Wiring a solid state relay

This one is:

Should work ok?


Nope. Doesn’t give you the 100mA. Also, power resistors of this type usually have to have a large heat sink to actually dissipate the specified wattage. Last time I needed one the heat sink was 1/4 inch thick aluminum 4 inches on each side.

So, you would just drop the resistor then?

If it were my machine, I would not use a resistor.

If you must use something try this:


1 Like

Hehe, I was thinking lamp also.
Maybe some smaller signal lamp/bulb to mount in the box wall.

This is my SSR:

You just have to make sure that whatever bulb you use meets the voltage and 100mA load requirement.

Hello all

From @AlanDavis drawing - I can see that the DC side of the SSR is connected to the Spindle (+) and Spindle (-) terminals of the Inventables Power Supply Interface PCB (Part # 30514-01)

I purchased the Inventables SSR ( part # 30406-04 ) I beleive is shown on the drawing.

Since I do not have the PCB - my question is: - do I need the PCB in order to be able to control I a 230 V Kress Spindle (router) so that I can start/stop from a G-code ?

If the PCB is not needed - where do I connect the DC side wires ? Will any V+ and V- from the 24V PSU do - or I need some additional electronics ?

Many thanks



You don’t need to wire it that way. The relay works with an input logic signal in the 3 - 32 VDC range. What you would need to do is to hook the spindle control wires from the Arduino/gShield to the SSR on the input side. If you wired your machine according to the Inventables instructions then you would hook the black spindle control wire to the - Input terminal and the yellow wire to the + Input terminal on the SSR.

The AC side of the relay would be wired as in the diagram.

1 Like

Hi @LarryM - thank you for your reply.

I am very much old school - I am used to the mechanical relays where you have 3 wires - V+, Ground, and the trigger - this SSR is too smart for me :smile:

So as I understand it (please correct me if I am wrong) - I should connect to the DC side of the SSR like this:

The positive terminal will be linked to “Spindle Enbl” on the Arduino - I.E pin 12

The negative terminal will be linked with the Arduino “Ground” - I.E pin 14

I reckon the Voltage will come from the cards power supply.

Hope this is what you mean - don’t want to fry anything unless it is dinner :wink:

Many thanks again



If you are running grbl version 8 then the spindle control is D12.

If you are running grbl version 9 then the spindle control is D11.

That is the control signal that you want to use (yellow wire).

The black wire goes to ground on the Arduino/gShield or any ground that is common to the Arduino. Looking from left to right on the connector with the homing switch/spindle control, it would be the 7th pin keeping in mind that 1 and 6 are empty.

The input to most SSRs is hooked to an opto-isolator which draws very little current. Somewhere in the 10 to 20 mA range. This amount of current can be supplied by the Arduino I/O pin.

1 Like

Thanks @LarryM

This is much clearer now.



alright @ErikJenkins how exactly do you have this wired? a photo if you could i thought i had it wired right and turns out i had it wrong nothing gave me the magic black smoke or anything but its wrong

I can’t really take a usable picture as the relay is at the bottom of my enclosure, So, that being said, using the relay at the bottom of this post to start and stop my Dewalt with my G-Code…here is how I have wired it up:

On the power supply where I marked an “A” the positive wire goes to the Arduino header pin 11. The ground wire goes to the Arduino ground.

On the power supply where I marked a “B” the positive goes to the positive relay input and the ground wire goes to the ground input on the relay.

Once wired, the switch on the power supply must be in the logic position, the router plugged into the relay in one of the “Normally Off” plugs, and the router power switch in the “On” position.

In G-Code, the M03 S12500 command turns it on (the router ignores the speed passed, but it needs it to trip the relay)
and the M05 command turns it off.

Makes sense?


1 Like

I got it , stayed up wicked late messing with it thank you

1 Like


Why do you do this? You take 5 volt DC PWM into the power supply terminal board to get 24 volts DC PWM out from the terminal board to control the relay.

The relay says that is takes 3.3 - 60 VDC as control. Why don’t you just run pin 11 to the relay?

That would probably work fine, I was just using the logic from the power supply…


I just thought of a function you gain by wiring the way you did. The switch on the power supply circuit board will let you turn it on/off manually or control it by the logic position.

Yeah, but he already had the PSU terminal board switch :smile:

1 Like

I did something similar with the SSR that Inventables sells. However, I found that to drive the SSR reliably you needed to drive the relay signal through a transistor with the appropriate resistive loads. I used a 2N2222. See the Fotek Datasheet page #2 “Applications Hints” NPN NO (normally open). The resistor between pin 4 and V+ should be about 10K ohms, and the resistor between the signal and the transistor base should be about 1K ohms.

Hope this helps.

@BennettMarks what source did you use for V+ in the NPN NO scenario?