Current limiter that doesn't

Hi all,

I'm stuck. I am trying to limit the current flowing through a relay by using two transistors and a current-sensing resistor. Schematic below. The Spice model works beautifully, but the real world circuit does not turn off. Ever. It just fires once the first time the timing pulse arrives, then stays on.

My goal is 70 mA through the 70 ohm load above the Q1 collector. The measured current through the load is around 100 mA - so straight off out of spec. The base of Q2 -- which in my mind is supposed to turn off Q1 -- stays at 0.7 V which is about what I would expect steady state. When I measure the base of Q1, I see it goes nicely back to zero once the timing pulse (V2) turns off. Why doesn't the Ic go back to zero in the real circuit?

Note: The power source on the high side of the relay is very noisy, which is why I modeled it as DC+pulses.

Thanks!

doug

Here is the LTSpice:

Version 4 SHEET 1 880 688 WIRE 416 -48 288 -48 WIRE 288 -16 288 -48 WIRE 416 48 416 -48 WIRE 288 96 288 64 WIRE 736 160 272 160 WIRE 736 192 736 160 WIRE 608 240 512 240 WIRE 624 240 608 240 WIRE 672 240 624 240 WIRE 416 272 416 128 WIRE 512 288 512 240 WIRE 416 336 416 272 WIRE 416 336 400 336 WIRE 608 352 608 240 WIRE 80 384 -112 384 WIRE 272 384 272 160 WIRE 272 384 160 384 WIRE 320 384 272 384 WIRE 336 384 320 384 WIRE -112 400 -112 384 WIRE 400 464 400 432 WIRE 400 480 400 464 WIRE 512 480 512 368 WIRE 512 480 400 480 WIRE 272 512 272 384 WIRE 400 512 400 480 WIRE -112 560 -112 480 WIRE 272 624 272 592 WIRE 400 624 400 592 WIRE 400 624 272 624 WIRE 608 624 608 432 WIRE 608 624 400 624 WIRE 736 624 736 288 WIRE 736 624 608 624 WIRE 400 672 400 624 FLAG 288 96 0 FLAG -112 560 0 FLAG 400 672 0 FLAG 400 464 Ve FLAG 416 272 Vc FLAG 320 384 Vb FLAG 624 240 Vb2 SYMBOL npn 336 336 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL npn 672 192 R0 SYMATTR InstName Q2 SYMATTR Value 2N3904 SYMBOL res 400 32 R0 SYMATTR InstName Relay SYMATTR Value 70 SYMBOL res 384 496 R0 SYMATTR InstName R2 SYMATTR Value 10 SYMBOL res 176 368 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 500 SYMBOL res 288 608 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 4.7k SYMBOL res 624 448 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R5 SYMATTR Value 4.7k SYMBOL voltage 288 -32 R0 SYMATTR InstName V1 SYMATTR Value PULSE(6 8 0.1 0.00625 0.00625 0.00001 0.025) SYMBOL voltage -112 384 R0 SYMATTR InstName V2 SYMATTR Value PULSE(0 2.4 0.1 0.00001 0.00001 1 2) SYMBOL res 528 384 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R6 SYMATTR Value 200 TEXT -88 24 Left 2 !.tran 5