Metal film resistors?
 

are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.


someone please help!

dr. x


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They're not wirewound. They consist of a thin film of metal deposited on
a ceramic tube, which is helically cut to leave a conductor in a spiral
pattern. A wirewound resistor has a lot of inductance because it's made
of many turns of wire. A metal film resistor has a very small amount of
inductance by comparison -- so little that it can be ignored for most
purposes.

Roy Lewallen, W7EL

James Bond wrote:
are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.


someone please help!

dr. x

James Bond wrote:
are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.


someone please help!

dr. x


Strictly speaking they are not "wire wound", but some are constructed
with the film "wound" around a substrate, thus having a lot of inductance.

If you dont't have the makers specs to tell you, the choices are to
measure the inductance or to remove the coating and see what's underneath.

--
Jim Pennino

Remove -spam-sux to reply.

are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.

Strictly speaking they are not "wire wound", but some are constructed
with the film "wound" around a substrate, thus having a lot of inductance.

If you dont't have the makers specs to tell you, the choices are to
measure the inductance or to remove the coating and see what's underneath.

Several manufacturers (Caddock and some of the Vishay divisions) make
metal-film resistors which are advertised as having about the same
amount of inductance as a straight wire the same length as the
resistor body. I believe that these have the film cut into a
back-and-forth serpentine meander pattern, rather than a spiral.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

James Bond wrote:
are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.

They can be suitable for some RF applications. If you are thinking of
the wire-ended ones up to 2-3W, they are a flat spiral of just a few
turns of the metal film, on a ceramic core.

Plugging the number of turns, diameter and length into the usual formula
suggests inductances of less than 0.1uH, even for the larger ones. This
means they have inductive reactances rising up to about 10 ohms at
30MHz.

For example, if you wanted to use say a 47 ohm resistor as a dummy load,
the SWR would be quite reasonable up to 30MHz.

However, the inductance of the higher-value resistors goes up much less
rapidly then the resistance does (they use a thinner film of a more
resistive material, but not a lot more turns). The inductance of a 470R
metal film resistor is not a lot higher than that of a 47R. So if you
were to use ten 470R resistors in parallel to make your dummy load, the
inductive reactance would be divided by 10 and the SWR would be much
better.

I have used multiple-paralleled 2W metal film resistors as high as
50MHz, and up to144MHz in situations where the inductance could be tuned
out.

The ones I tested can also be overloaded to red heat for several seconds
without significantly changing resistance when cold - unlike the cheaper
carbon film resistors which burn (as carbon does), or wire-wound
resistors which generally develop a hot-spot and burn out.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Wed, 24 Nov 2004 20:57:59 +0000, Ian White, G3SEK wrote:

Plugging the number of turns, diameter and length into the usual formula
suggests inductances of less than 0.1uH, even for the larger ones. This
means they have inductive reactances rising up to about 10 ohms at
30MHz.

Ian, I've just measured a couple of resistors bought as spares for the
FL-2100Z, 33 ohm 3W metal film used as a grid stopper, and 22 ohm 3W metal
oxide used on the anode, again as a suppressor, with a coil wound round it
(both of the originals were carbon).

The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the
aade.com meter. Both values (if the readings are correct) would represent
a high ratio of X to R at HF frequencies...

I have quite a surplus of them, so if you wish I can send one of each in
the mail for you to check on your VNA.

--
Duncan Munro
http://www.duncanamps.com/

I've used "Flame Proof" resisters to build signal splitters, the
outputs were within .1 db of a commercial unit (made by Suhner) @ 850
Mhz.

That ain't bad :).

Howard.

On Wed, 24 Nov 2004 18:59:16 +0000 (UTC), "James Bond"
wrote:

are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.


someone please help!

dr. x


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Duncan Munro wrote:
On Wed, 24 Nov 2004 20:57:59 +0000, Ian White, G3SEK wrote:

Plugging the number of turns, diameter and length into the usual formula
suggests inductances of less than 0.1uH, even for the larger ones. This
means they have inductive reactances rising up to about 10 ohms at
30MHz.

Ian, I've just measured a couple of resistors bought as spares for the
FL-2100Z, 33 ohm 3W metal film used as a grid stopper, and 22 ohm 3W metal
oxide used on the anode, again as a suppressor, with a coil wound round it
(both of the originals were carbon).

The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the
aade.com meter. Both values (if the readings are correct) would represent
a high ratio of X to R at HF frequencies...

I have quite a surplus of them, so if you wish I can send one of each in
the mail for you to check on your VNA.

Those values seem surprisingly high, so yes, please do and I will
measure them.

Based on the standard inductance formula (and there seems no reason why
that shouldn't apply in this case) it would seem to require very large
numbers of turns to achieve 1uH.

My posting was based on the ones I scraped the paint off in pre-VNA
days, so it would be useful to collect more data on typical 'coil'
dimensions and numbers of turns, together with the actual effective
inductances.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Thu, 25 Nov 2004 07:32:27 +0000, "Ian White, G3SEK"
wrote:

Those values seem surprisingly high, so yes, please do and I will
measure them.

Please post the results here when you've done so, Ian. I for one would
also be interested.


--

"What is now proved was once only imagin'd." - William Blake, 1793.

On Wed, 24 Nov 2004 18:59:16 +0000 (UTC) "James Bond"
wrote:

are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.

They are not WIREwound, but the resistive element IS usually a helical
film that is a layer on a ceramic body.

As someone else mentioned, there are some types which are serpentine
rather than helical, but these are always special and sold as such.

Film resistors generally have fewer turns in their helix than
wirewounds, so they will have more inductance than carbon or ceramic
bulk resistors but less than wire wound resistors.

-
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Jim Adney
Madison, WI 53711 USA
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