Introduction: Purple Squirrel Solid State Relay Kit

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.


Purple+Squirrel made it!(author)2013-08-19

Dream Dragon to be able to use this with European voltages (230Vac) will require you to use a different SSR such as the Sharp S208T02 (this part will fit in my board). To handle the current ranges you want will require you to use a heat sink (dimensions are in the supplier datasheet) as well as more of them (i.e. in parallel).. This part does use an optocoupler driven Triac output.

There is another SSR with Triac output from Sharp that will work and has a higher current rating, but the form factor is different from this can see it here:

If I get enough requests I will spin a board to accommodate this part or another supplier to handle the 230Vac range.

As for the 50Hz there should not be a problem with this part.

Dream+Dragon made it!(author)2013-08-23

Superbender, I thank you for your suggestion, but I've not yet been able to find a supplier for that particular component here in the UK.

Purple Squirrel, I have found this one. 5A may be sufficient.

I notice that it#s a slightly different form factor to the one you use, and I'm not at all sure about what the ancillary circuitry is intended to do, and I notice that at least some of these things appear to suggest a capacitor, as a "snubber", but it doesn't look like you use one. Is that deliberate?

Purple+Squirrel made it!(author)2013-08-24

Dream Dragon in regards to the current rating; if 5A will work just remember to be mindful of the datasheet chart that shows load current vs ambient temp. In the case of the one you listed, 5A of current is capable of being carried by the part to 30C (86F). After that the amount of current the device can carry starts to drop off pretty quickly.

Form factors vary as way to keep you locked into one or two suppliers parts.

The snubber circuit (usually a capacitor and a resistor) is used to stop unintended turn ons of the triac portion of the SSR. The reason for this is that 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.
Also, if you drive inductive loads, a snubber will almost always be needed due to the sudden changes in current due to changes in the voltage.

I did not include a snubber circuit as most people will have other circuitry across the triac portion of the device however your comment has caused me to put a note in the instructable about the snubber circuit and unintended turn ons and how to avoid them.

Dream+Dragon made it!(author)2013-08-18

Please tell me what I'd need to do to make this work with English Eclectrickery (240V, 50Hz, 13a) Though of course most things will use much less than 13 amps that's the standard power limit of the sockets here, (I'm not mad enough to load it onto something like an electric oven which has a 30A or 50A mains fuse). I do however want to switch pumps (inductive load) and Lights (resistive load).

Every single SSR I look at is aimed at American Eccentricity, and I don't understand the theory of operation properly so I can't figure out what difference it makes.

With that amount of power I THINK I'd want an optocoupler to isolate the computer from the mains power and if it could be "Zero Crossing" too (for the inductive loads) that'd be grand.

Any suggestions especially the correct part values or clues to how to find them would be REALLY helpful.

Superbender made it!(author)2013-08-18

FYI, an SSR ususally contains an optocoupler. Also if you need a higher load, I'd suggest you'd change the SSR to the correct voltage and power ratings. If the frequency is 50 or 60 Hz shouldn't matter, but the power output does of course.
Just google for S108T02 which seems to be the SSR in the kit. I found this link:
In the data sheet you'll see that a 240V version exists the S208T02. This version is also limited to 8A though, so you need to look around a little bit more than the 3min I spent to find the right SSR for your power needs.
Good luck.

About This Instructable




Bio: Just a guy who likes electronics. Come check out were I have open source electronics available.
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