How to build a current sensor?

[random robots]

Can you post shematics if possible? I would like the sensor to detect if the current to the motor is less than 3A.

Thanks,
Patrick

[Soeren]

Hi,

If you can afford to drop 0.7V, a resistor of 0R47 in series with the positive supply and a PNP transistor with the emitter to the supply side of the resistor, the basis on the other side and a pull down resistor from the collector to ground.
The collector will go positive with a current draw of more than ~1.5A and low otherwise.

[random robots]

okay, this is what I got from this.

resistor>PNP>pull down resistor>ground
current-/

Am I correct?
Also what does the pull down resistor do in the circuit?

Patrick
edit: no, i do not want to have a voltage drop at all if possible. thanks for the help though.

[Choco_liger]

If your worried about it damaging it, you could use a 3amp fuse.

Won't really measure it, but it can tell you if it's higher than 3amps.

[dunk]

If you can afford to drop 0.7V, a resistor of 0R47 in series with the positive supply and a PNP transistor with the emitter to the supply side of the resistor, the basis on the other side and a pull down resistor from the collector to ground.
The collector will go positive with a current draw of more than ~1.5A and low otherwise.

there are transistors designed for a purpose similar to Soeren is describing.
i use http://www.zetex.com/3.0/pdf/zxct1009.pdf .

it differs from a simple transistor in that instead of a simple high/low output,
the ZXCT1009 will give you an analogue output voltage proportional to the input current.

whether you choose to use the ZXCT1009 or not, it's datasheet will clarify the layout Soeren described.


dunk.

[Soeren]

Hi,

edit: no, i do not want to have a voltage drop at all if possible. thanks for the help though.

OK, This device is the closest to zero drop that you get.

[random robots]

dunk, your link appears to be broken

[Soeren]

Link appears fine, but I get a zero length file.

This works (darned, Zetex was sold )
http://www.diodes.com/datasheets/ZXCT1009.pdf

[random robots]

i can only get to the cached file.

[Soeren]

http://www.diodes.com/datasheets/ZXCT1009.pdf

[random robots]

thanks. new question- how do i connect the motor to the current sensor. does it go from v sense- to ground, or does it connect to the Iout as well?

Thanks,
Patrick

[Soeren]

The datasheet shows!

[random robots]

what do i use vout for? Scratch that. I think i got it.

Patrick