Dear Sirs,

I would like to design ac current source for injecting current into 230 V / 50 Hz mains outlet. The current frequency shuld be about few HZ (for example 3 Hz) and amplitude of injecting current can not exceed 30 mA. First I want to use high voltage source with selected low frequency and serial resistor (about 10 K Ohm) to limit current at 50 Hz and 3 Hz. I am sure that may be other idea better that suggested.

 

It's a much better idea to not mess around with electrical mains.

Possibly you could explain a bit about your high voltage source and what the purpose of the signal injection is.

 

High voltage source is built using op. amplifier with power MOSFETs similar as audio power amplifiers.
Maybe the current injection is not the right term, but I want load the mains voltage outlet with (passive or active) load that can cause current of about few 10 mA at frequency (say a few Hz) which is different from mains frequency (50 or 60 Hz). The load current at 50 Hz should be the same range as current at lower frequency, ideally is zero. The different frequency is used to distinguish between posibble load current at 50 Hz and test current at 3 Hz for example. This test signal will be used to measure mains impedance.

 

As long as you understand that the FET's are DC devices. They have an intrinsic diode structure that will cause uncontrolled conduction if reverse biased.

Use lots of interlocks - do not become a crispy critter.

 

High voltage source

Why on earth do you need a high voltage source to supply milliamps to mains wiring, which has impedances of milliohms to a few ohm?
All you will succeed in doing is recreating the big bang.

If you want to impress a signal onto the mains safely (yes it can be done) make sure you know enough about electricity and electrical safety first.

Connecting through a resistor is just plain foolhardy.

 

I want to measure line impedance. The line impedance of mains voltage supply can be accurately measured with a low test resistor (say about 10 Ohms, so in the system flows test current about few amps) which produce relatively high voltage drop on a system impedance. But I want to use much smaller test current (let say about few mA!). In this case the voltage drop on system impedance is very small and in most cases can not be distinguished with mains voltage fluctuations. In all those cases the test current of mains ferquency 50/60 Hz is used.
So, if I use low test current at frequency very different from 50 Hz the voltage drop on system impedance caused by new test current can be much easier distinguished from mains voltage fluctuations. But the current (if will flow in the system caused by this measuring sistem) must be also small, about few mili amps at the most. To limit current at 50 Hz to about 10 mA I use resistor of 23 kOhm, so the high voltage source (460 V) is used to cause test current (frequency of about 5 Hz) of 20 mA at selected serial resistor. Much smaller voltage source can be used, but I was mentoined earlier, the possible current at 50 Hz must be also very low (ideally zero).