe i should use different and varies resistor and capacitor that i can't find them in the curve and datasheet specification

 

[goldsmith]

Did you understand , that what i told really ?! i think not . the table is logarithmic , and you can select your desired values . in one table there are some curves .

 

[baby_1]

Dear goldsmith
i know what you say,excuse me that i persist on something that engage my mind.i only try to calculate the output frequency of Uc3842 with datasheet's formula

 

[godfreyl]

I think there is a mistake in the Texas datasheet. The graphs give the same answer as the formula, but the circuit has wrong values.

The NXP/Philips datasheet has a circuit with correct values. You can find it here: https://pdf1.alldatasheet.com/datasheet-pdf/view/19833/PHILIPS/UC3842.html

I looked at datasheets from ST and Fairchild too. They show the same graph and same formula, but no example circuit to compare.

 

[baby_1]

Yes godfreyl

your right.i calculate the NXP circuit with the formula that i indicate the frequency is 37210.2Hz or 37.2Khz