Understanding the Notorious Thermistor

[Pixl]

Lone Gunman asked me this question in a PM, I thought I'd move it to the forum for others to learn or add to it.

"It's my understanding that a thermister either works or it doesn't? Yes/no?

Reason I ask is I have two S9 receivers and was wondering if I need to do anything to them if I'm NOT seeing any problems now other than what others see that has been associated with firmware, ie, occasional freeze up once every couple of days that requires a reboot?

Your thoughts? "


The thermistor acts like a fuse in that it opens up and shuts off the power to the circuit it is connected to when it's rating (650ma) is exceeded, usually by a short. The nice thing about these is that they re-set themselves rather than requiring replacement after the short is removed.


They should run cool (room temp) under normal loads such as one or two lnb's, a switch, and a motor. Defective ones will run hot to the touch at normal loading and trip off prematurely and gets worse over time. So there is not a definite works or doesn't work, just that they get weak.


A simple test is to just feel it while it is on and connected to your dish system, should be cool. Known defective part# start with M60, or W60. Also one report of a R60 that was failing.

Lone, this may not be your freezing/booting problem. You might look to trying different FW if the thermistor is ok.

 

[AcWxRadar]

Pixl,

A thermister is a resistive device (just like a resistor) but changes its resistance with temperature. If the coefficient is positive, then as the temperature increases, the resistance increases. It can be utilized as a current regulating device because the more current flows through it, the hotter it becomes and thus the higher its resistance becomes, which limits the current flow.

However, it isn't designed to OPEN and remain OPEN like a fuse would, it just reaches a very high resistance value and eventually appears to be nearly an open circuit which stops all current flow.

RADAR

 

[AcWxRadar]

Looked this up on wikipedia, (PTC is positive temp coefficient and NTC is negative temp coefficient)

Applications:

PTC thermistors can be used as current-limiting devices for circuit protection, as replacements for fuses. Current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating. This creates a self-reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device.

PTC thermistors are used as timers in the degaussing coil circuit of most CRT displays and televisions. When the display unit is initially switched on, current flows through the thermistor and degaussing coil. The coil and thermistor are intentionally sized so that the current flow will heat the thermistor to the point that the degaussing coil shuts off in under a second. For effective degaussing, it is necessary that the magnitude of the alternating magnetic field produced by the degaussing coil decreases smoothly and continuously, rather than sharply switching off or decreasing in steps; the PTC thermistor accomplishes this naturally as it heats up. A degaussing circuit using a PTC thermistor is simple, reliable (for its simplicity), and inexpensive.

NTC thermistors are used as resistance thermometers in low-temperature measurements of the order of 10 K.

NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purposely designed for this application.

NTC thermistors are regularly used in automotive applications. For example, they monitor things like coolant temperature and/or oil temperature inside the engine and provide data to the ECU and, indirectly, to the dashboard.

NTC thermistors can be also used to monitor the temperature of an incubator.
Thermistors are also commonly used in modern digital thermostats and to monitor the temperature of battery packs while charging.

 

[KE4EST]

Great info, this definitely will help members to understand without taking an electronics course.

 

[Pixl]

Well said, also here is some more information from the "Little Fuse" Co. if you can get past the advertising.
Fuse or PTC? - Electronic Products

And a short excerpt......

Common PTC applications
Much of the design work for personal computers and peripheral devices is strongly influenced by the Microsoft and Intel System Design Guide which states that “Using a fuse that must be replaced each time an over current condition occurs is unacceptable.” And, the SCSI Standard for this large market includes a statement that “....a positive temperature coefficient device must be used instead of a fuse, to limit the maximum amount of current sourced.”

 

[chaskuchar]

so when i go to radio electroncs i just ask for a 650ma thermistor and that will suffice? i do have the literature on the one you replaced with. i think... charlie

 

[Pixl]

so when i go to radio electroncs i just ask for a 650ma thermistor and that will suffice? i do have the literature on the one you replaced with. i think... charlie

Well there are more to the specs than just 650ma. See the Digi-Key page
Digi-Key - F1990CT-ND (Manufacturer - 60R065XPR)

Or give your vendor the Little Fuse part number 60R065XPR.

 

[turbosat]

Don't get a blister
From your thermistor!

 

[rrob311]

So this device is like a solid state circuit breaker? I have a new to me S10 and every time I try to turn on USALS it resets the box. What should I check? The box is almost new.

 

[Pixl]

Well here is a long thread on replacing the thermistor.
http://www.satelliteguys.us/free-air-fta-discussion/279413-fix-your-openbox-s-9-58-a.html

However most people had the trouble of the lnb voltage slowly going low and loosing signal, not a sudden reboot. This might not fix your problem, but look at the part number and see if yours is one of the bad ones.

Here is a thread I posted about testing the S-9 voltages. Might help with testing your S-10.

http://www.satelliteguys.us/free-air-fta-discussion/277334-how-test-openbox-s-9-lnb.html


I'm working on a new post for yet another problem that has been discovered on the S-10. Stay tuned.



.

 

[AcWxRadar]

I'm working on a new post for yet another problem that has been discovered on the S-10. Stay tuned.

Hey Pixl,

Keep up the great work!

I don't have an S9 or S10 IRD, but I love to read your posts and all the informative hints/how-to's for these receivers. When I get my hands on a reciever that I think has potential, I get into it as deep as I can (Coolsat 5K and AZBox Premium for me). I view what you are doing with the S9/S10 as a sincere committment (not only to the receiver make/model/brand) but to your fellow satellite hobbyists here. It is a great thing that people like you go out of your way to procure information for others. It is what makes SatelliteGuys.US a superb site and why so many people come here!

RADAR

 

[N0QBH]

The PTC fuse issue cropped up in my S9 recently.
Symptoms were erratic switching between polarity and total weirdness when operating the dish mover.
Just out of curiosity, I measured the voltage drop across the bad PTC.
It was almost 2 volts sitting stationary, jumping to over 4 when trying to move the dish.
No wonder the polarity was confused...
After replacing the PTC, the voltage drop became a much more reasonable .18 volts stationary and .33 volts when moving.
Thanks to the folks who found this issue - made troubleshooting it a breeze!