Carbon film vs metal film resistors

[Viperel]

I had a few 5% 1/4W carbon film resistors blow up on me during my lifetime and i have always replaced them with the same type. Now i am wondering if metal film resistors could be used to replace them in sensitive circuits like switched-mode power supplies.

I understand that metal film has better reliability with age and less noise and 1% tolerance, are any of these elements going to introduce any issues though?

To sum it up, is there any advantage to using metal film resistors over a carbon film in PSU's?

[stj]

Re: Carbon film vs metal film resistors

metal film is a better replacement, but they cost a little more.
that's why they arent used originally.

[Viperel]

Re: Carbon film vs metal film resistors

Thank you, time to stock up on some resistor packs off ebay then

[eccerr0r]

Re: Carbon film vs metal film resistors

Are metal film resistors more inductive than carbon composition? Probably not an issue with SMPS but might be for radio apps.

[stj]

Re: Carbon film vs metal film resistors

neither should be inductive, both are simply a film coating on a ceramic rod.

[Longbow]

Re: Carbon film vs metal film resistors

The difference is almost non-existant except for the cost. 99.9% of the time, there is no good-better-best. You might also think twice about whether you are getting good quality metal film from overseas. The main improvement in resistors is really that the carbon composition style is no longer used.

[stj]

Re: Carbon film vs metal film resistors

actually, metal film handles more current for it's size.

a 7mm carbon film will do about .1/3w, a 7mm metal film will do .6w

[mariushm]

Re: Carbon film vs metal film resistors

Thank you, time to stock up on some resistor packs off ebay then

Forget about ebay, go to a site like http://www.TME.eu or http://ro.farnell.com and order everything you want .. you'll pay around 20 lei for shipping but you'll get everything in a few days and they're sometimes of better quality than what you get on eBay.

You can order 25-50-100pcs of what you want for lower prices and you pay shipping costs only once for the whole package so if you buy a bunch of parts the shipping cost is not that big of a deal.

[eccerr0r]

Re: Carbon film vs metal film resistors

resistors are slightly inductive, they have a slight spiral in them...

I'd imagine carbon composition resistors to be slightly less inductive as they need less "windings" to get the right resistance.

[montemcguire]

Re: Carbon film vs metal film resistors

The degree of spiraling depends only upon the resistivity of the deposited film, not whether the film is carbon or NiCr. An untrimmed 'slug' that's closer to its final value requires a shorter spiral, and vice versa.

In general, the spiraling of a through-hole resistor will add a pretty negligible amount of inductance. These days, if that was actually something that affected the circuit, an SMD resistor would be used instead of a through-hole part. An SMD resistor chip has less inductance simply because its terminals are closer together, so it's electrically shorter and thus less inductive. Remember, a straight piece of wire has inductance too - it's not just the spiral that contributes to inductance.

[linuxguru]

Re: Carbon film vs metal film resistors

... a straight piece of wire has inductance too - it's not just the spiral that contributes to inductance.

Correct - the greatest contribution to resistor inductance probably comes from the leads and end-caps. If they contain any ferromagnetic material (iron, steel, nickel, cobalt, etc.), their relative permittivity will typically be ~100x to 1000x that of non-ferrous leads (Silver, Copper, Palladium, etc.) and their inductance will also be higher by the same proportion.

It's usually not important in most applications, but steel and nickel-leaded resistors sound terrible in audio applications. For that matter, most Ni-Cr metal film resistors also sound too harsh, even if they have non-ferrous leads. The best-sounding resistors that I've heard are usually non-inductive wirewounds, closely followed by Palladium-Silver thin-film chip resistors (available packaged with Cu leads) from Japanese vendors like Susumu and Nikkohm. They're expensive (say 100x to 500x that of stock Chinese MFRs), but indispensable for certain locations in the audio signal path (including feedback networks).