Introduction: Purple Squirrel Solid State Relay Kit

About: Just a guy who likes electronics. While Purple Squirrel is still up and running we are now developing projects based on FPGAs, CPLDs, Arduino, and Raspberry Pi.

This instructable will detail how to assemble the open source Purple Squirrel Solid State Relay Kit (SSR); a small & simple kit for switching high current loads with a low current control signal (i.e. microcontroller or programmable logic device). This device is similar to the Purple Squirrel Relay kit except the relay is not mechanical but solid state. This kit can safely handle 2.5A as is without any problems; 8A with an appropriate heat sink..

This project is suitable for beginners. Some soldering tools are necessary but even if you've never soldered before this will not be that difficult. You can buy this kit from the Purple Squirrel website or you can gather the parts your self and using the Eagle files from the website get your own PCB(s) made and go that route.

Step 1: Getting Started

Before starting check your kit to make sure you have all the parts.

Your parts list (if you bought this from Purple Squirrel) should include these 10 pieces:
(1) PCB
(1) Solid State Relay
(1) 3 position terminal block
(1) 2 position terminal block
(1) LED, Red
(1) 2N3904 NPN transistor
(1) 10k ohm resistor
(2) 330 ohm resistors
(1) 150 ohm resistor

Step 2: Assembling the Kit

Once you have all of the parts, assembling the kit only takes a few steps. I've included pictures of each step.

Start with the PCB.

Step 3: Installing the Resistors

Next we are going to attach the 330 ohm resistors (the color bands will be orange, orange, brown). You may need to pre-bend the leads like picture to get the parts to fit.

Once the resistor is in the location it is supposed to be, turn the PCB over and bend the leads a bit as shown…this will hold the part in place while you solder it down

After soldering the leads down, trim them off so they look something like picture.

Repeat the above steps for the remaining resistors (150 ohm (brown, green, brown bands) and the 10k ohm (brown, black, orange bands)).

Step 4: Installing the NPN Transistor

Pay attention to the silkscreen marking on the PCB (it has the number 2N3904 near it) as it shows you how to put the part in. If you notice you will see a flat edge on the transistor. This flat edge of the transistor corresponds to the flat edge of the silkscreen on the PCB.
The leads on a NPN transistor when looking at the flat edge are EBC (emitter, base, collector) and the holes on the PCB correspond to this.

Step 5: The LED

The LED, a diode that gives off light, conforms to a polarity scheme as well. The longest lead is positive (the Anode) and the shorter lead is negative (the Cathode). The PCB shows a flat edge on the silkscreen marking for the LED. This flat side indicates the side that the negative lead or the Cathode should be installed in.

Step 6: Installing the Terminal Blocks

I installed the three position block first but could have installed the two position if had wanted. The thing to remember with these parts is that you cannot bend the leads to hold the part into the PCB for soldering. What I did was turn the PCB upside down, install one of the terminal blocks into its position and use the other to hold up the other side of the PCB while I soldered the leads down for the first one.

Step 7: The Solid State Relay

Installing the relay should be done last. Mainly because not waiting till last makes installing the other parts more difficult. Because this relay is not shaped like a mechanical relay you cant easily turn the relay upside down and rest the PCB on the relay…I steadied the board upright and soldered one leg of the relay to hold it in place; THEN I was able to flip it over and solder the leads from the bottom and keep the relay from falling out.

Step 8: All Done

When it is all said and done your relay kit should look like this when you are finished. Now go and switch some loads with your new relay kit.

Step 9: Get Eagle Files Here

Eagle Files for the Purple Squirrel SSR Kit.

Step 10: A Note of Caution

As most people may not fully read the datasheet for the Solid State Relay I need to point out (thanks to a question from Dream Dragon) that a snubber circuit may be needed. What is a snubber circuit you ask? A snubber circuit is a resistor and a capacitor in series with each other that are connected across the triac portion or load side of the SSR (see picture below) and is used to stop unintended turn ons of the triac portion of the SSR. They do this by reducing the size of the rapidly changing voltage signals

There are two reasons that you need a snubber.

Reason #1
You are driving an inductive load like a pump or solenoid.
If you drive inductive loads with an SSR, a snubber will almost always be needed due to the sudden changes in current due to changes in the voltage that are a result of the phase difference between voltage and current.

Reason #2
The forward current to the LED portion of the optoisolator in the SSR drops to less than 1mA.
In this scenario when the forward current to the LED in the opto portion of the device is below 1mA the triac portion could turn on if a voltage across it exceeds its dV/dt rating or in other words if the voltage across the triac portion of the device increases faster than the what the datasheet specifies.

So be safe and use the snubber circuit. To start use a resistor of 47 Ohms and a capacitor of 0.022uF. If you are switching AC loads make sure the capacitor is AC rated.