It's not nescessarily spikes that kill your circuits... it's the fact you are CONSTANTLY running them beyond their maximum ratings - they are just going to die, if not today, then tomorrow.

A lot of people take the 24vAC, rectify and smooth it, then apply that straight to circuits and regulators. Take a selection of typical 24vAC transformers used in industry (eg HVAC) and measure their off-load terminal voltage - you might be shocked. The smaller the VA, the worse they are - 30% is quite typical from some famous 'trustworthy' manufacturers. Factor in for example a transformer designed for 220-230vAC input, then apply 240vAC which is in use in some countries like the UK, and again this adds to overvoltage on the secondary.

Remember, even if it is EXACTLY a picture-perfect 24vAC, rectifying and smoothing gives you almost 34vDC... now go look at the Datasheets for LM324, CA3140, CA516E and all the others you're using... then come back and tell me what their Vmax is. Now also tell me why you are still surprised that your designs are flaming out.

If the world was flat, you're sailing right on the edge. Simple choice - either select devices with a higher Vmax, or, add additional circuitry to ensure the maximum device tollerances are not exceeded.

Melanie