For Want of a TRIAC, a Router is Running: Diagnostics and CNC Repair

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Forum topic by JAAune posted 10-23-2014 04:03 AM 2431 views 0 times favorited 10 replies Add to Favorites Watch
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1788 posts in 2284 days

10-23-2014 04:03 AM

This topic might have fit better in the blog section but I thought the CNC people would probably fail to see it and they are the folks who’d be interested in reading this.

I’m not the only one around here using a shop-built CNC router and sooner or later, everyone will encounter the situation none of us wants to face: malfunctions.

Last week I had the router running with a new program. I failed to spot a mis-aligned cut in the simulation and the result was a 1/2” bit trying to hog 7/8” of hard maple in a single pass. With the IPM the machine was running, the steppers stalled and the router almost did the same.

A hasty reset prevented excessive damage to the parts being cut but the router itself kept running which wasn’t right as it’s supposed to shut down on a reset. I soon discovered that the router would always run and at full RPM anytime I had the CNC turned on. No shut-off or speed control was possible from the PC.

Circuit boards are not my skill set but I did take a class on building the things and we had an evening of diagnostics lessons. So out came the multimeter and schematic documents.

The router has a 120AC line and a speed sensor wire going to a Super PID board in the control box. A laptop running Mach3 is visible at the edge and it connects via an ethernet cable. Basic electrical knowledge indicated that the router was getting full power when it shouldn’t have which pointed to a problem with the SuperPID device that controlled the flow of electricity to the router.

The control box is wired in an orderly fashion and all wires are numbered which helps a lot with diagnostics.

Here’s the SuperPID board. No visible switches so I looked up the instructions to see the schematics.

Since the machine runs with Mach3, the important terminals are “run” and “PWM”. Without power to “run”, the router will default to always on. PWM is the controlling signal from the computer. Both terminals are on the 5-volt system.

That’s the 5-volt supply.

As I learned in class, diagnostics consists mostly of using the multimeter to trace where power is going and where it isn’t until something is discovered that’s out of spec. The black lead gets touched to the negative on the 5-volt supply and the red lead is touched to various terminals on the 5-volt system until a reading that fails to match the schematics is discovered.

I was mostly interested in “run” and “pwm”. “Run” tested out fine at just over 5-volts. “PWM” also tested out properly since it showed 5-volts when I gave the spindle on command from Mach3 and showed 0 volts when I gave the spindle off command.

So the SuperPID gave every indication of functioning properly from the control side. The problem had to be on the 120v side that sent juice to the router. Without electrical knowledge, I had no idea where the switch was and went to Google to run searches on variations of “SuperPID” and “router always running”. An hour of searching unearthed a forum discussion that brought up the TRIAC.

One more search and a YouTube video on TRIACS was all the information I needed to make the diagnosis. TRIACs are devices used to control the amount of current passed on to an electrical device like a motor. The excessive cut I’d subjected the router to likely caused the SuperPID to send more power through the TRIAC which created a permanent short.

That is the TRIAC. It has the part number printed on the top so another Google search brought me to Mouser Electronics where the part was sold. $8 for a pair of TRIACS and $15 for a tube of thermal paste plus shipping was the total cost of the repair.

The TRIAC was swapped out this afternoon and the CNC is fully functional once again. Hopefully other owners of home-brew CNC equipment find this little experience of mine helpful should they ever encounter that moment when their machine malfunctions.

-- See my work at and

10 replies so far

View exelectrician's profile


2327 posts in 2395 days

#1 posted 10-23-2014 04:23 AM

Very well written example of logical troubleshooting, good job.

-- Love thy neighbour as thyself

View timbertailor's profile


1594 posts in 1392 days

#2 posted 10-23-2014 01:04 PM

Always nice to share experiences like this so others can profit from your hard work.

Thanks for posting.

-- Brad, Texas,

View johnhutchinson's profile


1243 posts in 1597 days

#3 posted 10-23-2014 03:01 PM

I tried to follow this, but … :(

I see that you had a problem at the beginning and you solved it at the end. Congratulations !!!

-- John - Central Ohio - "too much is never enough"

View DocSavage45's profile


8520 posts in 2810 days

#4 posted 10-23-2014 05:02 PM

In my 20’s I worked for IBM, and in bio electronic services. i followed you. LOL! Glad you shared it but it might not be a hand-tooler’s first read? LOL!

Looks like you learned what you needed and solved your problem. Could have been an expensive and time consuming fix if it was serviced externally?

Hope you are a one trial learner…LOL!

-- Cau Haus Designs, Thomas J. Tieffenbacher

View JAAune's profile


1788 posts in 2284 days

#5 posted 10-23-2014 05:39 PM

Having an outsider service the machine would have been very expensive since there’s no manufacturer to offer technical assistance.

-- See my work at and

View oldnovice's profile


6773 posts in 3335 days

#6 posted 10-23-2014 09:54 PM

Troubleshooting PWM and triacs is not straight forward but you seem to be right at home without any major issues.

Good work!

-- "I never met a board I didn't like!"

View Pezking7p's profile


3217 posts in 1619 days

#7 posted 10-24-2014 12:08 AM

Did you design/build the unit? Cabinet wiring looks pretty good.

-- -Dan

View TheFridge's profile


9249 posts in 1453 days

#8 posted 10-24-2014 12:22 AM

I’ve dabbled in a little electronics. Looks like a nice setup you have there.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View JAAune's profile


1788 posts in 2284 days

#9 posted 10-24-2014 12:35 AM

I’m afraid I can’t take credit for most of the design or the setup of the controls and wiring. That was done by John Knight who taught a CNC class at Marc Adams School. He’s a co-owner of a company that retrofits old CNC equipment with new controls so that’s his specialty.

-- See my work at and

View Pezking7p's profile


3217 posts in 1619 days

#10 posted 10-24-2014 12:38 AM

I would love to spend the winter building one of these. No idea what I’d use it for but it looks fun to build.

-- -Dan

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