PWM + SSR relay

Development Boards Arduino
1

I built a basic relay circuit using an AC solid state relay to turn on an AC pump. Send 5v into the SSR and the pump turns on.

I also built a basic button controlled PWM circuit using my arduino. For each button tap it increases the PWM duty by 10%. I connected LED’s to test and as expected the LED’s gradually increase in brightness with each press.

I then connected the PWM output on the arduino to the 5v trigger on the SSR expecting it to control the pump speed but nothing happened. I can turn the output to HIGH and the pump will turn full on, and the SSR is rated to as low as as 3v input trigger but still nothing happens no matter what speed the output is set at.

I could really use some suggestions as what to try to fix this and make it work. Thanks!

http://pcmofo.com/pictures/ssrpwm.png

2

pcmofo,

Your design appears to depend on two things in which I have little confidence:

  1. that your motor will allow substantial range of speed setting by varying the input voltage and

  2. that your PWM control scheme produces variable-voltage AC power.

The former you should be able to ascertain from the motor specifications.

As to the latter, I suggest you search for information on using SSRs for AC motor speed control. A critical difference between controlling DC and controlling AC is that DC power does not have an inherent time component. You can use PWM to control DC and the average voltage will be the product of the input voltage and the duty cycle.

With AC, things are much less straightforward. In AC, the voltage varies (typically in a sine wave) between positive and negative values. The synchronization between the AC power and the SSR are critical.

Two questions about what you’ve done so far:

  1. When you tested with LEDs, were they connected to the input or output of the SSR?

  2. At what frequency are your PWM and AC power?

Happy Hunting,

Eric

3

Some SSRs don’t switch quickly, nor can the control the AC signal’s duty cycle using zero-crossings.

You may need a TRIAC type circuit.

4

I think the problem you are having is due to your use of PWM to control an AC motor. Nothing will happen unless your SSR is always on or running perfectly in phase with your AC supply. This is because your SSR may be in it’s ‘on’ state as your AC signal (mains) passes through zero volts and as AC current goes both ways the over all current flow will be zero. For this to work you should be using your SSR to ‘chop’ up a stable DC signal to limit the current supplied to the motor ( and a DC motor to drive you pump as well).