Introduction: Make Your Own Solid State Relay

Picture of Make Your Own Solid State Relay

In this project we will have a look at solid state relays, find out how they work and when to use them and in the end create our own DIY Solid State Relay. Let's get started!

Step 1: Watch the Video!

The video gives you all the information you need to create your own Solid State Relay. In the next steps though I will present you some additional information.

Step 2: Order the Components!

Picture of Order the Components!

Here you can find a parts list with example seller:

1x MOC3020 Optocoupler:

1x BT138 Triac:

1x 330Ω, 3x 1kΩ Resistor:

2x PCB Terminal:

1x Heatsink:


1x MOC3020 Optocoupler:

1x BT138 Triac:

1x 330Ω, 3x 1kΩ Resistor:

2x PCB Terminal:

1x Heatsink:

1x MOC3020 Optocoupler:

1x BT138 Triac:

1x 330Ω, 3x 1kΩ Resistor:

2x PCB Terminal:

1x Heatsink:

Step 3: Build the Circuit!

Picture of Build the Circuit!

Here you can find the very simple schematic of my DIY solid state relay along with pictures of my finished board. Feel free to use them as a reference.

Step 4: Success!

Picture of Success!

You did it! You just created your own Solid State Relay!
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gm280 (author)2017-11-20

Mechanical Relays, Solid State Relays and even MOSFET switches all have their place in the scheme of things. Picking the proper one is the option of the person designing the circuit. They all have their pros and cons.

kgiri (author)gm2802017-12-14

you are absolutely right.

kschmidt2 (author)2017-11-25

Now I'm glad I didn't buy one of these (DC one though) for the ignition system on my bike, though I might try and make something with lower losses if I can. Standard Relay's are fine but I'm looking for a lower overall current usage to take some load off the 40 year old alternator.

TexomaEV (author)2017-11-21

Love this, please show us a DIY option for switching DC loads. Thanks again for this, because I knew China solid state relays usually don't hold up to the ratings published.

JohnC430 (author)TexomaEV2017-11-24

you switch DC loads with transistors or relays. For a DC load the device must be reverse biased; so a capacitor is connected to the anode and then switched negative with respect to the cathode so the cap conducts all the load current during that pulse time.

schmitta (author)2017-11-21

Are you an EE? If so where did you go to school?

JohnC430 (author)schmitta2017-11-24

at the local barber shop

martinroberge (author)2017-11-23

how to calculate R2?

JohnC430 (author)martinroberge2017-11-24

dont bother about that.

JohnC430 (author)martinroberge2017-11-24

check the spec sheets for the diac and the triac. they will tell you how much current is needed to fire the triac and the diac Vbr. when the diac fires at Vbr the current flows thru the resistors to the Triac gate.

CarraN (author)2017-11-21

Hi! are they indicated to control audio signals?

JohnC430 (author)CarraN2017-11-24

absolutely NOT!!!!

attanayaka (author)2017-11-20

i want to control 230v 15A water pump.i can use it?

SeanB10 (author)attanayaka2017-11-21

I suggest using a relay for the benefits mentioned in the video. Slow but nothing complex about them. Just use a flyback transistor across the solenoid coil and use an NPN transistor between the coil and ground to switch it. 15A is big, pumps switch slow, and ratings are reliable. Sainsmart relay boards are nice because they have optoisolators, protection diodes and relays, and little leds, all packaged pretty cheap. To get a sizable relay for your use, spec it out.

JohnC430 (author)SeanB102017-11-24

"Just use a flyback transistor across the solenoid coil" you mean use a diode across the solenoid coil

dcalado1 (author)attanayaka2017-11-21

You can use BTA41, not for sure in this project.

jimvandamme (author)attanayaka2017-11-21

Be careful of current ratings. Measure the peak current of the pump as it turns on, and the steady state current. You need to pick your triac for both conditions. The instantaneous current rating will take the starting current, then the heat rejection will deal with the steady load.

I'd think that the 15 amps is probably the inrush current rating of the motor, so the 12 amp continuous rated one might work.

Don't expect a fuse to protect your triac. As a rule, solid state blows in nanoseconds "to protect the fuse". At least use a spike protector (MOV, a LC filter, or both) to keep voltage below dangerous levels.

jfranke (author)jimvandamme2017-11-21

Good comments! However, a fuse is useful if the load fails and or the triac fails in a mode that maintains enough power to cause a fire or other damage. When things go bad, it is best to remove power at the source and that is where the fuse should be. A fuse is used to protect life and home, not the triac and or load.

SeanB10 (author)jfranke2017-11-21

My experience with zero crossing detectors and triacs to switch any device that induced a phase shift would destroy the triacs and leave them running, or short them out so to speak. The problem is that the triac is switch on at the wrong time because zero crossing detector couldn't handle the phase shift. This is motors, old CRTs, the like. You could tune the detector for a given phase shift. My solution was a relay.

KEDam (author)attanayaka2017-11-20

You would need to finde a more powerful triac, this one is only rated for 12amps but rest would be the same.

JohnC430 (author)2017-11-24

I just thought of this: how do you connect them in parallel? can you do that? do we need to have balancing resistors in series?

blabbersnitch (author)2017-11-21

A 40A Fotek solid state relay for a few bucks? I don't think so. That is a counterfeit unit. It is no surprise that the triac inside is underrated.

jfranke (author)2017-11-21

The BT138 seriesTriac tab is connected to the heat sink and the ac line. This is an extremely dangerous condition! The author merely glanced over the issue. The video shows many examples where the heat sink is exposed, the author's hand is close to the heat sink, and the problem is not addressed. The problem can be easily cured with a thin insulator under the tab and a nylon mounting screw or a metal mounting screw and an insulating sleeve as used on TO-220 style transistors.


ivak245 (author)2017-11-21

Good lesson on what is inside these little beasts. I have used several dozen of these, and have had several go "bang"! After seeing that the triac could not switch the rated current, I know why!

mani.fc (author)2017-11-20

Great instructable thanks.

misterxp (author)2017-11-20

Great instructable thanks!

acheide (author)2017-11-19

I have built several of these. Less expensive and more reliable than some import units. One of my imports did not work, and when i did an autopsy, it turned out to be dangerously wired internally. Thanks.

Aditya Tripathi (author)2017-11-19

just watched the video. and just read the intro as always good in every aspect.

About This Instructable




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