Greg, I'm not sure I could go along on the 555. I use them alot but I can't figure an easy way to use one for BigB's application. I have never used one for timing at the same time power was applied with a trigger event. It's the same with the LS7210 that I like to use for switched timing. It too, has a negative trigger and I wouldn't trust it at power up. By the time we add all the goodies to make the timer fit the app, I'll bet the original SCR schemes we used are as good or better.
I thought of other means to do what he wants, using bipolars and TMOS (BS-170) transistors. Then I started wondering how stiff his supply might be and what would happen when the relay picked. Now we have to add hysteresis to account for that. I'm betting that BigB just needs a little time for things to settle down and wouldn't mind a little extra. I keep coming back to the SCR method being cheap, simple, reliable, and probably the best.
Scott, promise not to laugh at me. I use Generic Cadd 6.0 It was the last version before AutoDesk gobbled up Generic Software. I bought 3 packages for work and a co-worker and I bought it for home at $65.00 each. It was our department's first use of CADD. Since then, everyone else has switched to the latest version of AutoCAD. They try to get me to switch over but I resist. The AutoCAD 2004 in my computer isn't even working now because it's confused over a license file that must have been moved. I just export files as DXFs if we need them for AutoCADD. Fills are the main problem. I'll send you a sample so you can see what I mean.
Greg, you make a fine point. I'll freely admit guilt. The NEC or U/L don't even cross my mind when I'm thinking about 12 volt relay TDO circuits. That could certainly come into play, depending on the application.
Joe
