Accomodating resistor tolerance for reasonable accuracy ? - Page 1
Forum section: electronics / general

author: Sarvesaa
author_rank: Contributor
timestamp: April 28, 2023, 03:07:38 pm
content: Hey friends,I am building a lab power supply. I am using a LM358 as a current sense amplifier and feeding the output to a micro-controller(Atmega328p).Sad thing is that I cannot find 1% resistors locally in the market, they only have 5% tolerance resistors.The gain of the current sense amplifer is 1.I measured the resistance of the resistor and it was about 17.5k instead of 18k as expected,could I substitute the actual read resistance in the gain formula and change the code accordingly ?.Need your help on this. And the actual current sense cirucit is based of EEVBlog usupply Rev C.Thanks

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 28, 2023, 03:23:30 pm
content: Yes you can, but you must be sure that your multimeter is reading correctly.That is all you need, after that with 5% resistors you can put them in series/parallel to obtain mostly any value.

author: tom66
author_rank: Super Contributor
timestamp: April 28, 2023, 03:33:54 pm
content: One thing to be aware of is temperature drift. Metal film resistors (most likely that is what you have) are quite stable over long periods of time, at least for most applications, but can have a large enough temperature coefficient that can impact the performance of a system. This means if room or device temperature varies, the gain could change.You could minimise the effect by keeping the resistor away from hot things like the heatsink for the main transistor/regulator and, if necessary, having a cooling fan in the product.Though at the end of the day unless you are building a precision product the tolerance is likely fine. A typical worst-case figure is 100ppm/degC worst case which means a 30C rise means a 0.3% additional error. If you have designed the product around a 1% toleranced 18kohm resistor, then adjusted it for a real world 17.5kohm resistor, consider if the additional error is consequential or not to your application.

author: Sarvesaa
author_rank: Contributor
timestamp: April 28, 2023, 04:06:54 pm
content: Thanks tom66 and PartialDischargeI thought of temperature coefficient and I will make sure it stays constant.And I am using EEVBlog Brymen 256 multimeter.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 28, 2023, 06:55:00 pm
content: Quote from: tom66 on April 28, 2023, 03:33:54 pmIf they are axial leaded resistors they could also be carbon film resistors which have much higher temperature coefficient. It's still not likely to be a problem, but it would be if the resistor were handling significant power and self-heating.

author: David Hess
author_rank: Super Contributor
timestamp: April 30, 2023, 11:30:29 am
content: Quote from: Sarvesaa on April 28, 2023, 03:07:38 pmYou could do that, or even better most circuits rely on the ratio of two or a set of resistors, so you could go through some resistors of the same indicated resistance and select a set of two or four which closely match. This removes the accuracy of your resistance measurement as a factor to be considered.