# Proportional Valves and Dither - Using a 555

There are a lot of posts on dither over the current control of a proportional valve, but few actual solutions

In my case the valve requires a max of 2.4A, so through a 1Ohm resistor I get a 2.4V signal to apply to an op amp which drives a Tip120 in current mode... Lots of examples out there of constant current drives.

My analog output to drive the constant current opamp is 3.3V max so a 5K/13K resistor divider gives me 2.38V at full output, close enough to 2.4V

However the spec for the valve requires a 50HZ dither, but no indication of value other than to show that there is little or no movement in the first 0.30A ( or 0.3V on the resistor) to the valve. So I chose 0.3V as the amplitude which means that when the AO is below 0.15V then the dither will not affect the valve.

Enclosed is a LTspice simulation of the the dither adder circuit onto the AO.
The 555 controls to 50HZ by setting Rx and Cx. The RC = 4.7K/1uF provides 50HZ smoothing and attenuation

The output voltage Vattenuationis about 250mV
Vmix is shown with the average of about 2.4V and the superimposed 50HZ dither

I gained most of my understanding from this excellent tutorial AC/DC Superimposed Circuit Analysis - YouTube

At the time of writing the design is finished but not implemented in real life till I get a PCB built. I' keep you posted

Any comments to design or thoughts would be welcome