Cheap Chinese metal film....Or are they?

[Electrobumps]

I wanted a bunch of resistors to have on hand. Ali express discount and free delivery ended up 1300 1/2 w delivered for just under $5. How could I resist?

Thought they looked large for a 1/2w metal film. They are the same size and shape as 1/2w carbon comps I have. Google found a few posts of people saying they paint carbon comps in the style of metal film blue and sell them as metal film.

I scraped a couple back and they all look the same to me. I can't find any information on how I determine what the film is made of. Any suggestions would be greatly appreciated.

I've attached a picture 1st and 2nd are the ones in question. 3rd is a known carbon film and 4th is a known metal film. They all look the same to me.

 

[Electrobumps]

So I have worked this out. I'm posting how for anyone who needs a solution.

Carbon film resistors will drop in resistance as they warm up. Metal film will stay stable. So I got a metal film 100K and carbon film 100K and warmed them with a hot air gun. Metal resistance stayed solid carbon film dropped from 100K to 96 K.

Did the same test on a 100K from my questionable resistor pack and there was indeed 4K drop in resistance.

With this test and package size and also being very cheap from China! I'm confident that these are carbon film and not the advertised metal.

 

[KLDVOX]

that handy to know,

 

[Rob Flinn]

I bought a questionable pack of 1% resistors a while back. They weren't 1%. A lot of the time this isn't a massive problem but it's nice to know one is getting what one paid for ...

 

[cyrano]

What did you expect?

5$ for 1300 resistors isn't even remotely believable. Hec, it's even too cheap for carbon film ones.

 

[JohnRoberts]

I believe he has figured that out... Resistors are typically the cheapest component in most circuits so not ripe for cost savings.

For decades I used ROHM carbon film and gladly paid fractions of a penny more for the good reliability and high quality.

I was happy to see that Peavey was already using ROHM carbon film when I started working there in the mid 80s. For most of my 15 years there I resisted sundry cost saving suggestions to switch to a cheaper vendor. Finally I had my arm twisted hard enough to approve a fraction of a cent cost savings which amounted to real money at the volume we were using. I made sure engineering rigorously tested and approved the new vendor.
===
[TMI warning] I've told this story several times before... Back last century a different purchasing puke was twisting my arm to approve a new cheaper vendor for small mixer potentiometers. I finally said OK but only if they passed a proper production test putting thousands of these test parts into real mixer boards and running them through factory production. Of course these "too cheap to work" parts were a colossal failure. Now the factory workers and supervisor were angry at me, because they had to rework all those PCBs to swap in good potentiometers. (It wasn't all that many PCB because they used tens of pots each, but it was still a costly PIA to manually rework them all.)

I invited the purchasing agent to a meeting I scheduled out in the factory with the production supervisors and a few line workers so he could explain to them why we did the test run. Hopefully he learned a lesson about pursuing "too" cheap components.

FWIW These pots were only costing us something less than $0.20 ea., but the cost saving can look significant with the large volumes these parts are used in.

JR

 

[ruffrecords]

Many, many years ago, when metal film resistors first came out, they were known as high stab (high stability) resistors because they were much less temperature sensitive than carbon types.

Cheers

Ian

 

[Electrobumps]

What did you expect?

5$ for 1300 resistors isn't even remotely believable. Hec, it's even too cheap for carbon film ones.

I do like to expect what I order! I took a punt in the knowledge that It is free returns. I had a new sign-up deal with it should have been $15 + postage.

They don't want the product returned, which is normally the case with low-value products. So I do have 1300 free carbon film resistors.

 

[KLDVOX]

this opens up a nasty can of worms. so if building an LCR network of some type do we now have to do all the calcs for a vast range of temperatures or do we just settle on blind faith? most of us are not working in a cryo or deep space environment. thankfully.
if i was doing nasa stuff they would not even get to the launch pad

 

[KLDVOX]

I do like to expect what I order! I took a punt in the knowledge that It is free returns. I had a new sign-up deal with it should have been $15 + postage.

They don't want the product returned, which is normally the case with low-value products. So I do have 1300 free carbon film resistors.

1300 x 1watt resistors could be usfull bet there all 1/16 watt

 

[Electrobumps]

1300 x 1watt resistors could be usfull bet there all 1/16 watt

They are pretty big so I hope they are 1/2watt. I'll test these and let you know.

I'm doing a bunch of effect pedal stuff and want to be able to try different values and have most values to hand. So these should be just fine.

 

[KLDVOX]

for most low voltage stuff 1/2 watt 5% carbon film are fine. i would guess if you were doing very smart stuff looking for low noise, close tollerance could be justified.

 

[My3gger]

They are pretty big so I hope they are 1/2watt. I'll test these and let you know.

I'm doing a bunch of effect pedal stuff and want to be able to try different values and have most values to hand. So these should be just fine.

More surface area means better cooling, guitar pedals normally don't have much power to dissipate. Having 1300 free carbon film resistors isn't bad deal if they are always measured before soldering. I do this with metal films just in case...

 

[cyrano]

I do like to expect what I order! I took a punt in the knowledge that It is free returns. I had a new sign-up deal with it should have been $15 + postage.

They don't want the product returned, which is normally the case with low-value products. So I do have 1300 free carbon film resistors.

OK, the sign-up deal changes it a bit. I checked, an assortment of 390 1/4W 1% resistors is 15,99€ incl. VAT @ Conrad:

https://www.conrad.be/nl/p/tru-comp...iaal-bedraad-0-25-w-1-390-stuk-s-1565936.html
Still about three times as expensive, as there are less resistors in the package.

I've had no troubles with ordering from China, but I never choose the cheapest one. And often I look for stuff on Alibaba and end up buying from a source in the EU. Avoids VAT and tax trouble. In my experience the Chinese sellers who ship from Europe (with VAT) can be trusted.

 

[Electrobumps]

I have some 1/4 “metal film” these are smaller package and looks wise are much better. Ordered when I wanted some parts quickly (Amazon prime). Measured fine in tolerance and look fine. Guess what happens when I put the heat gun on them, they drop resistance.

Either I’m unlucky or there is a high chance unbranded metal films are often actually carbon film.

 

[Tubetec]

I think the carbon look a bit different , its basically a cylynder of carbon loaded material with a helical groove cut to set the resistance , the metal film is deposited on a ceramic base .

I picked up the 1500 piece resistor lucky bag from China , measured values looked good ,but i never tried a heat gun to check stabillity .

 

[AnalogPackrat]

Back in 2005 I bought 10,000 1% metal film 1/4w resistors for $50 plus $13.50 shipping. It was a selection of 100ea of 100 values on cut tape. Clearly surplus. They are real and I've used many in various projects over the years. There are deals to be had, but I'd avoid foreign sources.

 

[JohnRoberts]

The only time I ever saw a small through hole resistor fail open circuit was inside a cheap Chinese made battery charger for a mobility scooter. My 80 something YO neighbor (now RIP) had one that broke down regulary, and I eneded up repairing it bacause I was that neighbor who could.

I didn't bother to dissect it to determine the resistor's failure mode, it just measured infinite resistance. I didn't have the exact value sitting around but replaced it with one that was close enough to get the charger working again.

JR

 

[SoundBlaster]

I just looked at a BOM for a product I make in China. Resistors (EIA 0805, 2%) were listed at $0.007 each....

 

[samic]

I have just bought audionote tantalum resistors for £6 each for a tube microphone preamp project....
am I crazy? Yes
I like this ? yes