Does this voltage/waveform look strange?

I am trying to build a sprinkler system controller and you can read further detail about that here.,86500.0.html

In the process of developing the final circuit I built the following prototype.

The voltage from the regulator through the 680 Ohm resistor is supposed to mimic the voltage from one of the arduino's digital output pins in the final circuit. When I hookup the circuit like this the solenoid turns on. (Hooray!)

If I disconnect the regulator from the 680 Ohm resistor and just measure the voltage between the regulator output and ground I get a nice flat 11.9 Volts When I put the oscilloscope probe on the wire between the 2 resistors I get the following output.

First of all does that output look unexpected to anyone else, what is causing the fluctuation? Also will the fluctuation in voltage damage my arduino when I hook up the output pin to the 330 Ohm resistor as in the final circuit? Finally will the fluctuation in voltage damage the TRIAC over time?

Thanks in advance for any insight into what's happening here.


Think it is caused by the fact you are using AC voltage and the one diode only allows 50% of the sinus-wave to pass through. use a complete wheatstone bridge ...Or switch to DC .

robtillaart: Think it is caused by the fact you are using AC voltage and the one diode only allows 50% of the sinus-wave to pass through. use a complete wheatstone bridge ...Or switch to DC .

I agree the fluctuation in the voltage is because of the ac voltage flowing through the triac. I'm just curious how a ac voltage in a nice even sine wave flowing through the mt1 and mt2 on the triac and the flat dc voltage (I checked this and after the diode gives the half rectified voltage the cap smooths it and the regulator knocks it down to 11.9V in a nice flat line) flowing out of the regulator are combining to create the wave shown in the picture. But perhaps more important is will the 1V fluctuation cause damage to the triac or the arduino?

Compare the input to the output with and without your connection to the 680 ohm resistor.

When you disconnect the 680, you have almost no load on the regulator. When there is current flowing through it, there is a load. Since you are only half rectifying the signal, it is very possible the 330uF cap isn’t enough to continue providing power when the regulator is loaded.

If you do the calculation its obvious 330uF is enough - what's happening at the gate of the triac is that its swinging from its positive to negative bias voltages across the AC cycle. Calculate the voltage and its varying from +1.2V to -0.4V at the gate. Just what to expect from a TRIAC thats positively biased.

Ok so I did a couple more measurements while under load and the capacitor doesn't seem to be the problem. Under load the input to the regulator never drops below 32V with not a lot of ripple. And under load the output of the regulator is flat right at 11.9V I don't see the fluctuations until after the 680 resistor. So I agree this has to do with the triac. Perhaps I need to study more how triacs work. My more important question is will this be damaging to the arduino when I start using it to drive the triac?

So I've spent the last little while trying to figure out triacs and they are clearly more complicated that I thought ( I thought of them as transistors that could switch ac current). So I started considering other options and wanted to know what people thought of changing to solid state relays Would something like this work as a replacment.

A triac is essentially 2 SCR's with the cathodes to opposite MT pins and a common gate, during the negative halfcycle MT2 will pull the gate negative to line common hence the superimposed AC that is clipped. IF you want to use DC as a gate control, use a SCR across the DC terminals of a bridge rectifier in series with the load or use a photo triac optocoupler such as the Vishay IL410.