Phil Allison wrote:

"Cydrome Leader"
Phil Allison

** The lower the resistor's value, the more significant inductance
becomes.

Egs:

I use a pair of 100W rated, 8 ohm, tubular, wire-wound resistors as a
dummy
load - wired in series for 16 ohms and in parallel for 4 ohms. The
ceramic
tube is about 1 inch dia and 4.5 inches long with 34 turns of flat strip
conductor would along it, so the inductance is about 6uH.

At 20kHz, the error caused by inductance is under 0.5% while at 100kHz,
the
error is still only 10%.

For the 1 ohm wire-wound sense resistor, the inductance was about 200nH.
However the ringing frequency was around 3MHz making the inductive
reactance
almost 4 ohms - four times more than the value of the resistor !!!

well, it's known that giant ceramic wire wound power resistors are not the
choice for RF use.


** Sure - but this is a current sense resistor in SMPS circuit where most
of the current is at 100kHz but with a strong harmonic at about 30 times
that frequency. Problem being, the over-current trip was being activated
prematurely because the peak value of that harmonic was being exaggerated by
a factor of 3.

I agree with this- for the power supply problem- who'd have expected this
to happen?

While 1W metal film resistors have low enough inductance to work OK in the
circuit, typical wirewound types do not.

BTW:

I could have added a RC network ( ie a Zobel ) across the 1 ohm wirewound to
bring it line at 3MHz and well beyond. A 47nF cap and any 1 ohm, 0.5W film
type would do fine.

Same goes for the 100W 8 ohm power resistor, a Zobel using 68nF and 8.2 ohms
corrects it to beyond 5MHz if that was ever needed.

I wonder how much that would confuse the next repair tech?

Bizarro repairs are always fascinating to come across.