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I am currently using a DC-AC ERI Solid state relay (Model number 001J) with a PIC16F microcontroller to control the operation of an LED street light. I have dumped a PWM code onto the microcontroller. I'm able to see a variation in the voltage on the input of the Solid state relay (Corresponding to the code). However, the intensity of the LED street light head (which runs on 220V AC mains) connected to the output of the Solid State relay remains the same. Am I doing something wrong, or is there a device I can replace the Solid State Relay with, to ensure smooth operation?

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    \$\begingroup\$ We need some more information here. Are you switching AC or DC through your solid state relay? SSRs will remain on as long as current is flowing through them, regardless of the signal on the gate. Also, there are SSRs that are TRIACs, which are quite slow, not very good for PWM applications, and SCRs, which may be faster but only conduct in one direction. Perhaps a schematic, or the type of LED light you are trying to control would make the question clearer. \$\endgroup\$ – Brendan Simpson Apr 29 '16 at 19:21
  • \$\begingroup\$ To add to @BrendanSimpson comment, would you please post part numbers and links to datasheets for the LED and Solid state relay. Otherwise we are just guessing. \$\endgroup\$ – gbulmer Apr 29 '16 at 19:30
  • \$\begingroup\$ I am using an ERI DC-AC Solid State Relay, Model number 001J. I have a commercial LED street light head, which runs on 220V AC mains. I hope that clarified any doubts regarding my question. \$\endgroup\$ – Abhishek Vasu Apr 29 '16 at 19:35
  • \$\begingroup\$ Are you sure the Street light is designed to be dim-able? Normal LED lights for home use are not all dim-able, because the AC-DC circuitry needs to be designed specially to do so. I imagine the same is true for the big lights. \$\endgroup\$ – Brendan Simpson Apr 29 '16 at 19:45
  • \$\begingroup\$ I'm not sure of that. I went with the assumption that LED street lights are dim-able, as there are a few companies offering the service of brightening and dimming of LED lights in accordance with the time of day. \$\endgroup\$ – Abhishek Vasu Apr 29 '16 at 20:09
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Not going to work. LED emitters themselves are inherently constant-current devices.

However, the street light is probably a packaged commercial product, designed to accept a constant voltage, and it includes a constant-current power supply. These are usually switch mode, which tend to be inherently multi-voltage, because when they see a lower voltage, they simply hold the switch open for a bit longer, which draws more current.

Aside from working on world-market AC power (i.e. 100V Japan through 277V USA 3-phase), this also gives them a natural ability to ride through brownouts at full brightness.

It probably has some some capacitors too, to get across those AC zero-crossings without shimmering the LED (which annoys people in motion relative to a point light source, some car taillights do this.)

So, as you dim it by messing with its voltage, it's just chopping harder and pulling more current to stay at full brightness.

It will never work. You need to disassemble the light and replace the power supply with one that is friendly to dimming.

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You have supplied a partial part number and no link to the datasheet. This one says it could be either zero-cross turn-on or random turn-on.

enter image description here

Figure 1. ERI SSR datasheet extract.

If it's zero-voltage turn on then it will always switch on as the mains crosses the zero volt point and give full mains cycles. You can't dim using that kind of SSR.

You want one that allows variable (or "random" in ERI's terminology) turn-on.

enter image description here

Figure 2. Phase angle control using triac / SSR.

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I would suggest:

  1. Try changing the PWM values. If your're expecting brightness change for a particular PWM value, then you might not get that upto certain PWM value.
  2. If you are using high intensity street light that might be using an AC input to drive the LED's then you might not be able to control the brightness as the internal rectification circuitry might take in specific input range only and you can vaguely change avg. voltage gathered through PWM cycle change.
  3. Still, you can put the ac input to a lamp dimmer circuit as shown here.Though I'm unsure for an Led setup.
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