Smoked Ignition Coil

[hsdropout]

Got engine wired up to run in basic form similar to a test stand (see picture, less the tachometer wires). On/off switch and push button for starter. With coil unplugged from dist., turned engine over with push button, everything good. Repeated a couple time, turn engine over for several seconds each time.
Leaving switch in on position and checking with tester to coil, ok 12V+. After the switch was left on for approx. 1 minute, the coil starts sizzling and smoking! Killed power but I'm afraid I fried it.
Any clues to why this happened?
Ran red "hot" wires to positive at coil from distributor and on/off switch; black from distributor to negative on coil and grounded to firewall (not to engine as shown but tested ok).
Coil should be able to take constant 12V, correct?
Summit 850500 remote coil for HEI distributor.
Wasn't plugged in to distributor.
Anybody have this happen before? Thanks for any help or comments..........