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?