# how to detect a specific current value?

Discussion in 'General Electronics Chat' started by aris, May 14, 2010.

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1. ### aris Thread Starter Member

May 5, 2010
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Hi all,

my next target is to build a circuit that will be able to detect a specific level of current out of a 12V DC power source (e.g. 500mA or below) and when this is reached to get a 12V output signal. Does anyone have any idea of how I could do that?
Thanks once more!!

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
You can use a low value resistor (0.1 R or less) in series the power source and measure the voltage across it. Since the voltage across the resistor is proportional to the current flowing then you can measure the current. The disadvantage of this is that lot of power is lost in the resistor at high current. Another solution is to use a hall effect current sensor.

3. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Measure the voltage drop across a low-value resistor in the load return path using a comparator.
E=IR. If R=0.1 Ohms, then 500mA * 0.1 Ohms = 50mV.

[eta] Whoops, Mik3 - I'm slow today

4. ### aris Thread Starter Member

May 5, 2010
45
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and how would I sort of select the values of 500mA and below in order to get an output?

5. ### mik3 Senior Member

Feb 4, 2008
4,846
63
What do you mean?

6. ### retched AAC Fanatic!

Dec 5, 2009
5,201
312
Ok, you want to wait until 500ma is used then close a switch?

So once 500ma then sw=1 ?

You may be able to use a capacitor. Once the cap absorbs 500ma, it releases it, closing a switch.

Im having a little trouble understanding the question.

You may be able to us a resistor in parallel to your circuit to allow a signal to pass once the circuit reached 500ma.

Am I throwing you off?

7. ### aris Thread Starter Member

May 5, 2010
45
0
What I want is when the current is reaching a certain level (e.g. 500mA) to get a 12V output which I want to use in order to trigger a delay which is based on CMOS 4000 series IC.I want to do this in analogue (without a micro-controller). Roughly what retched describes with closing a switch.

Dec 5, 2009
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9. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Use a comparator IC to compare the voltage of the current sense resistor with a reference voltage which will determine when the comparator outputs a signal.

10. ### aris Thread Starter Member

May 5, 2010
45
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this looks like it...can I ask just how can I vary the amount of current I want to be the threshold for this circuit?is there any simpler way to do this?shunt resistor ?

11. ### mik3 Senior Member

Feb 4, 2008
4,846
63
One input of the comparator will be the sensed voltage from the current sense resistor (V=I*R). Determine the desired current and then the voltage V, then set the reference voltage of the comparator (second input) to that value (V).

12. ### aris Thread Starter Member

May 5, 2010
45
0
a bit more of asiking from me! could someone provide a schematic for such a comparator?
once we are talking about a slight change between the two voltages would this be reliable?
should I use a schmitt trigger configuration instead?

thanks guys!!!!

Apr 20, 2004
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