Bad resistors
in [Design]
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From: DrParnassus on 23 Jun 2010 12:45 On Wed, 23 Jun 2010 08:50:37 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >We recently needed a 100M metal-film resistor to use in a photodiode >TIA thing. We tested a bunch of thickfilms and all had too much excess >noise; You can't just solder it in, and not clean the board completely and start relying on test data. It being clean is the whole key. You can't do that at the bench without a brominated solvent or hot alcohol and a heat gun... hell... not at all. It needs to be clean. Working with HV and with feedback loops, etc. we established a method of building the SMD circuit with the least leakage paths. It MUST be CLEAN. 'Bump soldered' It allows your precious degreaser to get all the way under the parts, and then encapsulation seals the path, which was how we finished off our HV sections. But the gap is the point being made. part> ___ bump>___n_n______ << Where the bumps are placed first, and then the assembler places the part while keeping it parallel to the PCB. This ensures that flux, VOCs, and any other debris that might reduce a resistance path's desired value, is gone once you clean the board, and in certain circuit locations, it makes the circuit far more repeatable, and solves long sought after 'buggy behaviors' (though once learned, we no longer had any such issues).
From: John Larkin on 23 Jun 2010 13:02 On Tue, 22 Jun 2010 23:21:52 -0700, DrParnassus <DrParnassus(a)hereforlongtime.org> wrote: >On Tue, 22 Jun 2010 20:04:34 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >>Fingerprints are no problem at 1 Gohm (that's a thousand megohms to >>you.) Well, depending on what's on your fingers. > > You're an idiot. No, I just wash my hands more often than you do. I deliberately fingerprinted a Pomona dual banana plug that had a 1G resistor attached. I couldn't see the measured resistance change. Try it. John
From: John Larkin on 23 Jun 2010 13:12 On Wed, 23 Jun 2010 09:28:26 -0700, DrParnassus <DrParnassus(a)hereforlongtime.org> wrote: >On Wed, 23 Jun 2010 08:50:37 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>Dale says they're expensive because hardly anybody buys them. > > > If they have to do a lot run simply because one order triggered it, >they lose money if it isn't a pretty big order, and SMD has expiration >dates due to termination oxidation issues so they cannot simply store the >over-production stock either. Though I will take old parts because they >are usable for proto builds. As I said, they are axial RC07 types, not SMD. They do stock 22M, available from distributors at a more sensible price, and 33M, also distributor stock maybe. The 50M ones are apparently not much in demand. All *are* 1% parts. And we can measure them accurately. I need a small constant current, and it's a battle between Johnson noise current and shot/excess noise. It's not easy to generate a nA level quiet constant current. The alternative was ten 10M surface-mount thinfilms in series, which we don't have room for. John
From: John Larkin on 23 Jun 2010 13:13 On Wed, 23 Jun 2010 09:30:03 -0700, DrParnassus <DrParnassus(a)hereforlongtime.org> wrote: >On Wed, 23 Jun 2010 08:50:37 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >>Too bad about the noise. A single 0603 100M thickfilm would have been >>cheap and a lot less hassle. > > 0603 and 100M. That just sounds funny. Like simply looking at it could >change the value between the nodes. Stop being silly. There is nothing special about 100 megohms. John
From: Phil Allison on 23 Jun 2010 13:48
"crasic" "Phil Allison" > > ** Though pretty rare, I have come across wrongly marked resistors in a > piece of commercial equipment !! > > Dozens of resistors colour coded as 3.3kohms were in fact 4.7 kohms. > > The correct value WAS 4.7 kohms - so the equipment maker knew about the > wrong coding !! > The bands can change color, especially with age or heat. ** Brand new equipment with the resistors dissipating only milliwatts. Or it could have been a batch of mismarked resistors that the equipment maker got for cheap ** Exactly my conclusion. The maker musta saved about 5 cents ( or a few UK P) on a $1000 product. .... Phil |