555 Timer, Astable Mode Question

[micro571]

Hi,

When I'm using the 555CN timer in astable mode, the first pulse is about 3x longer than the sequential pulses. Could someone tell me why that happens and how to minimize it?

Thanks.

 

[Roff]

Well, I don't think the theory predicts more than about 1.6 (instead of 3 times), but anyhoo...
This should get you pretty close. If you need it more precise, we could add several more parts and get really close. If you add a large cap from pin 5 to GND, similar problems could befall you. Keep this cap to 10 or 100nF, or smaller if you are running at high frequencies.
Oh yeah, It happens because the capacitor is initially discharged, but subsequently cycles between VCC/3 and 2*VCC/3. The first time, the charge cycle is from 0 volts to 2*VCC/3. The additional circuitry causes the capacitor to start charging from just below VCC/3 on the first cycle.

 

[micro571]

Ok. Thx for the circuit.

I'm running at low frequencies (4s / 2s), and I wasn't using any cap on pin 5.

First I'll try adding the cap, then building the extra circuitry if it isn't good enough.

 

[Roff]

Ok. Thx for the circuit.

I'm running at low frequencies (4s / 2s), and I wasn't using any cap on pin 5.

First I'll try adding the cap, then building the extra circuitry if it isn't good enough.

The cap on pin 5 won't make the problem go away (as you will discover).