Electronics > Projects, Designs, and Technical Stuff
I built a bridge
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Vgkid:
Did you heat sink the resistors while soldering?
BradC:

--- Quote from: Vgkid on October 10, 2018, 02:17:04 am ---Did you heat sink the resistors while soldering?

--- End quote ---

Yep. Each resistor lead had a goot clip on it.

I suspect it's all to do with stress relaxing and initial settling. I only had to drop the trim 10 ohms this morning.
These metal film resistors seem to have quite a bit of hysteresis when exposed to differential heat, as while the heat clips do a good job there's still several 10s of degrees across the resistors as the leads are soldered.

Remember, we're talking 10s of PPM on resistors that are rated way higher than that, so I'm really using the wrong tools for the job and the process is not particularly well managed (a steep part of the learning curve). As in "oops, shouldn't have done that, now I'll need to wait a week for things to settle again".
BradC:
An update. The resistors continue to drift. The trim is down to 3.11k now.

I did a quick set of tests/calculations and this indicates the 2 reference resistors have moved 205ppm since I started this, and they continue to slowly diverge. On average it's about 2-3ppm per day. Some days it's none, some days it's 7. Still well within the 0.1% tolerance, but interesting in that it's relatively reliable and consistently in the same direction. It is also slowing with time. I suppose that validates the little of what I know about resistor ageing.

My biggest issue is I don't know if it's the 10K wirewound or the 1M metal film causing the drift as I can't separate them. The stand-alone wirewound doesn't seem to move much based on a relatively inaccurate absolute measurement (hp3457a).

I wish I'd started collecting and date-stamping the data as this would produce an interesting graph.
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