Analog AC to DC

I have a CT wired up to a 50 ohm resistor and it is making a 0-5 volt AC signal, I want to feed it into an analog input. I hook it up to a bridge recitifier and that makes it DC, but now I need to figure out what size cap I need to make it smoother. And if there is any way to further filter it to a steady DC signal that is still varies with what the CT is sensing?

Not too sure what a CT is?

But for filters see the end of this:-

CT= current transformer. A low pass filter after your rectification should give you a good DC measurement value. The frequency of the AC signal will be either 100 or 120hz (assuming 50 or 60hz power) because your using a bridge rectifier, so the filter cut-off frequency needs to be lower then those values.

Note: For donut type CT, be sure there is no way the 50 ohm resistor can ever become disconnected from the secondary winding of the CT while under load. The turn ratio is very high it CT's and the secondary voltage will skyrocket if the load resistance is lost, often damaging the insulation of the CT's windings. Usually the load resistance is connected right on the CT's secondary connections. Also note that you may have to compensate in your analog reading values a peak to RMS conversion as the DC filtering can charge to the peak value of the current flow rather then the RMS value. RMS = .707 X peak.


A problem with your setup is that the bridge rectifier will lop off 0.6 volts and render it useless at low powers. Presumably its for measuring the current flow in a conductor, What you need to do is have the CT floating with one end tied to 2.5 V dc and measure the AC output at the other end which you manipulate to swing between 0 and 5V and feed it to an analogue pin.

Heres a circuit that works for me :

You need to sample the waveform several hundred times a second to get meaningful readings. I gave up trying to measure power consumption with mine, unless you have nice clean resistive loads, its difficult to translate the spikey mess coming out into a mean voltage. Little things like screwy power factors and switch mode power supplies ensure its its not going to be nice most of the time.

Heres a live picture of the output from mine

A somewhat out of date writeup of my setup :

Its a nice smooth sine wave when someones in the shower…

Whatever happens, make sure the burden resistor is in place, it will produce dangerous voltages without it. :)

Lefty's already said it, but it can't be repeated too often..........

Isn't 50 ohms very high for a burden ?, The VA rating of these things is pretty small and since the resistance pushes the voltage up it it can saturate the core and give inaccurate readings. I have 0.33 ohms on my 100:5 CT .

I use a reflective object sensor on the spinning disk in the meter, much more accurate and far less dangerous. ;)

My CT has a secondary of 230 turns, so I figure with a max 20 amp load on the conductor I would have 86.9565mA. Looking for a 5 volt output at that point leads to a 57 ohm resistor.