Maybe this is a very stupid question, so pardon me. I was recently trying to buy an assortment of resistors from Digikey, and the sales rep responded by asking for the type of resistors. So far I have known that discrete resistors are made from carbon. Is there any other material that is used to make these ? I was only looking for ordinary 1/2 W, 1 W and some 5 W resistors.
Wirewound , metal film!
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Maybe this is a very stupid question, so pardon me. I was recently trying to buy an assortment of resistors from Digikey, and the sales rep responded by asking for the type of resistors. So far I have known that discrete resistors are made from carbon. Is there any other material that is used to make these ? I was only looking for ordinary 1/2 W, 1 W and some 5 W resistors.
Look around on their parametric search.
Carbon composition still exists, surprisingly, but most small resistors are carbon film. Beyond that, power (>= 1W) are usually metal oxide, metal film (thick/thin film), ceramic, wirewound...
The most basic tradeoffs you'll see are power, tempco, inductance and peak power capacity. Most times, this data is poorly specified; if you require specs for a purpose, best to find one with specs.
Tim
-- Deep Friar: a very philosophical monk. Website:
Most surface mount resistors lately are "cermet" aka "thick film", which is a metal oxide paste that's screnned fired onto a ceramic substrate and laser trimmed. They are commonly 5%, but sometimes 1%, with power from 1/10 watt to 1/2 watt or so.
Most of the cheap leaded parts are still carbon film on ceramic tubes, spiral cut like one of those holiday hams. You can get leaded cermets, too.
Really good resistors are "thin films" or "metal films", 1% and better.
Plus a zillion other exotics: wirewound, metal foil, whatever.
We buy mostly 1% surface mount, 0805 being our preferred size. There's no point in buying 5% surface-mount resistors any more.
This is a basic starter kit:
We got this one for our lab, 34,000 resistors!
John
0805 is probably the smallest that you'll want to use for soldering down with naked eye.
Given a halfway decent assembly microscope, a basic temperature controlled iron (I have a Weller WTCPN) and a reasonably steady hand,
0603 works just peachy -- and you need that assembly microscope for quite a few semiconductors, anyway.Check eBay for microscopes. They're there to be had.
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
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"we" used to deal with the 0402
:-((((
There are various types. The most common TH parts are are carbon film, metal film, or wirewound. As someone said screen printed Rs are also used for SM parts. There are other types too. If your specs arent especially demanding, the cheapest type for general use is carbon film.
NT
No fooling! I spent this morning tweaking a photosensor preamp full of
0603s, under my Mantis, and didn't enjoy it.I dropped one replacement resistor out of its tape and of course it landed upside down. So I pressed it with my finger to attach it, lifted it up a few inches, and dropped it. It landed upside down again. SIX times in a row.
I found a great way to reduce the bias current error of an opamp:
+2.5V | | 100K | OPA843 | | in------+---------- +infb ---- -in
John
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.Tweezers let you choose which way up it is when it lands, and they don't smear the surface with sweat.
You didn't reduce the error, you just found a way of making the error range closer to symmetrical about zero current.
says that the typical bias current is -20uA, worst case is -35uA at
20C, -36uA for )to 70C and -37uA for 0 to 85C. It's exactly the kind of inadequate data sheet that I've come to expect from TI over the last forty years. They've obviously got some good engineers, but their entire marketing department seems to need exterminating and the offices burnt down - to make sure that whatever they've got isn't passed on to their replacements. It's not so much that they are dishonest, as that they are criminally stupid - we get to find out about the less desirable featrures of their devices fast enough, and it's much easier to design around them if the data sheet has told you about them before you solder them into a real circuit - or designed them onto a printed circuit board, which tends to be a good idea for a 800MHz gain-bandwidth part.This is a slightly unusual op amp, and you might have mislead the some of the less sophisticated lurkers.
-- Bill Sloman, Nijmegen
If this is for hobby type pth work you might find better value looking on ebay (defiantly *not* recommended for professional work)
-- We have failed to address the fundamental truth that infinite growth is impossible in a finite world.
I sure reduced the DC error that the bias current makes in my sensor.
Inadequate? How? There's also a graph of typicals over temperature.
They've obviously got some good engineers, but their
Differental NPNs in the front and, nothing unusual except that it's fast, with corresponding power consumption and bias current. Transient response is gorgeous.
It's a pretty nice amp, a voltage-mode opamp that's faster than many current-mode amps. The current-mode amps I used, AD8009, are rotten for DC performance, and my customer is now telling me that his PEM photosensor may put out 20 millivolts... I need a lot of gain, and the AD8009 bias current, more like 100 uA, and offsets are getting in the way.
The resistor to cancel most of the Ib is an obvious kluge, but will let us ship some stuff. I measured 25 uA Ib on these amps, so a 100K resistor should cancel most of the input current. Next board spin, I'll have to use a fet opamp, ADA4917 probably.
Really, TI makes a lot of very nice opamps, especially fast stuff with high voltage rails.
John
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That might be nice thanks.
The opa847 looks like a 'goer' too.
I was going to say, I have no application at the moment.
But I wonder if I could hang a piece of wire on it and make an FM antenna?
George H.
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A nice quite 2.5 volt supply. (obviously)
I thought it looked like a good spec sheet. (though I only browsed it.)
George H.
Heck, you can do serious RF with opamps nowadays.
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I think you said this before, but it's like a golden age of analog. Great opamps, 0.1% resistors (and better). Why no 1% NPO ceramic caps? .....went to the shop for a beer...... (I guess everyone digitizes things so RC times are not so important.)
George H.
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That's one way of looking at it - not a particularly sophisticated one.
No upper and lower limit on the bias current - if typical is -20uA and worst case is -35uA, you can probably infer that the other extreme is about -10uA, but TI has to have collected enough data to have been able to give an explicit figure. Maybe they are hoping that they can get higher current gains in the input transistors in a couple of years, and don't want to have to re-write the data sheet.
And the graph of typical bias current, off-set current and offset voltage - half-way down page 8 on the left hand side of the page - displays a range that seems to run fom -18uA to -22uA on graph that covers +/- 25uA, which is singularly unhelpful
Bias current that high are pretty unusual - if not that that kind of brute force fast op amp.
I did say that at least some of their engineers seem to be okay - I'm bitching about the people who write the data sheets.
-- Bill Sloman, Nijmegen
I remember beeing told that carbon composition resistors have poor thermal conduction and are useful for thermally isolating temperature sensitive components from the rest of the pcb. I've never actually tested it myself.
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The thermal resistance of resistors (an ohm and greater) is mostly the thermal resistance of the substrate. I guess since carbon comps have no substrate, they would be the best. I once tried to determine how much of the heat generated inside a through hole metal film comes out of the leads. (compared to into the air) And got an answer that was very close to zero. But it was a very crude measurement.
George H.
How's your Baxandall oscillator coming along? Bought any parts yet? I could donate a couple of resistors to speed the project along, save you a year or two.
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You've got a funny idea of the design process - which we've known for some time. Farnell has a perfectly adequate line of resistors, and they deliver directly from warehouses in the Netherlands.
I'm stuck on doing a detailed design with gEDA. It's not so much that it's not user-friendly - it's certainly no worse than Orcad, and I could work with that - but that I've got to fire up my Linux partition to get started, and there's always something else more important to be done in Windows - like booking the train ride to Lyon and back for our
3-star Christmas dinner.-- Bill Sloman, Nijmegen
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