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| references:resistortrimming [2024/07/30 02:52] – created - external edit 127.0.0.1 | references:resistortrimming [2024/07/30 03:33] (current) – external edit 127.0.0.1 | ||
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| Assuming you have a standard carbon film resistor at hand, place it between two alligator clips on your multi meter probes thus. | Assuming you have a standard carbon film resistor at hand, place it between two alligator clips on your multi meter probes thus. | ||
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| Then with a small needle file, file through the coating on the outside of the resistor till you reach the resistive material inside. Then gently file away the resistive material and you'll notice the resistance climb. Of course you can only make the value rise so you need to choose a resistance value lower than your target value. When you reach your desired resistance value, coat the filed out area of the resistor with some nail polish. Where would the electronics industry be without nail polish. I'm sure using it on nails was an after thought. :) | Then with a small needle file, file through the coating on the outside of the resistor till you reach the resistive material inside. Then gently file away the resistive material and you'll notice the resistance climb. Of course you can only make the value rise so you need to choose a resistance value lower than your target value. When you reach your desired resistance value, coat the filed out area of the resistor with some nail polish. Where would the electronics industry be without nail polish. I'm sure using it on nails was an after thought. :) | ||
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| It is important to note that you don't need to use much pressure on the resistor. Partly because it's relatively easy to file past the target value but also because you don't want to place too much mechanical stress on the resistor. You need to be gentle with the resistor. Take it slowly and note that the contact of the file it self might temporarily change the resistor' | It is important to note that you don't need to use much pressure on the resistor. Partly because it's relatively easy to file past the target value but also because you don't want to place too much mechanical stress on the resistor. You need to be gentle with the resistor. Take it slowly and note that the contact of the file it self might temporarily change the resistor' | ||
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| **But wait there' | **But wait there' | ||
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| - | '' | + | //Trimming (filing) in circuit would be the big deal. Imagine tuning a filter bank. Measure capacitors, then do some calculations and select resistors, or combinations of 2 resistors in series. Lot of measurement and calculation involved. If you can trim the resistors in the living circuit, just feed in a sine with the right frequency and file until the amplitude at the output reaches the peak. Same as with trimpots, only that you have better long term stability and tempco.// |
| This is a great point. By filing in circuit you could measure the response of the circuit until it comes up to the exact conditions you need for the exact response. But there is the concern about getting conductive filings into the circuit board which may cause problems or damage. You would have to be careful here and blow or brush the filings from the board to prevent this from happening. | This is a great point. By filing in circuit you could measure the response of the circuit until it comes up to the exact conditions you need for the exact response. But there is the concern about getting conductive filings into the circuit board which may cause problems or damage. You would have to be careful here and blow or brush the filings from the board to prevent this from happening. | ||
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| Now the conventional wisdom here is that you can't customize a metal film. Only carbon film resistors can be customized. But this is not necessarily so. I have successfully trimmed up quarter watt Metal film resistors as well. The resistive material in metal film resistors is tougher than that of carbon. Carbon is like filing through some kind of chalk. Metal films are like filing through aluminium. Because you need to file it more you can be more accurate. But be warned. | Now the conventional wisdom here is that you can't customize a metal film. Only carbon film resistors can be customized. But this is not necessarily so. I have successfully trimmed up quarter watt Metal film resistors as well. The resistive material in metal film resistors is tougher than that of carbon. Carbon is like filing through some kind of chalk. Metal films are like filing through aluminium. Because you need to file it more you can be more accurate. But be warned. | ||
| - | [mailto: | + | [[mailto: |
| - | '' | + | //Well metal films are (or at least *were*) ceramic tubes, coated on the outside with the resistive film. Sometimes , for the higher values I suppose, this film would have a helical groove etched into it so that the resistive film actually ended up being like a ribbon wrapped round the tubular substrate. Filing one of these at the wrong angle or in the wrong spot could give you an open circuit fairly quickly.. so you can file 'em but sometimes you might get a bit too much extra resistance all of a sudden// :-) |
| So it's important to note. You should only file it in one spot on the resistor. You should never file around the resistor. | So it's important to note. You should only file it in one spot on the resistor. You should never file around the resistor. | ||
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| Finally as mentioned before, a resistor has a power rating. The standard size and most commonly used in small signal, low voltage electronics being one quarter watt. When you file the resistor, you are actually lowering the power rating of the resistor as well. Ever so slightly may be but it's worth noting. If your circuit required a resistor right on tolerance, you could be in trouble. | Finally as mentioned before, a resistor has a power rating. The standard size and most commonly used in small signal, low voltage electronics being one quarter watt. When you file the resistor, you are actually lowering the power rating of the resistor as well. Ever so slightly may be but it's worth noting. If your circuit required a resistor right on tolerance, you could be in trouble. | ||
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| Finally Robin also points out thusly: | Finally Robin also points out thusly: | ||
| - | '' | + | //What you're doing is essentially the same as the automated 'laser trimming' |
| This has been used in a number of electronic devices. Most notable precision trimmed A/D converters, multipliers and digital audio controls. Basically resistive elements are laid out in silicon. Before the dye is mounted in the IC package it is inserted into a testing and measuring machine. The resistors are hit with a laser which burns off layers of the resistors until the value comes up to the optimum. Thus pre-turning the circuits before they are packed and shipped. | This has been used in a number of electronic devices. Most notable precision trimmed A/D converters, multipliers and digital audio controls. Basically resistive elements are laid out in silicon. Before the dye is mounted in the IC package it is inserted into a testing and measuring machine. The resistors are hit with a laser which burns off layers of the resistors until the value comes up to the optimum. Thus pre-turning the circuits before they are packed and shipped. | ||
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| So when you need a custom resistor, get your little bastard out and start filing. | So when you need a custom resistor, get your little bastard out and start filing. | ||
| - | Email the author: [mailto: | + | Email the author: |
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| Thanks also go out to the following people for their contributions to this page: | Thanks also go out to the following people for their contributions to this page: | ||
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