LMC555: Frequency/Duty Cycle

Ritter,

Try again to attach the schematic. For 1Hz timer, propagation delay will not be a factor. Capacitor tolerance and leakage will be a biggest factors for accuracy.

Ritter,

The formulas and resistor and capacitor components look good.

What is the biggest care about for output waveform [high time, low time, or frequency]?
Do you have a timing capacitor picked out? It is key to output timing accuracy.

The Zener diode should work, but why was it chosen over a simple diode like 1N40001 or 1N4148?

The biggest concern is high time because of current concerns using optocoupler CPC1788. I am using a zvs capacitor charger set at 400VDC to directly charge a 500v, 100uF capacitor which will be pulsed via the 555 through the opto.
I was planning to use an electrolytic 50v, 4.7uF cap. Any other suggestions would be appreciated.
The zener was chosen because of availability. I also have 1N4001 if that would be more applicable.

I thank you for your time thus far. Much appreciated.

Ritter,

I put the values into a TI-Tina TLC555 model. LMC555 and TLC555 will perform the same in this app.

TLC.LMC555 15ms.TSC

I saw a repeating 17ms high time. The diode voltage drop makes the high time longer than the calculation.

The very first high time was 26ms because the timing capacitor starts at zero.

The capacitance value tolerance of the timing cap will affect timing so chose tolerance that is acceptable.

If longer first pulse is unacceptable, then a 1Hz timer followed by a 15us one shot can be used.

 

Another question on this same circuit if I may. I built this circuit on a bread board to start, for testing using a 9V battery. It gave me the 1Hz pulse that I was looking for. I then soldered it on a perf board and connected the 12Vdc to it. The results were as follows;

Bread Board 9V battery 1Hz (per multimeter Hz setting) LED flashing at 1Hz
12Vdc wall wort 5kHz " " " " LED flashing at 1Hz

Perf Board 9V battery 50Hz " " " " LED not flashing (on constant)
12Vdc wall wort 4kHz " " " " " " " "

Any idea what I am experiencing ?

Ritter,

If the power source changes performance then focus on the supply voltage to make sure the supply voltage is good. Check for AC voltage on supply. Keep in mind that multimeter frequency reading may have issue with low frequency as higher frequency noise on supply may be read by meter instead. A low pass filter between signal and multimeter should help.

Ron,

I am getting excellent results for about 15-20sec, then drift and then erratic results from this 555 set up.  You mentioned above using a 1Hz timer followed by a 15us one shot.  What do you suggest for "each" and would there be any appreciable drift from such a set up?  The high (15us) is used to discharge a capacitor so that is not critical but consistency and repeatability is for any high and low settings.  I am using this method to automatically charge and discharge TI's demo board #TPS65560 EVM-165 without pushing the buttons.  Attached is a schematic.  I show a external power mosfet but I can still use the on board IGBT.  The PNP is used for charging the cap with a 3.5v signal and the NPN is used for discharge the cap with 3.5v via jumper pins on J5 on the demo board or using the external mosfet with a 10-12v signal.  Bottom line, I need to be able to generate approx. 1.25sec output of 3.5v to charge the capacitor and for that signal to end and another signal of 3.5v or 12v for X seconds to discharge the capacitor.

Thank you in advance.

   

Ritter,

Do you know the resistance setting for R1? Perhaps it is too low.
Did you want a high time near 15ms or 15us?

Sorry to bother you with such trivia. R1 was 4.7K, and I have moved it up to 77K with R2 at 307K. I was shooting for 15ms but a this point I will take anything in that area that is consistent.

Ritter,

4.7k and 307k should be about 15ms high time and period close to 1 second.
77k and 307k should be 277ms high time and period close to 1.27 second.

For debug, use oscilloscope to check timing cap voltage (pins 2,6) and output (pin 3) to make sure the timing is working correctly.