Wrapping the resistors with the thermocouple wire is not a good idea because that thermocouple winding will pick up the magnetic flux variations from the current flowing in the resistors and the induced voltage will swamp the thermocouple signal approximately 100000 times.
See the scopeshot below for a 10Ω ¼W THT resistor wrapped with 7 turns of wire while a 1MHz 340mAP-P square AC is flowing through it.
Green trace is the current through the resistor and the yellow trace is the voltage induced in the 7 turns of wire (4.8VP-P).
Dear Verpies
I'm trying to figure out how you got (7 turns) from the picture I posted, since it only shows one half turn of the
twisted portion of the junction. Also you do not explain what
load you had on the 7 turns. As you know an
unloaded coil will have quite a self resonant rise and tend to exaggerate the voltage produced across it, even by just capacitance coupling from a nearby object.
1) I never suggested anyone should use 7 turns of TC lead wire around the resistor, I don't know where that came from. I use only one half turn of the twisted portion that forms the thermocouple junction. Since it is twisted it is immune to current in the resistor except perhaps for some common mode signal capacitively coupled. The common mode as well as series mode AC signal is rejected by most good
DC uV meters, as generally it would be very high frequency, low level. The output of the thermocouple is DC voltage or current depending on how it is used.
2) The resistor you used may have been "spiral cut" to trim it's value. This creates a few turns on the resistor film which can indeed inductively couple to the 7 turns,
As you know, a wire passing through the center of a coil cannot induce current into the coil by Faraday induction because it is at right angles 90 degrees to the coil. It can however excite the turns capacitively, which is what I see in your scope shot.
3) As I said earlier, there are improvements that I would discuss at a later time so as not to overly complicate the basic device. A couple of the improvements are to add ferrite beads to the thermocouple leads both series mode and common mode to attenuate any capacitive or inductive coupling effects. I did not complete the fine points of advanced construction yet.
4) Depending on the skill of the constructor at forming a tiny thermocouple junction, it as not at all necesary to wrap the junction or it's associated wires around the body of the resistor. A tiny bead of epoxy will thermally connect the tiny junction to the resistor.
5) There is a well known technique that should the constructor wish to add a turn or two of the wires around the body of the resistor, it can be done in a in a non-inductive manner. It is the same method used to make non-inductive wirewound resistors. Why would an extra turn or two help?
The value of the extra turn is more precise temperature reading by also heating the thermocouple wires leading to the junction so that the leads do not adversely drain heat from the junction.
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There seems to be an intent on killing this method before it is "off the ground". Trying to warn others that it is a bad method is probably working, having it's effect by now.
I have not yet had a chance to post the finer details of the method such as the HF rejection curves.
Sorry for that, I have a lot of domestic and health related issues to deal with right now.
P.S Brad
As I mentioned in the beginning there are many ways to sense the temperature of the resistors e.g. thermistors, Thermal IC chips, diodes. You could build a bridge that uses any of those devices but it will not be passive, will need a power supply, and will have a propensity to pick up HF noise that would not be easily filtered due to the non-linearity and rectification of some of the devices.
I chose thermocouples because I am quite experienced in using them and they solve a lot of the problems that would be introduced with other temperature sensing methods and it is a passive method, not requiring a power supply, which could introduce additional capacitance coupling.
Also I have a lot of good uV meters and measuring instruments in the shop. Nowadays they are very common. Back 50 years ago they were more difficult to construct, usually requiring chopper stabilised DC amplifiers.
When I get time I will try to demonstrate using a passive analog 50-0-50 uA meter.
To each his own.
I'm gonna take a break from this jousting session, it's bad for my health.
"Secrecy, secret societies and secret groups have always been repugnant to a free and open society"......John F Kennedy
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Topic: Flea Power Measurements (Read 8328 times)
