[sdiy] OT delay circuit needed

[David Brown]

This is way off topic but at least the circuit is related.

I have two electric space heaters in my shop that go out nearly every 
year.  They're pretty simple:  three 240V AC heater elements and a 
control relay, and a 24V transformer for the thermostat circuit which 
activates the control relay and a thermal delay relay for the 
fan.  The thermal delay relay is a simple device consisting of a 
heater element and a bi-metal contact to control the 240V AC fan.  It 
delays slightly on startup so that the fan doesn't blow cold air, and 
continues the same amount of delay after the heater elements shut off 
to cool the heater down.  I haven't measured the delay but it's 
probably somewhere between 30 and 60 seconds.

These thermal relays go out every year.  The heater overheats and the 
safety circuit cuts out.  They're also expensive.  I've decided to 
design this part out and replace it with a simple delay circuit.

I need to control a 240V fan so I thought I would use a solid-state 
relay.  I'm looking for the simplest and most reliable circuit that 
uses few parts since I have to retrofit this into the heater.

What's nice about the thermal delay is that the thermostat simply 
turns it on or off with 24V AC.  What's unique is that the delay 
continues after power is removed.

I could live with a circuit that has no delay on startup (e.g. the 
fans start right away) but I need the 30 - 60 seconds of delay AFTER 
the power has gone away.

I was wondering if I could charge up a capacitor that would have 
enough current to keep a solid state relay on for that 
long.  Otherwise I have to build a small power supply from the 24V AC 
to power the delay circuit.

Anyone have some simple and reliable delay circuits or ideas that 
work when the power goes away?

Dave