homework relay question

[maie]

so i got this paper work to do, and i got stuck with something.
this is the circuit-


that my Vin-


relay is Normally Open.
L=1Hy, rL=100 ohm, closing current=40mA, opening current=20mA.

transistor:
VCEsat=0.2V, VBEon=0.7V, B=50.

all idial.
V+(VA=12) = 8V , V+(VA=0) = 4V.

and the questions-
1) specify the periods when the B-C contacts of the relay-
a- brief
b- open
2) R4max

anwers are-
1a)
29.82 mSec > t > 10.8 mSec
1b)
29.82 mSec > t < 10.8 mSec
2)14.12 Kohm

help?

 

[BradtheRad]

There is more than one part to calculating the answer.

You must determine at what times the input makes the op amp output go high, and whether it's briefly or a long time.

Transistor response also plays a part. Is it saturated or not? The transistor is biased by the op amp, but only some of the op amp signal goes to the transistor.

Another part is to determine how many mSec it takes for the relay coil to reach 40 mA after power is applied. The coil is 1 H and 100 ohm resistance, and you must also add transistor Resistance_On.