Thermistor on Motor Control Board

[MacLaddy]

I work for a company which performs repairs on exercise equipment. One of my biggest problems is a constant failure of a Thermistor resistor on a certain brand treadmills motor control board. I am guessing poor design, but I am hoping somebody can clear up exactly what a thermistor resistor is for. I wiki'ed it, and I realize that it is a resistor which changes it's values according to temperature, but I don't understand why that would need to be applied.

These little junk Thermistors are a headache, and I would love to figure out a work-around. They are constantly frying, and they are placed right next to a fuse on this board. Any thoughts would be appreciated.

I am not an electrical engineer, engineer, or even an eng; so laymen terms will be very much appreciated.

 

[es1]

It will be tough to recommend a workaround without knowing what the thermistor is for. I would be especially hesitant to recommend something because thermistors are often used in safety circuits and I would hate to recommend bypassing some safety measure.

Perhaps it is failing because there is an unsafe condition with the treadmill!
Which leads to the next question: Are you sure it is the thermistor failing that is the root cause? Perhaps something else is damaged and that in turn cause the thermistor to fail.

If many treadmills are all failing in the same way it is likely a bad design or perhaps some component of the treadmill is outside of its lifetime. Does the manufacturer have any recommendations or insights?

 

[MacLaddy]

It will be tough to recommend a workaround without knowing what the thermistor is for. I would be especially hesitant to recommend something because thermistors are often used in safety circuits and I would hate to recommend bypassing some safety measure.

I understand completely, and I apologize for making it sound as if I am looking for someone to recommend a redesign for me. I understand that would be unsafe, especially without even seeing the board.

The question that I am actually looking for is what is the purpose of a thermistor resistor? In what sort of capacity would one be used?

Typically on a treadmill when the walking belt and the deck it rides on gets too old it will start drawing far too many amps through the board, causing failure. However, the new motor control boards are failing in the same component (the thermistor) as the old, but with a far less amp draw. It is a high percentage of these treadmills, so it is a poor design by the company.

Here's my best guess... If the treadmill starts pulling too many amps and starts to overheat the motor control board, then the thermistor resistor is designed to shut off power to the board? What's strange is the next connection in line after the thermistor resistor is a fuse, and instead of that blowing it is frying the thermistor...

Anyhow, I appreciate any help you can offer. Again, I am really only looking for someone to explain in what capacity a thermistor resistor would be used.

 

[dlgoff]

Once I helped a friend who owned a fitness center. I worked on his treadmills and to keep the motors from "over amping" lots of maintenance on the belts was required. That is, clean the deck and lubricate often (these were always in use). That said, have you checked that the fuse has the proper value? I would think that the thermistor would not be needed with a fuse.

 

[MacLaddy]

Once I helped a friend who owned a fitness center. I worked on his treadmills and to keep the motors from "over amping" lots of maintenance on the belts was required. That is, clean the deck and lubricate often (these were always in use). That said, have you checked that the fuse has the proper value? I would think that the thermistor would not be needed with a fuse.

These treadmills absolutely need a lot of maintenance. Between cleaning and re-waxing the decks I use up most of my days. However, as time goes on the belt & deck's really just have to be replaced; and I work for a company that has no budget. So, I get stuck with work-arounds.

I attached a couple of photo's of the issue. The picture on the right shows the black Thermistor which always fries, and an in-line "wire" fuse right next to it. As you can see the Thermistor is what blows out, whereas I rarely have to replace either one of the fuses.

Besides all that though... Was I right about my guess on what the thermistor is probably used for?

 

[dlgoff]

How do you know if the fuse doesn't blow when it appears that it has been replaced with a wire? Maybe the motor is drawing too much current and since there is no fuse, the "thermistor, or what ever it is, is opening up.

 

[MacLaddy]

The two wires which are in the place of fuses, (I had assumed they were a type of fuse) come from the manufacturer.

Nobody has changed any design on these controllers. So they are being shipped from the warehouse with these wires/fuses on them.

 

[dlgoff]

Do you have or can you get the schematic from the manufacturer so we can have a peek?

 

[MacLaddy]

No, the manufacturer doesn't want us to know what's going on the inside because they charge us $500.00 for replacement, or $250.00 for repair. That's why I figured out how to repair these for a $2.50 thermistor.

 

[floopz]

No, the manufacturer doesn't want us to know what's going on the inside because they charge us $500.00 for replacement, or $250.00 for repair. That's why I figured out how to repair these for a $2.50 thermistor.

I am facing a similar problem, and have also been quoted $500 for a replacement board. I can see the thermistor has a chunk missing but have no idea what sort it is as it's unmarked. As a result, I've got no idea where to begin :uhh:. Any help would be appreciated.

 

[Phrak]

floopz. You're posting to a thread that's 2 years old, but if you have a motor control board, the thermistor is an over current protection device. It's typical of applications where the motor is expected to stall when the mechanism hits a stop (such as when the electric windows on a car hit the end of their travel but you keep your finger on the button).

If it's not coated, it's a dark-mat gray disk of material.

It's called a positive temperature coefficient thermistor. It gets more current; it gets hotter. It gets hotter; it gets more resistive. It gets more resistive; the current is reduced. The current is reduced and your motor doesn't burn up.

The required specs of such a thermistor depend upon your application.

 

[floopz]

Thanks for the tip, that gets me half way there, I couldn't work out whether it was a PTC or NTC thermistor in this sort of application. In this instance, it's a dark green matte one.

 

[Phrak]

Why in the world would it be NTC? Is it in series with the load?

 

[floopz]

Not sure, just I don't have any experience with thermistors

 

[MacLaddy]

The thread may be two years old, but they are still a constant annoyance.

Thanks Phrak, you actually answered my OP. Didn't realize it had been two years already. We finally came to the conclusion that a lower quality replacement belt was to blame, as it was drawing far too much current.

Floopz, I don't know if you are using it for the same treadmill/application as me, but I found the part here.
http://www.newark.com/ametherm/sl32-2r025/ntc-thermistor/dp/72J6846?Ntt=72j6846

Remember that you are dealing with potentially lethal voltages, which may not only be dangerous to you, but anyone that uses the equipment. So if you aren't 100% sure that this will work for you, and that it is the correct application, than it may be better to fork out the $500. That's a lot cheaper than a lawsuit.

PM me if you have any manufacturer questions.

 

[MacLaddy]

Why in the world would it be NTC? Is it in series with the load?

Yes, it is in series, which is why I find it so bizarre that it is fused in series directly after the thermistor.

Can't answer much more than that though, as it's reaching the limited boundaries of my electrical know-how.

 

[Phrak]

MacLaddy. OK, it's certainly a PTC if it's in series. The thermistor in this application has a different function than a fuse. It is used to limit start-up current when you turn the machine on. This has the side effect that the full line voltage doesn't get applied. Whereafter, as it warms up it will let alow nearly full line voltage into the circuits.

floopz. You might take a chance and replace it wth the same part number MacLaddy is using. If its the same or an equivalent part the thermistor shouldn't blow up again if you take some precautionsone simple precaution: Don't cycle power off then quickly on again. Wait thirty second for the thermistor to cool before turning on the power switch. And don't let the motor stall by stopping or overloading the belt. (Can you lube the belt with talc (baby powder)? I don't known anything about the care and feeding of treadmills.)

However it's also possible the thermistor blew because the electronics or motor shorted. Is the fuse also blown?

 

[stevar]

Without an circuit diagram we are guessing:

A NTC, Negative Temp Coef, thermistor can be used in a control ckt to increase the current with n increase in temp, enough to blow the fuse. Very crude, but cheap.

A PTC, Postive.., would require a voltage switch, which could be the holding coil of a relay, but relays cost.

Shame to use either, as thermal cutoff switches, and avoid the dependence on the characteristics of the current levels of fuse or relay coil , are available for a dollar.