Post by thread:More AC current

STeve Childress asked for more information. My attempt at ASCII art was aborted. If you can read MACDRAW II documents then let me know I can send *real* diagrams. I have tried to clarify the explanation from my previous posting and add a bit. Steve, if you are still confused and not blessed with Apple Mac, then email me your fax no and specific queries and I will try to help on paper. Explanation:- One lead carrying the load current passes through the transformer. Define this as the primary. Either choose one of the existing windings as a secondary or wind your own with insulated wire to scale the current by the turns ratio. The OP amp will create a virtual earth (see classic op-amp theory) at the negative input at a dc voltage offset of the bias put on to the positive input. This will be about 2.5 volts. For extra precision and temperature tracking, this value can also be sampled by the PIC ADC and subtracted from the value derived from the op-amp output. Monitor the op amp output with a scope and ensure that it does not limit over the range of load currents required. Adjust feedback resistor and turns ratio if needed. Also ensure that the op amp used is suitable for operation on low supply voltages and with an output swing close to both supply rails. Kalle suggested fitting a resistor across the secondary of the transformer to avoid near infinite votages being developed if the secondary is un-loaded. Whilst this is theoretically possible, it rarely happens because the transformer core saturates and limits the voltage. It would however be a good idea to fit parallel reversed diodes across the secondary to protect the op amp under surge conditions. Someone commented about errors with non-sinusiodal currents. This needs to be taken into account where appropriate. If you anticipate such loads there are two choices. 1) If the current waveform remains a constant shape the an estimate of the form factor can be made and used to scale the peak current measured into an RMS equivalent figure. 2) The proper way to do it is to calculate the rms value. After all we have got a processor and we can use the square root algorithm that has filled the PIClist a week or so ago. Remember your AC theory - RMS means ROOT MEAN SQUARE and is calculated as the square root of the mean of the squares of as many sucessive samples as possible. Many samples per cycle is best but you could be processor time limited and so instead, sample with an interval that is NOT equal to factors of the mains period and average for as long as possible before taking the square root. Final comment: if the application is destined to measure *power* consumption then the phase shift in reactive loads must be understood. This is really beyond the scope of this list and risks boring everyone else. I recommend that the application be talked over with a competant analogue engineer. It could be worth reading the two part article in Everday Practical Electronics February and March 1996 issues which features a power consumption meter based on a 16C84. They can supply the PCB, software and a source of preprogrammed PICs if required. Regards to all Safety First Bob Minchin spam_OUTbob.minchinTakeThisOuTroke.co.uk Romsey UK