Mills wire vs. ceramic power resistors

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AndrewT

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Joined 2004
Determine the average power dissipation required of the resistor.
Use that to determine what the Power Rating of the resistor should be.
Check the overload capability of the chosen resistor and compare to the worst case transient current that you expect to pass in normal operation, not including abusive accidents.

Now model the temperatures of the various parts of the "whole" resistor at that average power to see if you consider those temperatures to be acceptable.
Due to a severe lack of information in resistor data sheets you may just end up measuring an actual resistor in a test jig before you make the final decision.

Me, I just buy job lots of cheap 5W ceramic cased wirewounds. And use them for many different jobs.
I have lots of 0r39, 10r, 20r, 50r. And I buy 600mW 1% 1r0 metal film by the 1000. again lots of uses.
 
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Greebster

Greebster

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Joined 2010
Am wondering if anyone has thought to use or has tested Nichrome wire for use as a resistor. Unless coiled would not be inductive. Not sure of the noise vs standard or temperature coefficiency.
Grew up with this around the house, grandmother and father used for various projects like bending plastics (plexiglass) >40 years ago. Have a small spool here somewhere, hiding in a box I'm sure ;)
 
infinia

infinia

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Joined 2005
AndrewT said:
I suggested it, but got rubbished in the response.

It's worth trying.

Look up non inductive windings. Perry somebody and others.
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Its OK to make yer own resistors ( they got a learning curve too)
IMO It's just better to focus on the job on hand, IE speaker design.
rule of thumb for power resistors is to specify at least double the average power esp with no airflow.
running near limits you can use wide PCB traces to wick off some heat, or other attached metal bits, IE heatsinks
 
Greebster

Greebster

Member
Joined 2010
AndrewT said:
I suggested it, but got rubbished in the response.

It's worth trying.

Look up non inductive windings. Perry somebody and others.
Click to expand...
Bifilar and all that jazz, been there done that back in the '70's :)
It was just one of those things I never got around to checking years ago and was wondering off hand (and out loud ;) Other than thermally insulating don't see an issue. Don't have a speaker need for, might use in a toroid start ckt. When I started cutting foam wing cores used nichrome wire, but found steel fishing leader to work best as it has a higher tinsel strength and doesn't stretch as much when hot. Cuts a much cleaner line. :)
 
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AndrewT

R.I.P.
Joined 2004
Greebster said:
Bifilar and all that jazz, been there done that back in the '70's :)
It was just one of those things I never got around to checking years ago and was wondering off hand (and out loud ;) Other than thermally insulating don't see an issue. Don't have a speaker need for, might use in a toroid start ckt. When I started cutting foam wing cores used nichrome wire, but found steel fishing leader to work best as it has a higher tinsel strength and doesn't stretch as much when hot. Cuts a much cleaner line. :)
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I used piano wire because a competition glider designer suggested it. Enough resistance that a simple battery charger could do the heating, (6V or 12V each with high and low, gave 4 heating options).
Very hard and very strong so it holds the "straight" shape while being dragged through the melting plastic.
 
Greebster

Greebster

Member
Joined 2010
infinia said:
Its OK to make yer own resistors ( they got a learning curve too)
IMO It's just better to focus on the job on hand, IE speaker design.
rule of thumb for power resistors is to specify at least double the average power esp with no airflow.
running near limits you can use wide PCB traces to wick off some heat, or other attached metal bits, IE heatsinks
Click to expand...

Yes the learning curve... here was a thought I had some 30years ago. Make fiberglass posts and web the nichrome wire alternating front to back within the port in a zig zag, bare wire (easy to calculate). As power levels increased port velocity would cool the wire adding a bit of head room. Noise would be minimal.

OK I said it, shoot me now :p
 
infinia

infinia

Member
Joined 2005
logon said:
I resumed this "ancient" thread because I found this post very interesting and also I think it might still be so for other people: thank you!

Since I would like to try Vishay RCH 50W resistor then my question is related to its needed heatsink.

I read from datasheet that "in air" or "unmounted" a 50W Vishay RCH resistor dissipates only 5.5W so I wonder how it can even be used at its full power inside a loudspeaker and most of all which type of heatsink would be suitable.

Many thanks for any appreciated reply.
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the problem with crossover resistors it's difficult to determine what the average power will actually be IE music levels ( so most over specify to what 2% of users will actually use)

if you post a crossover schematic where the resistor is and amount of audio power used , more ppl can give an opinion on how much metal you should bolt onto an RCH if any.
 
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AndrewT

R.I.P.
Joined 2004
I used nichrome to adjust the Q of a speaker.
It needed a few tens of milliohms to correct the Q to the value I wanted to match it's partner in the stereo pair.

I inserted the nichrome in the bass driver speaker cable.
It ran alongside the return wire as a part of the twisted pair. It added NO EXTRA inductance to the cable. Bakers fluid was used as the flux to attach it to the copper cable.
 
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AndrewT

R.I.P.
Joined 2004
I have never done that.
I just match ready made ceramic cased wire wounds.

I have some commercial amplifiers where the output device emitter resistors are hand wound resistance wire. Deliberately stretched, presumably to reduce capacitance.

A length of nichrome stretched out along a 1m (100cm) rule makes a VERY GOOD variable resistor in a Wheatstone bridge.
 
infinia

infinia

Member
Joined 2005
Greebster said:
Do you leave flux on your connections? That is the only issue... Learning curve bro.
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cleaning up flux ... no not always, not on purpose anyways.
my experience with using acids at the workplace is not my best memories!
I used to have in house assemblers solder RF PCB to brass housings, sent one home for 2 days for fumes from residual acids. learned something bro, glad I ddnt have to by going to court
 
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Greebster

Greebster

Member
Joined 2010
infinia said:
cleaning up flux ... no not always, not on purpose anyways.
my experience with using acids at the workplace is not my best memories!
Click to expand...

Have used powerful acids in various applications most of my life and always clean all fluxes. Have a minor in chemistry, so always use caution. Flux removal is from my NIST PMEL ECM experience and lessons learned growing up with a pyro father with a "enough to get you into trouble" chemistry background. Pops tried to invent PU glue, what a mess that was :eek:
 
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