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Electronics => Electronics => Topic started by: random robots on July 10, 2010, 01:59:31 PM

Title: How to build a current sensor?
Post by: random robots on July 10, 2010, 01:59:31 PM

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

Title: Re: How to build a current sensor?
Post by: Soeren on July 10, 2010, 02:47:10 PM

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.

Title: Re: How to build a current sensor?
Post by: random robots on July 10, 2010, 02:55:05 PM

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.

Title: Re: How to build a current sensor?
Post by: Choco_liger on July 11, 2010, 03:29:32 AM

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.

Title: Re: How to build a current sensor?
Post by: dunk on July 11, 2010, 04:32:44 AM

Quote from: Soeren on July 10, 2010, 02:47:10 PM

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 (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.

Title: Re: How to build a current sensor?
Post by: Soeren on July 11, 2010, 06:29:58 AM

Hi,

Quote from: random robots on July 10, 2010, 02:55:05 PM

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

OK, This device (http://www.lem.com/hq/en/component/option,com_catalog/task,displayserie/serie,FHS%2040-P--SP600/output_type,/) is the closest to zero drop that you get.

Title: Re: How to build a current sensor?
Post by: random robots on July 11, 2010, 06:50:05 AM

dunk, your link appears to be broken

Title: Re: How to build a current sensor?
Post by: Soeren on July 11, 2010, 06:52:23 AM

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

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

Title: Re: How to build a current sensor?
Post by: random robots on July 11, 2010, 06:57:46 AM

i can only get to the cached file.

Title: Re: How to build a current sensor?
Post by: Soeren on July 11, 2010, 07:00:38 AM

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

Title: Re: How to build a current sensor?
Post by: random robots on July 11, 2010, 07:11:11 AM

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

Title: Re: How to build a current sensor?
Post by: Soeren on July 11, 2010, 09:16:16 AM

The datasheet shows!

Title: Re: How to build a current sensor?
Post by: random robots on July 11, 2010, 02:37:11 PM

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

Patrick