measuring mA AC using uC - Page 1
Forum section: electronics / projects

author: hussamaldean
author_rank: Unknown
timestamp: June 04, 2019, 02:59:02 pm
content: Hi allI am currently working on AC power analyzer. However, I am using ACS7212-30A which can't measure less than 1mA and I want to measure as low as 1mAis there any way to measure such current? like using current transformer or current shunt resistance?regards

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 04, 2019, 06:22:17 pm
content: Quote from: hussamaldean on June 04, 2019, 02:59:02 pmWell certainly a shunt resistance would get you there assuming you can measure the voltage across it at 1ma and you can reference it in a way that doesn't cause issues.At the extreme end you could think about the small voltage across a small shunt resistance as a thermocouple voltage and look at techniques used to measure those.

author: hussamaldean
author_rank: Unknown
timestamp: June 04, 2019, 06:28:35 pm
content: Quote from: cur8xgo on June 04, 2019, 06:22:17 pmthinking about using 1Ohm 1Watt resistor and short it when I need more current like using relay or something similar

author: Kleinstein
author_rank: Super Contributor
timestamp: June 04, 2019, 06:57:36 pm
content: For AC one can use a current transformer if needed also for small currents. It gives insulation from the measurement circuit, which can be a big help.A shunt is also possible - usually one would have something like 10 -200 mV as the maximum voltage at the shunt. So a 1 Ohms resistor would be Ok up to about some 200 mA. With AC one can do amplification relatively easy, as there are less offset problems. So less voltage may be acceptable with AC than with DC. Also AC accuracy is usually not that good anyway.For the 1 ohms shunt one could use diodes in parallel to protect it from too much power / current.

author: hussamaldean
author_rank: Unknown
timestamp: June 05, 2019, 04:37:34 am
content: Quote from: Kleinstein on June 04, 2019, 06:57:36 pmI will buy 100A current meter which commonly designed for control panels which comes with current transformerwhat value of resistance shall I use for 1Amp range,10Amp and 100Amp range ?

author: NorthGuy
author_rank: Super Contributor
timestamp: June 05, 2019, 05:08:13 am
content: Quote from: hussamaldean on June 05, 2019, 04:37:34 amFor 100A, you don't want a big shunt because it'll waste energy. Commonly used values in solar installations are 0.1mOhm, or even 0.05 mOhm. Keep in mind, that at such currents, wires create significant voltage drops, so your ground is likely to be different from the shunt ground. Then you would use an instrumentation amplifier to increase the voltage range to 3V (or whatever your ADC needs).For 100A AC, a current transformer is a much better idea.

author: hussamaldean
author_rank: Unknown
timestamp: June 05, 2019, 05:40:12 am
content: Quote from: NorthGuy on June 05, 2019, 05:08:13 amwhat about 1Amp using current transformer, will same 100Amp current transformer work with 1Amp if I change the resistance ?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2019, 06:50:06 am
content: In principle the 100 A current transformer would still work at 1 A. However there can be some extra errors (e.g. due to core hysteresis). Not all current transformers are made the same. I would increase the shunt at the secondary, but not by a factor of 100 to get the same ouput voltage, but maybe only 10 times so that the core would not see so much field.Depending on the construction one may be able to use more primary turns (e.g. 5 turns with the outer part of the turns to 4 different directions).A good starting point for a mall DIY current transformer would be a common mode choke (e.g. from an old SMPS / PC supply). For not too high demand these cores are not that different from the ones in current transformers.

author: hussamaldean
author_rank: Unknown
timestamp: June 12, 2019, 05:31:26 am
content: Quote from: Kleinstein on June 05, 2019, 06:50:06 amwell, I got myself 100A/100mA current transformer what the values of the resistance shall I use for 1A,10A and 100A ranges ?regards

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2019, 04:50:37 pm
content: Quote from: hussamaldean on June 05, 2019, 05:40:12 amCurrent transformers have a maximum volt*second specification given for their secondary before they saturate so raising the secondary shunt resistance, while proportionally increase the sensitivity, also limits the maximum current before saturation. So changing your 100 amp current transformer into a 1 amp current transformer by increasing the secondary resistance by 100 times means that it will be limited to accurate measurements of up to 1 amp.You can safely overload a current transformer however it may be necessary to limit the secondary voltage with diodes or something similar and I have sometimes seen this done.