# Help needed in calculating Rt and Ct of 3524 to generate ...

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#### nino220022

##### Newbie level 4
Help is needed in calculating the value of the resistor and capacitor (Rt and Ct) to generate an oscillation frequency of exactly 50Hz from the 3524 IC .

I can't in anyway figure out what those 2 values should be !!! eventhough the formula seems easy but .. i don't know !

F =1/RT*CT
Where
RT is in kilo Ohms
F is the frequency in Hz

Thanks

P

#### pauloynski

##### Guest
Re: Help needed in calculating Rt and Ct of 3524 to generate

The datasheet shows a graph relating R,C and the oscilattor period. Take a look at https://www.national.com/ds/LM/LM2524D.pdf . Try starting using C = 1nF and R = 8k2 and adjust if necessary.

#### VVV

Re: Help needed in calculating Rt and Ct of 3524 to generate

Pick a resistor that is within the allowed range, say 100k.
Then calculate the cap as 0.2uF for 50Hz.
Other values will also work if you stay within the limits.

So now that you have established a starting point, change the cap value to a standard 220nF=0.22uF and recalculate the resistor, because it's easier to get resistors with "strange" values, than capacitors.

So the resistor becomes 90.9K, which happens to be a standard 1% value.

#### nino220022

##### Newbie level 4
Re: Help needed in calculating Rt and Ct of 3524 to generate

Hello and Thank's for the reply, if we apply the values you are giving against the given formula it doesn't match for 50 Hz :

F=1/(R x C)

1/(100 x 0.2) = 0.05 Hz
1/(90.9 x 0.22) = 0.05 Hz

What do you say ?

Thanks
==============================================
VVV said:
Pick a resistor that is within the allowed range, say 100k.
Then calculate the cap as 0.2uF for 50Hz.
Other values will also work if you stay within the limits.

So now that you have established a starting point, change the cap value to a standard 220nF=0.22uF and recalculate the resistor, because it's easier to get resistors with "strange" values, than capacitors.

So the resistor becomes 90.9K, which happens to be a standard 1% value.

#### VVV

Re: Help needed in calculating Rt and Ct of 3524 to generate

I say your frequency is in kHz, not Hz.
Look at the formula, it's just the regular f=1/T. So for f=50H, T=20ms. Now multiply 90900 ohms by 0.22E-6 and see that you get 19.998 ms or a frequency of 50.05Hz.

Look at the picture, that came out of TI's SG3524 datasheet. For 100k and 0.1uF you get something above 100Hz, so chances are you will get the right frequency with the values I suggested.

The problem seems to be that 0.1uF is the maximum value and 100k is the max for the resistor. So you would be operating outside the guaranteed limits, but my guess is it will work. Check out the datasheets from other manufacturers, they may allow wider limits for the timing components.

Or, set it up for 100Hz and use only one output, because the frequency should be half in push-pull mode.

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