why does swapping the test leads around give an apparent resistance change?
From near short to ~double the expected value?
Could severe overheating oxidise a surface and create an oxide rectifier?
Then in one direction it passes current showing a near short.
But in the other direction it blocks currents and sends the measure around a different route?
Guessing. Any other ideas?
From near short to ~double the expected value?
Could severe overheating oxidise a surface and create an oxide rectifier?
Then in one direction it passes current showing a near short.
But in the other direction it blocks currents and sends the measure around a different route?
Guessing. Any other ideas?
If there is a shorted turn, it may be forming a rectifying contact. Put another 1 ohm to ground in series with it, and see if the current is sinusoidal. You may be only stressing half your amp.
As I said: thermoelectric effects.
And how do you explain the increase in resistance?
For me it looks like the resistor has not failed and the DMM just gets confused by thermoelectric voltages. Measure the voltage across the bare hot resistor and see how many mVs you get. Also do the measurement procedure as suggested by me earlier in this thread.
Samuel
The average of both readings stays constant, right? Another clear indication that we have thermoelectric effects at work. In fact more advanced resistance measurement systems reverse the DC current polarity and take the average of both readings to cancel these voltages.
Samuel
A small experiment
6.8 ohm 25watt ali body resistor unmounted
13.8V 2Amp (27.6 watts 🙂 )
measures 6.9 on my DVM both ways Cold
measures 5.8 and 7.3 on my dvm at 140C
at 140 its taking 13.8v at 2.01 amps from the bench supply so its real resistance is still 6.8 ohm
all readings will be to the tolerance of the meters - none are calibrated.
Looks like a thermoelectric effect.
I'll stick to fan cooling 🙂
6.8 ohm 25watt ali body resistor unmounted
13.8V 2Amp (27.6 watts 🙂 )
measures 6.9 on my DVM both ways Cold
measures 5.8 and 7.3 on my dvm at 140C
at 140 its taking 13.8v at 2.01 amps from the bench supply so its real resistance is still 6.8 ohm
all readings will be to the tolerance of the meters - none are calibrated.
Looks like a thermoelectric effect.
I'll stick to fan cooling 🙂
Yes, thermoelectric emf is confusing the meter. This means that internally there are different metals with temperature gradients. You would have to ensure that the resistor and its leads (internal and external) were perfectly symmetric to avoid this, or just keep it cool. However, it won't affect your amp tests unless the amp has a peculiar response to a few mV of DC fed back into it.
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