Discussion in "Project Help" started by    Help    Jun 16, 2007.
Wed Jun 20 2007, 08:28 pm
Ya, thank you very much. You let me understand alot of thing.
I also no plan to use ADC08034. Because i can't get this IC in my country and i think ADC0834 also mostly the same.
I already order LT1787HV and LTC1286 will coming soon.
Ya, may i know ADC can measure Negetive voltage and if negetive supply then how can i measure the current?

Thank You.
Wed Jun 20 2007, 10:56 pm
well.. according the rule of electronics.. if the voltage is -ve then.. the current must be flowing in opposite direction.. so current measurement must be the same way.. just we get -ve value.. coz of opposite direction. maybe not sure..
i think there must be some provision to support -ve voltage in the IC.

off-topic: where are you from by the way?
Mon Aug 25 2008, 11:10 am
hi...I also doing the project about measure the current by using 8051 microcontroller but I have no ideas for design the circuit.
Thanks :-)
Mon Aug 25 2008, 07:52 pm
Hello Christie,

welcome to our forum !

measuring Current using 8051 is simple, it is done same way as measuring voltage.

first we need to convert the current flowing in the circuit to voltage, for that we use either : a sense (shunt) resistor,a non contact torridal transformer or a Current transformer (often called CT).

for simple current measurements we can use Sense resistor of any value from 0.1 ohm to 1 ohm. now the sense resi has to be placed in the target circuit to read the current flowing, this resi can be placed in the negative path ( i,e between the load and ground) this is called as "low side sensing" and when we place the sense resi in positive path ( i,e between positive supply and load) this is called as "high side sensing"

usually, low side sensing is easy and requires low components, so we'll stick to this type of current sensing for time being, the following figure shows this setup :

now lets recollect our famous Ohm's law, V= RI , for eg : in the above circuit the sense resistor 's value is 1 ohm and when 1 ampere of current is flowing thru the circuit , then 1 Volt will be developed across the sense resistor.

V = R x I

I = V / R

here I = 1(volt) / 1(ohm)
i,e = 1 (ampere)

so we read this voltage developed across the sense resistor using a ADC and micro, this voltage signal can be fed to a Op-amp and then fed to a ADC to remove noise. the power dissipated in the Sense resistor needs to be calculated; its given by I2R ( here it is 1 watt, so to be on safe side we can use 5 watt wire wound 1 ohm resistor)

for larger currents the sense resistor method does'nt work( becoz of large amounts of heat generated in resistor) , thats where we use CT or Torroid CT.

Arun

nischay kumarchristiekurrodu like this.
Mon Aug 25 2008, 08:38 pm
In industries, for high AC current measurement,C.T.s are used whereas for the high DC currents a DC shunt resistor is used. the shunt resistance is designed such that the output voltage from the shunt is always 75mV/150mV for full scale operating current.e.g. if u buy a 50A DC shunt, then the potential difference across the shunt when 50A is passed through it is 75/150mV . this DC voltage can then be given to a suitable op-amp amplifier to amplify the signal to the required level and given to the ADC as explained by arun.

kurrodu like this.
Wed Aug 27 2008, 11:43 am
i have try to draw low side current sensing circuit but i cannot get the output in AC form.
and i not sure that my output is correct or not.
thanks

Wed Aug 27 2008, 01:08 pm

Where is the load ? and what is the value of Sense resistor ?

Arun
Wed Aug 27 2008, 02:43 pm
the load is R5 then the sense resistor is R4 n value is 1 ohm
Wed Aug 27 2008, 07:40 pm
what do you call this connection ? ( i have marked ) high side or low side sensing ?

pl.refer to the diagram i posted earlier and connect accordingly.

Arun
christiekurrodu like this.
Wed Aug 27 2008, 07:55 pm
@christie

maybe you haven't noticed the "thanks" button in the posts, its there for some purpose, so use it for the people who are spending time to help you, our Moderators especially Mr.Arun has explained the concept in great detail and also Pdi33, so you owe them Big time.

( i have seen in this forum, folks from Malaysia are seldom Thankful and very active in posting their problems)
christiesachudam like this.

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