Attempting to repair treadmill circuit board

Semiconductor Discrete Semiconductor Products
1

I’m novice with circuit board repair so please bear with me; this is about a thyristor, but I’ve got to give a little back story first. Over the thanksgiving holiday my wife and I were gifted with a used NordicTrack C2100 treadmill from family, I plugged it in, and everything seemed to work fine, flashing error codes, the display lit up and functioned properly, everything was fine until I attempted to start the motor to run the belt. The motor instantly ramped up and shut off within a second, that’s all the motor would do and me being me I started troubleshooting. Doing some research, I found another person talking about the exact board I have and its common failures, and one of them being the motor will either not turn on at all or ramp up then shut off which was an exact match to my problem.

The two components in question were the MOSFET that controls the output to the motor and the thyristor which I know is on the AC side of the system, but I know a little less about its exact function, they said the two work in tandem and if one fails the other will fail as well. I ordered a new mosfet and thyristor, I installed the mosfet first just to see what the results would be and to my surprise when I started the motor this time it worked fine, the speed controls worked and it seemed to be fixed however the second I put my weight on the belt and applied a load to the motor I hear a faint pop, the motor ramps back up and shuts off and again I’m back to where I was before with the mosfet shorted closed. Now mind you at this point the thyristor had no physical damage and given how the motor did work properly leads me to believe the thyristor was working but just not well.

I removed the board, installed a new mosfet and replaced the thyristor, however this time there was a much different result, the second I attempted to start the motor the thyristor catastrophically failed with the housing blowing apart and a nice flash. At this point I’m extremely confused because the thyristor I removed was not blown apart, and now this new one has failed for an unknown reason. I figured maybe I had accidentally damaged the one of the legs when bending them to install it, so I installed another thyristor and I got the same result, immediate catastrophic failure the second the motor is commanded on. The rest of the board seems fine, the treadmill never loses power or anything! The thyristor’s I bought were randomly off amazon and I’m not sure their source, I’m thinking I should buy some from a reliable source however I’m afraid of it popping again. Does anyone have any advice on what I should do? Do I try another thyristor? Why is the thyristor now popping, would the ones I bought somehow be rated for a lower voltage/amperage even though I searched by the numbers on the front of it? Should I just give up trying to fix the board and buy a replacement?

The model number of the treadmill is: NTL1075.0
The board number is: MC_2100
The thyristor information is: S4025L
The MOSFET information is: 1RFP250N

2

Hello,
Welcome to the DigiKey TechForum. Could be lots of things going on here. Are you sure the thyristor you put has the same specifications as the original one and with the same pinout ?

3

Well that’s what I’m concerned about and believe is wrong, the thyristor I bought has the same number on the front S4025L but being that it was a cheap amazon purchase I’m not sure its ratings or pinout, I’m thinking about purchasing from a known dealer and trying again I just wanted to make sure that before I wasted the money on more components I wasn’t missing something obvious or wasting my time when I could try to buy a whole new motor controller.

4

It’s very hard to say, as there are many things that could go wrong. However, there’s definitely a greater risk of counterfeits when not ordering from a franchised distributor of an electronic component.

In the case of the S4025LTP, of particular note is that it has an isolated thermal tab (the metal tab is electrically isolated from the internal silicon). This allows it to be physically in contact with grounded metal without shorting out the device. If the part you had ordered from Amazon was not what it purported to be, it may not have the isolated tab, which could be an issue, depending on mounting. Of course, it may not have the same electrical specs either.

For long-term durability, using a thermal interface material (TIM) or heatsink grease are recommended. They fill in the micro gaps between the metal tab of the thyristor and the heatsink, allowing much better thermal conductivity.