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Post by radioholic on Dec 17, 2021 15:13:02 GMT 12
Hi All I bought some 1-watt resistors from RS components. But they seem to be VERY small compared to the 1W resistors I am used to. See picture. I called RS and they were quite sure there is no mistake. Any thoughts - should I trust they are actually 1-watt?  |
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redxm
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Post by redxm on Dec 17, 2021 21:09:01 GMT 12
Id be more worried at the voltage rating. I bought some 1/4 watt resistors from Mouser last week. They are the smallest non SMD resistor Ive seen. I get my resistors for valve stuff from justradios.com/ They deliver to Aust, so NZ shouldnt be an issue. They have a kit of 'old school' values |
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Post by Richard on Dec 18, 2021 7:45:21 GMT 12
Have you looked up that code on the packet with RS ? I'm just thinking that "the picker" in the warehouse may have got the wrong packet or its been placed in the wrong bin? I've experienced that with Jaycar in the past.
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Steve
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Post by Steve on Dec 18, 2021 8:25:08 GMT 12
Hi James, from what I can see they are 1W Vishay metal film resistors, rated to 350V (the 2W version is rated to 500V). The size is just a product of modern technology... Datasheet: docs.rs-online.com/40d8/0900766b801b8af1.pdfIt's why I use 2W resistors... Because they look more like they belong. Carbon resistors tend to be bigger too. I've heard but never tested that modern metal film resistors can include inductance into a circuit that carbon resistors do not... Although I doubt that would have much of any effect on the broadcast band even if it were true. However, that is another reason I stick with carbon. The final reason is the color... Carbon tend to be brown, metal film tend to be blue which is a little more 'sticky-outy'. I have some 3W resistors somewhere that are the same size as a 1W resistor was 20 years ago... It doesn't seem right but I assume they're designed properly. Unless you need them all, you could test one to destruction? Cheers, Steve |
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Post by Peter Walsham on Dec 18, 2021 20:53:33 GMT 12
Metal film resistors are physically smaller than their carbon counterparts, and the RS (& Vishay) datasheets state that these are rated at 1 Watt. I have used metal film resistors for years without any problems. They appear to have a better short term overload capability than carbon resistors do as well. Typically, they are very stable too. However, Steve is right about them being inductive (as are carbon film resistors). The resistance value is created by cutting a very narrow helical spiral around the resistor - effectively making it in to a coil. The more 'turns', the higher the resistance. I doubt that there would be a problem using them in the RF & IF stages of a domestic receiver - even on the short wave bands, but I wouldn't use them in VHF circuits. Having said that - most (but not necessarily all) resistors in the RF & IF stages of a domestic receiver are only at DC and non-signal anyway (like screen dropping, cathode bias & AGC filter resistors), so resistor inductance will, most probably, never cause any problems. We used to use Philips carbon film resistors in TV tuners (at VHF) & IF stages without problems.
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Post by radioholic on Dec 18, 2021 21:52:36 GMT 12
Thank you all for these insights. The datasheet does indeed state that they are 1W resistors.
However, the 350V max voltage rating is now puzzling me.
If 350V is applied across a 1 megohm resistor, the power dissipated is P = (V^2)/R = 0.12 W. So how can it be rated as a 1-watt resistor?
Conversely, if 350V is applied across a 1K resistor, the power is 123W (which would of course cause immediate failure of the resistor) !
So what does the 350V rating mean?
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