This is the coil from a solenoid that is used in a dental counter top sterilizer. When it is energized it pulls a plunger away from a valve seat and lets water flow through the valve and into the sterilization chamber. While I was working on a sterilizer a while back I got some really strange readings on my ohmmeter when I attempted to verify continuity in the coil of this solenoid. Further tests on other coils in other sterilizers showed that my unusual readings were consistent with this style solenoid.
As one can see from the label this coil is designed for 120 Volts 50 to 60 Hz and is rated at 14 Watts which would indicate a current draw of approximately 100 mA and a DC resistance of approximately 1K Ohms.
The Ohms reading I was getting however from various digital meters was more in the area of 500K Ohms. I hooked the coil up on the bench and applied 115 Volts AC to it and got a current reading of about 100 mA as expected. Still the digital meters all said the resistance was 500K Ohms. When I tried using one of my analog Ohmmeters I got a resistance reading of about 800 Ohms. The coil was acting very much as if there was a semiconductor junction in series with the coil. I had seen this before when I have attempted to read polarity on a diode using the Ohms scale of digital meters. The digital meters do not use a high enough test voltage to overcome the junction energy level and therefore do not give an accurate reading. The analog meter on the other hand uses 3 volts and therefore can read the resistance past the semiconductor.
I continued my tests by reversing the polarity of my test leads and found that there was no noticeable polarity discrimination. I got the same 800 Ohms on the analog meter and 500K on the digital meter regardless of the polarity of the test leads. When I switched the digital meter to Diode Test it told me that the energy barrier that I was dealing with was about 1.9 Volts. This would be close to three Si diodes in series with the coil and since there was no polarity preference there must be an additional 3 Si diodes reverse polarity in parallel with the first three diodes.
Now I have no way of knowing if these are standard diodes or some other semiconductor such as a NTC or PTC Thermistor or some other type of semiconductor. Whatever it is, it is molded into the plastic shell that is formed around the coil itself and not visibly evident.
I attempted to find data sheets on the coil from the manufacturer but there was nothing to indicate that it is anything other than a simple coil.
My question is: Does anyone have any idea of what type of semiconductor might have been put integrally in line with the coil of this solenoid and what purpose would it serve? Does my analysis of this actually being a semiconductor in series with the coil make sense? Are there any other possible explanations?
The solenoid, when in operation, must dissipate the 14 Watts that is generated by it and it is in an already hot 80 degree C environment. The fluids flowing through it are as hot as 100 C. Other than being in a hot environment and handling hot fluids there is nothing really unusual about the solenoid. Also in recent models of this sterilizer a different solenoid is being used that does not have the anomalous readings of the older style solenoid. The new solenoid has a roughly 1K DC resistance that is readable by both the Digital and Analog Ohm meters.
Any insights will be appreciated.
John
