I had been using this 24 volt power supply I pulled from a laser printer, many years ago. I heard a "POOF" one day when I was experimenting. Opened it up and found a fractured ceramic resistor. It was a weird shape, I didn't recognize - so I ordered 10 of the same watt rating (5 watt), 2-ohm - no big deal, at $1.50 for 10 (with delivery cost), I might actually find a use for the other 9 at some other time.
Days later I was browsing for cheap gizmo's on eBay, and found the same shape resistor, was labeled as "NON INDUCTING" resistor. I think they wanted like $8.00 EACH for them!
The $8.00 isn't going to throw me into bankruptcy, but it's too much money for me to spend on a spare power supply that's 8 years old.
For educational reasons, I am a little curious WHY someone would want or use a NON-INDUCTING resistor?
Because you want it to be a resistor, not an inductor. If it was being used at DC it would not matter, but if you want it to be a resistor at high frequencies you want it non-inductive.
Low value resistors are often wire-wound around a ceramic former or core. If the wire is wound in a continuos direction the result is both resistive and inductive, which will have an effect in high frequency circuits.
To create a non-inductive wire-wound resistor, half of the resistance windings are wound in one direction (say clockwise) then the wire is folded back on itself and the rest is then wound in the opposite direction (say anti-clockwise) These two windings are inductive but the two inductances so created "cancel" each other out (from an inductance viewpoint) whilst the resistance is the value determined by the full length of wire.
DocStein99:
I had been using this 24 volt power supply I pulled from a laser printer, many years ago. I heard a "POOF" one day when I was experimenting. Opened it up and found a fractured ceramic resistor. It was a weird shape, I didn't recognize - so I ordered 10 of the same watt rating (5 watt), 2-ohm - no big deal, at $1.50 for 10 (with delivery cost), I might actually find a use for the other 9 at some other time.
Days later I was browsing for cheap gizmo's on eBay, and found the same shape resistor, was labeled as "NON INDUCTING" resistor. I think they wanted like $8.00 EACH for them!
The $8.00 isn't going to throw me into bankruptcy, but it's too much money for me to spend on a spare power supply that's 8 years old.
For educational reasons, I am a little curious WHY someone would want or use a NON-INDUCTING resistor?
Hi,
Did you mean "non inductive" resistor? I guess you could call it the other though too if you like.
As others have been saying, the resistor is made so that it has very little inductance but still has the right resistance. That means it is wound a different way. In the circuit it helps when there is a high frequency or fast rise and fall signal. It has low inductance so it does not react as badly as a regular wire wound resistor would. In some cases it is very important to have one, in other cases it doesnt matter. It's usually a matter of frequency.
The label on the broke resistor is "MPC75", I found a document that explains, low inductance. Inside this gizmo are bands in a zig-zag from left to right, soldered at the top and the bottom. Interesting.
Mine marks "2.2 OK" (where "O" is the OHM symbol). I thought it was 2.2k ohm? Confused. This document claims it's available in "2,20 ohm" (I assume the comma means decimal). They offer other values, .01 .05 .08 - but nothing in 2.2k range. I do not know why there is a "K" stamped on my broken resistor or what that should mean since this manufacturer doesn't seem to supply 2,200 ohm low inductance 5w resistors.
Thanks for the information. These resistors are rare, and they only make certain ohm ratings. Well, I'll just go buy another junk HP laserjet 4 from another flea market and have all the rest of the goodies with the power supply. The resistor is just too expensive.
My broke resistor does not physically resemble this design. I checked again. It's a ceramic housing, 2 leads sticking out. A z-shaped zig-zag of metal, which is soldered at each vertical point.
JohnLincoln:
Here is the datasheet for the MPC75 series of resistors.
Yes. That's the same one I've been looking at. I was surprised to find it was actually easy to look up. I knew it was a resistor, and ordered 2.2 ohm 5w resistors - after reading only the chart section of the datasheet. Ironically, just a few days (without looking for it, just stumbled on it while searching for something else) I found "non inducting resistor" with a picture of the same one I broke.
Two methods of making an non-inductive wirewound resistor:
The two left-hand images are inductive windings, the right-hand images are non-inductive.
In the top one, the wire is wound in one direction, does a u-turn, and is rewound in the new direction. A practical construction will probably have several u turns to make sure the direction changes are evenly distributed throughout the entire winding.
In the bottom one, the wire is split and the two branches are wound in opposite directions and merged at the end.
Electrically, the two windings act like a 1:1 transformer with the primary and secondary winding in series and oriented so the voltages they produce is inverted. +1V from one winding is exactly cancelled by -1V in the second coil, so the net effect of the inductance of the "two windings" is 0.
Then it's not wirewound and will inherently have pretty low inductance. Thos construction techniques are really only applicable to wire-wound ones. Like the name implies, those kind of restors are constructed by just wrapping a certain amount of wire around a spindle and encasing it in a protective material like cement. The wrapping naturally forms a coil, and if you don't wrap it the right way there will be significant parasitic inductance.
Other construction techniques and materials don't necessarily suffer from the same issue.
If it blew up, how do you know it's 2 ohms
If it's a zigzag of metal then could it be a current sense resistor and hence of extremely low value
If so, the 2?K may mean it's actually 0.002ohms
Hi. The 'K' is most probably the tolerance of the resistor. Off the top of my head the characters J, K, L and M represent tolerance values of 2%, 5%, 10% and 20% respectively. If the K was to represent thousands as you suspect, the value would probably have been written 2K2O (again where O stands for the ohm symbol). So you have a 2.2 ohm 5% tolerance reistor. Another variation is to use 'R' instead of the d.p. to signify ohms - as in 2R2.
Jim.
PS - first post. Just had to sign up to let you know this!
Well, that's interesting. How is inductance measured? I wonder if I can just make a 2.2 ohm resistor out of this stainless wire, or the nichrome samples I have?
Regular resistors have a very small inductance. This is no problem in circuits up to 10 MHz when the current is in the milliAmpere range.
Power supplies nowadays use switching technologies. This implies currents of 10, 20 or even more Amperes.
And the signals are digital (5KHz - 50KHz). Rectangle. Only in the converter section of the circuit of course.
The voltage conversion is still done with coils. However in an other way than in standard transformers.
So a current limiting resistor should have no impedance.
If the current limiting resistor has some impedance of its own, it will reduce the current flow of a rectangle signal. Manufacturers do everything to keep the power supplies small and extremely low cost. If a resistor can improve efficiency by 2-3% the will use it, even if it cost 5 cents more (manufacturers will not pay more than that).
A standard resistor will simply reduce the power efficiency of your power supply.
If you are not taking more than 70% of the specified power from your power supply you do not have to worry about inductive / non-inductive resistors.
