555 astable drift

[NMNeil]

I'm still messing around with a CDI ignition for my tractor and as part of the project have ordered a few IR2151 self oscillating half bridge drivers.
While waiting for them to arrive I though I'd build the circuit as modules from discretes as a learning exercise. Put together a normal 555 astable circuit using a trimmer to get a 20KHz output, then swapped the trimmer for the closest 1% resistor. Ended up being 19.5KHz. Close enough for me.
Next day the frequency has dropped to near 14KHz!. Ambient temp is much the same, so it's not that. Left the 555 running for a few hours and it had changed again by a significant amount. Swapped the ceramic cap for tantalum, changed the 555 for a different brand, but the frequency keeps shifting.
Any ideas?

[Audioguru]

You are using fake 555 ICs from ebay?
Cheap resistors or capacitors from ebay?

I have used American Name Brand NE555 and TLC555 ICs and their frequencies are very stable, even if the temperature changes.

[Zero999]

I'm still messing around with a CDI ignition for my tractor and as part of the project have ordered a few IR2151 self oscillating half bridge drivers.
While waiting for them to arrive I though I'd build the circuit as modules from discretes as a learning exercise. Put together a normal 555 astable circuit using a trimmer to get a 20KHz output, then swapped the trimmer for the closest 1% resistor. Ended up being 19.5KHz. Close enough for me.
Next day the frequency has dropped to near 14KHz!. Ambient temp is much the same, so it's not that. Left the 555 running for a few hours and it had changed again by a significant amount. Swapped the ceramic cap for tantalum, changed the 555 for a different brand, but the frequency keeps shifting.
Any ideas?

This is hardly surprising. Did the supply voltage change much by any chance?

It's more likely the capacitor that's at fault than the 555 or resistors, irrespective of where you bought them from. Initially you used a ceramic capacitor which will have poor stability in the values required to make such a low frequency. Tantalum will be better but still poor. Try a polypropylene or polyester film capacitor.

If stability is important, then you shouldn't be using a 555 RC oscillator. Even a microcontroller with an internal RC oscillator will give better accuracy and stability and if stability is important, you need a ceramic resonator or better still, a crystal oscillator.

[NMNeil]

You are using fake 555 ICs from ebay?
Cheap resistors or capacitors from ebay?

Guilty on all counts I'm afraid

[NMNeil]

I'm still messing around with a CDI ignition for my tractor and as part of the project have ordered a few IR2151 self oscillating half bridge drivers.
While waiting for them to arrive I though I'd build the circuit as modules from discretes as a learning exercise. Put together a normal 555 astable circuit using a trimmer to get a 20KHz output, then swapped the trimmer for the closest 1% resistor. Ended up being 19.5KHz. Close enough for me.
Next day the frequency has dropped to near 14KHz!. Ambient temp is much the same, so it's not that. Left the 555 running for a few hours and it had changed again by a significant amount. Swapped the ceramic cap for tantalum, changed the 555 for a different brand, but the frequency keeps shifting.
Any ideas?

This is hardly surprising. Did the supply voltage change much by any chance?

It's more likely the capacitor that's at fault than the 555 or resistors, irrespective of where you bought them from. Initially you used a ceramic capacitor which will have poor stability in the values required to make such a low frequency. Tantalum will be better but still poor. Try a polypropylene or polyester film capacitor.

If stability is important, then you shouldn't be using a 555 RC oscillator. Even a microcontroller with an internal RC oscillator will give better accuracy and stability and if stability is important, you need a ceramic resonator or better still, a crystal oscillator.

The voltage was spot on 12v.
As I said I just built the circuit as a learning exercise but was surprised just how much variation there was in frequency even with the dubious quality of ebay components.
It does make you wonder about the logic of the knock off chip makers. They go to all the trouble of setting up a manufacturing facility, which can't be cheap, then produce substandard chips. Just as easy I'd imagine to make them of comparable quality to TI or NEC

[wraper]

Y5V/Z5U ceramic capacitor?