produce digital signal, based on a fixed Amp threshold?

[settra]

Hello forum . i want to build this circuit, and i need some help! :


basically, what i want to do, is this :
the "LOAD" circuit, will usually draw , lets say 100mA. But some times, it will draw 300-400mA. (this numbers are not exact, but i can make them exact, with current limiters i suppose).
I want the "SENSE" circuit, to detect, when the current is 300mA+ , and only then, produce a signal, so that i can oparate the relay !

Any simple way , that i can make the SENSE circuit?? thanks !!

 

[MikeMl]

Here is a simplistic solution:



Plot shows V(out) vs load current at temperatures from 0C (green) to 30C (lt. blue).
It drives a relay with a coil resistance > 100Ω.
Its trip point is ~0.65/R1 Amps.
It is somewhat affected by temperature.
Biggest problem is the peak ~1.5V drop across R1 when load current is 400mA.


Ratchit See Ratch, you worry about minutia; I just use LTSpice and get the job done...

 

[settra]

cool ! thanks for the quick reply !!
the problem is, that most propably , the resistor coil will be ~60 Ohm (automotive relay)..
will this transistor be able to drive the coil?? ..

the voltage drop at 400mA is not a big problem, becouse the "peak" current, will always be "momentary" , and the "load" arledy works with vRreg at 5v !

But anyway, if i use a smaller, 5W resistor, between 2-3 Ohm, the voltage drop will be even smaller. correct?
thanks again!

 

[MikeMl]

Here is a version that drives an automotive sugar-cube relay. I added the Darlington to drive the higher current coil. The sim shows the effect of varying R1 through the standard values of 1.2, 1.5, 2.2, and 3.3Ω

 

[MikeMl]

Here is how to "fine tune" the trip point.

 

[settra]

thanks man! you are the best !!!

 

[settra]

hey ! so i am experiancing the following problem :

the relay indeed does not close for small currents
(relay coil is 72ohm btw)
BUT, if the relay closes for some reason (etc , larger current), then after the current is removed, the relay does not de-energize... the voltage on its coil is around 5volts on that time..
any idea what might be causing this?? i used the 2.2ohm R1 !

edit : NVM, the Q1 was propably burned/not working correctly. now everything works! thanks again !

 

[MikeMl]

...
edit : NVM, the Q1 was propably burned/not working correctly. now everything works! thanks again !

I was going to suggest that Q1 might be bad or leaky. The relay will provide some hysteresis; its release current is less than its pull-in current, but the slope of the red plot in post #4 indicates that the on/off differential should be fairly small...