Hi,
Thanks for saying time needed, etc. Three seconds is huge/an eternity. Sorry, I can't see your schematic in detail, it's blurry when enlarged. What I'm not understanding from your thread is whether the relays turn on at power-up, or a delay is needed before they turn on.
Now to make a fool of myself with my babyish idea #1...
Explanation(s):
I was thinking last night whilst running through three variations on the same theme and contemplating as many 'No, that won't work because...' as I can with my knowledge on how to do this: temperature ruins timing, trying to compensate for temperature becomes a Sisyphean task full of frustrations and disappointments, at least for me. Then it hit me - why fight against temperature when it might be possible to use it to one's advantage...
My thought #1 is: A dirty little current source that will more or less track the change in the 4,300uF capacitors charging timing as the dirty little current source will also speed up with temperature. KISS principle, I suppose. As your timing requirements are not in the us or ms range but seconds, then milliseconds more or less should be irrelevant (?). I mentally factored in that the TL431 is a pretty loose voltage reference and it has some drift with temperature. I kept the comparator hysterisis as small as possible, LM193 goes from 0.3us to 1.3us switching across temperature. 10R resistor is because otherwise the relay output didn't switch off.
Excuse the odd schematic (and I had to cut short the exploration of the idea as I need to do some work now, so it's a simpleton's sketch of a simpleton's idea, not a turnkey project): 5V supply is presumably a voltage regulator if you have a 24V DC supply - that way you shouldn't need to worry about possible voltage droop. The 100V DC VS3 was just for the relay, I know it should be 265V AC. The 20V DC VS2 is because it saved me putting a Zener on the gate of the NMOS if it were 24V DC (I guess your relay coils operate on 24V DC). Current source capacitor and resistor are both 100ppm, by the way (and I personally feel the 2N2907A model is almost too good compared to reality, but still, what would I know).
For some reason, in the simulations I did, the relay turns on at 0ºC but at 50ºC, 100ºC and 150ºC it doesn't turn on. The coil does turn on and off, as you can see from 'NMOS in'. I'm not so bothered about that as I don't understand how the relay from the simulation tool is supposed to be configured, it doesn't say anywhere what the maximum voltage is on either side of it, just 35mA hold current and 25mA dropout current and stuff about 100mOhm series resistance...
How preposterously far-removed and useless to you is this idea as a general idea?