Electronics > Beginners
is it possible to mechanically trim resistors?
Gyro:
That very much depends on the situation - whether the E24 resistors are 5% or 1% for instance. It's very easy to go overboard but instances that come to mind:
1. 50R termination resistors in E24
2. Fine trimming things like voltages references where you are padding a precision resistor with a much higher value, less critical resistor to bring it to spec without a pot.
3. Production trimming of voltage regulators where you cut out resistors to achieve trim (there's a NS app note about that somewhere).
4. One-off home designs where you want to fine trim something.
5. Gone blank... but there are others.
As I said, it's easy to go overboard using low quality resistors to try to achieve close precision, but it is a useful tool.
jancumps:
In the end, when using resistors with the same tolerance, isn't this exercise virtually moot?
edit: this message wasn't a reply to the last one - It's about trying to improve resistor precision within a given tolerance range: does scratching up a 47 Ohm 5% resistor so that it measures 50 Ohm improve anything?
David Hess:
With the coating removed, thin film and metal foil resistors can be trimmed by rubbing them with a pencil eraser but they tend to continue drifting with their passivation layer removed.
mzzj:
--- Quote from: schmitt trigger on August 10, 2018, 04:57:09 pm ---Many moons ago, when the most common resistors were thru-hole, 1/2 watt carbon composition types, one could use a triangular file to slightly increase its value.
--- End quote ---
Many moons ago there were also vertical resistors that you could cut shorter to trim the resistance value. Snapsistors or something like that.
Never seen them in use, just in old advertisements.
JS:
I've done it for fun with a few metal films, just trim a tiny bit, like selecting some 999Ω from the 1k batch to trim down to 1.000k few years after I read exactly the same, but only 3 zeros, I've just received the 121GW so I could try something with 4 zeros.
I used some small files and went slowly, turning the thing as I went so I extracted as little material from a single place as possible and then coated them with some epoxi. I guess higher values would be harder as the strip is much thinner so there's a better chance of extracting too much material easily.
The hard part is that you are heating and getting conductive dirt onto the conductive layer as you file, so you need to clean, wait temp to stabilize and then read, that make the thing tedious and slooow. For the final trim you could go slow, blow some ambient temp air and read faster, once they settle make a good clean, put some epoxy and they might be good, age them for a bit before trusting them.
Or just buy better resistors to start with. Some pretty usual way is to build a network with a trimmer in it, but the trimming range being pretty small, so the tempco of the trimmer doesn't affect much the tempco of the combined resistor.
Carbon might be easier to carve but as they come in really horrible tolerances you could very well select a value from a batch somewhere off the nominal. If you need 10X divider, 910 and 82 820 and 91 makes a pretty exact value, you just need to select the resistors from a batch and you will find some. X2 is easy, to equal ones, time 3, probably use 3 equal ones and do series of 3 (that makes self heating more even than parallel) or get some 1k and 2k. All standard values of E24 series and there might be a perfect combination for each ratio you want, just grab the calc and do the math... or an excel sheet.
JS
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