AC low current measurement

Status
Not open for further replies.
N

nima_1981

Member level 3
Joined
Apr 22, 2010
Messages
61
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Location
Ocean Mind
Activity points
1,877
Hi My friends.
I have a problem with measure ac current , I have a 100 to 200 ma AC @ 1KHZ current that should be measure via ADC , first step i use acs715 but low current cause very low voltage plus noise on acs712 output so i can not use acs712 for this range of current .
anyway i should use very precise shunt but important issue i can not find precise shunt in this region i can not find that , did you have any idea can measure that range of ac current without shunt

Thanks ,

output of acs712-5


my shunt ( 10 % Error)


Current across the shunt
 

the shunt value you have chosen looks to be in the 10 ohms range.

reduce that to very low value , based on the permissible shunt drop.
 

Dear srizbf
this method Dependent on shunt error .for measure that we need precise shunt do you have any method can measure current without shunt resistor as i ask that in previous post ?
 

Any current measurement method involves errors. Shunts with 0.1% initial accuracy are available and will outcompete medium accurate current sensor like Allegro ACSxx in any case. For higher AC currents, current transformers are first choice. But the secondary current will be usually converted by a shunt...
 

200 mA peak to peak or RMS?
What is the required resolution?
What is the maximum allowable burden?

An example:
Assuming 200 mA pk-pk: What I would do, is to employ a 1 ohm resistor, which can be found in 0.1% tolerance. That will provide 200 mV pk-pk.
Assuming your ADC has a 5 volt supply and a 4.096 volt reference, you require to amplify it 20.48 times, with a low noise, R2R-output-opamp. There are dozens of modern opamps which can achieve that.
Because even the best R2R opamps do not swing all the way to zero volts, it may be a better idea to amplify it only 20 times, and give yourself some headroom. The actual value conversion can then be achieved on software.
 
Hi,

200mA is not "low current". Indeed it is a very good measurable value.

You need a good quality shunt. No 10% type. And don't use a wirewound one, because it has a lot of series inductance.
Use a low inductance one. If you are worrierd about the name "shunt"...it's just a low value resistor. Any low value resistor is O.K.
(Low inductance, low tolerance)

The value should be between 100mOhms and 1Ohms, maybe more if the voltage drop and the permissible power is no problem.
Then there are special current sense amplifiers, you could also use an Opamp to amplify the signal to your desired value.
Use low pass filter, then an ADC.

I recommend at least 10kSmpl/s. And true RMS current calculation with software.

Klaus
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…