| Electronics > Beginners |
| is it possible to mechanically trim resistors? |
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| KL27x:
--- Quote ---1) sometimes I need an "odd" value (like 50 Ohms, while only 47 or 51 are available) --- End quote --- Assuming this is true where you live (or that you don't want to buy and wait for a resistor for a one off): It might be easier to trim the resistor with other resistors. Use the 51 and whip out the calculator to find the value you want to solder in parallel to it. 2.55K. In this case a 2.5K in parallel would get you to around 49.98R. If you need bang on, you will be trimming anyway. Say all you got is a 3K. Add that in, recalculate, and you need to add 17K in parallel to that. Repeat with the closest higher value you got until you get as close as you want. Or just tack on 1 meggers until you get as many zero's behind that decimal point as your heart desires. Gyro beat me to it. But you just need a calculator that has a X^-1 button. You gots a scientific calculator, right? :) I have more than once made low ohm power resistors/shunts with a pile of 1206's and a piece of copper clad. |
| Zero999:
I use two resistors in series or parallel and the site linked below, to calculate the resistor values, if I need an odd value resistor. http://jansson.us/resistors.html Another way to trim is to choose a resistor with a higher value, than needed and place many much higher value resistors in parallel with it, the PCB track can then be cut, to remove one of the higher value resistors, until the desired value is reached. This method can be used for trimming the output of a voltage regulator of offset nulling. |
| jeroen79:
--- Quote from: jancumps on August 10, 2018, 06:23:20 pm --- --- Quote from: jeroen79 on August 10, 2018, 05:44:25 pm --- --- Quote from: jancumps on August 10, 2018, 05:16:19 pm ---... one of the reasons why your desired resistor isn’t there is because of the tolerance. That’s why the Exx ranges of resesistors are there. They are covering the whole range, taking tolerance in account. --- End quote --- True. You could just buy 100 resistors close to the desired value and then pick the one that is closest. --- End quote --- If you have to do that, you're probably buying a wrong tolerance series? --- End quote --- Probably, but what if even the E192 series does not have the exact value you need? |
| Gyro:
I've just remembered another resistor combination calculator from this very forum - Resistor Fun: https://www.eevblog.com/forum/projects/fun-with-resistors/ Much more featured - All resistor series, selectable number of resistors, voltage dividers with multiple taps, analysis of the effect of TC etc. |
| Twoflower:
Does any parallel or series circuit actually make any sense at all? As the tolerances are so wide that a E24 100Ohm resistor can be anywhere between 104.5Ohm and 115.5Ohm. The next higher E24 resistor (120Ohm) spans from 114Ohm to 126Ohm. As you see they actually even overlap. So doing any magic with E24 resistors to get a 115Ohm resistor (as example) would fail at the tolerances. Unless you select them. In that case you might consider going to an higher E series. At least that's my understanding. Series or parallel circuits might make sense for example to reduce the number of different resistor types in your BOM or increase the allowed voltage or power ratings. |
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