I am attempting to repair an older piece of equipment and have found what looks like a damaged component. There are two identical sections on this circuit board and each have this component. The component designators on the board start with R and all other normal looking resistors on the board also start with R.
The one I think is good measures 25K and the one I think is damaged measures around 5K. They are light yellow discs and measure approximately one inch across and are very thin, around .065" They each have two legs. The good one appears to have the marking 40/3 and appears to be newer and to have been changed before. Both were wrapped in a heavy piece of heatshrink and dated 91/01/07 with a marker.
If I heat the good one up the resistance drops but not dramatically 25K to around 16K when too hot to touch. These components are part of an output power transistor circuit and they each have a diode in a TO16 package bent over and siliconed down to the top of these discs. The discs were mounted flat against the PCB. I assume the diodes are some type of thermal protection and are also found siliconed down to some power resistors on the same board.
Any help identifying or finding a suitable replacement would be appreciated.
thanks for your time,
ED
Could these be some older style NTC inrush limiters or what???
The good one measures 25K
The damaged one measures 5K
Resistance is way too high to be an inrush current limiter, they are usually less than 500 ohms. Aslo ICL’s increase in resistance, PTC, with an increase in heat(current). It’s most likely an NTC Thermistor that adjusts circuit parameters based on the diode’s body temperature.
Assuming the 25K value is at 25°C, there are four R/T curves avaialble for these devices at DigiKey. https://www.digikey.com/short/ppzhzq
I’m pretty sure over the decades many other curves were offered for NTC thermistors. So if you can’t find out any more part info you’ll need to measure the resistance at 85°C to determine which curve type you need.
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Thank You for taking the time to answer my question.
I will give the thermistors a look. I have a thread running over on the EEVblog also and the suggestion was made that they could be MOV’s as the legs are usually soft soldered to a metal disc and ooze out when overheated.
Would the numbers 40/3 mean anything if they where MOV’s
I agree after a little research that the resistance is WAY to high for an NTC current limit.
I think I may try and draw this part of the circuit and see if I can learn more about there function.
thanks again for your reply!
Eric
They can’t be MOV’s unless they are both broken because MOV’s read near infinite resistance (open circuit) until the varistor voltage is reached. When the varistor voltage is reached they clamp at that value (dampening the surge) and stay clamped similar to a zener diode but also working for AC as a well as DC.
Given your measurements and mounting description I’m pretty certain they are NTC thermistors that adjust the analog circuit performance based on a diode’s or resistor’s body temperature. This was very common in precision analog circuitry in the past before analog IC’s routinely had built-in temperature compensation.
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Yes. What you have said makes sense.
I looked at the link you provided but did not see anything that looked close to these size wise. These discs are pretty large and measure almost exactly 1 inch. They do behave like a thermistor. At least I get a pretty good swing hooking them up. Heat causes a drop in resistance so NTC I think.
I guess, following your logic, the diode should be in line with the power output components and the feedback from the Thermistor would go back to an analog drive section of the circuit. I’ll see if I can tease out some of the connectivity later today and try and at least get a partial schematic.
thanks again for your help
