# Help 555 timer problem

1. Jan 17, 2008

### N468989

[SOLVED] help 555 timer problem

hi all, i am designing a 555 on astable mode. it is designed to operate at 100kHz with a duty cycle of 95% ( and i get 48Khz and duty cycle 93.5%).

i calculated all the values necessary:

Ra = 12.987 kOhm
Rb = 721.5 Ohm
C = 1nF
F = 1.44/ ((Ra + 2Rb)*C)

i used thes formulas :

tH = 0.693*(Ra + Rb)*C
tL = 0.693*(Rb)*C

T = tH + tL

i searched everywhere and found out the max frequency is 500Khz so im not sure where the problem is....i need the 100kHz because i am designing an infrared modulator and the central frequency for my filters is 100kHz

can anyone help me thanks.

Last edited: Jan 17, 2008
2. Jan 17, 2008

### mgb_phys

1nF is a bit small, capacitors this small aren't very accurate and the effects of stray and gate capacitance start to matter, you might want to redesign it with at least 10nF.

3. Jan 17, 2008

### N468989

just tested with 10n and 100n ...got 40Khz this time....ive double checked everthing.

i just simulated it on multisim and i got 45.9kHz ....but by my calculations it should be 100kHz

any ideias? i think this is very odd

Last edited: Jan 17, 2008
4. Jan 18, 2008

### N468989

--SOLVED--

the multsim 555 design wizard is not correct. there is a problem with it... in the lab i changed the IC (it was a bit damaged) and i managed to adjust it to my desired frequency / dutycycle.

btw.... thanks for trying to solve my problem