Resistor types with inherently bad VCR? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 18, 2022, 07:44:42 pm
content: I know carbon composition resistors have bad VCR (Voltage Coefficient of Resistance), but modern ones have far too good VCR specs to be any fun anymoreI know I could use some simple BJT or FET as a variable resistor to achieve the same effect.but what options do I have with just resistor types (and yes I know it depends a lot more on the size and value of the resistor most often)I was thinking maybe LDRs would have bad VCR since they're... terrible in everything else, but I couldn't find any datasheet which listed a spec for VCR.and also, I see positive and negative values used for VCR, sometimes having negative ppm/V and positive %/V specs in the same sentence but in the next sentence it's negative %/V...so, does a carbon comp resistor have positive resistance change per voltage (+ %/V?, + ppm/V?) and would increase in resistance the more voltage is applied to it, or the other way around?

author: Benta
author_rank: Super Contributor
timestamp: August 18, 2022, 10:34:39 pm
content: How about a VDR/Varistor?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 19, 2022, 01:57:58 am
content: Wind yourself an old school mica resistor using fine nickel wire. It's used for temperature sensing, so should do what you want. Or buy-https://www.harlingen.cz/index.php/en/niklova-teplotni-idla/ni-5000Looks to be 5000 ppm/K

author: Berni
author_rank: Super Contributor
timestamp: August 19, 2022, 09:33:30 am
content: Yes they make resistors with a high voltage coef, they are called Varistors.In practice you will mostly find them as MOVs (Metal Oxide Varistors) being used as surge protectors because they can absorb a large amount of energy before they blow up.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 19, 2022, 12:04:10 pm
content: If you miss carbon, get some pencils.Remember that bag of briquets from last winter?Got a bale of fencing wire?Mmmm.If you've run out of these high end components, then put some nails in a potato.

author: Berni
author_rank: Super Contributor
timestamp: August 19, 2022, 04:49:28 pm
content: What are you trying to do in the first place?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 19, 2022, 05:26:45 pm
content: Quote from: ELS122 on August 19, 2022, 05:18:42 amTrue, but you design the resistor to heat up! Pure voltage coefficient is harder to find and may not be very large. I worked with some high voltage amplifiers decades ago and we learned that MOX resistors performed poorly when used for feedback because the voltage coefficient was high. That likely isn't true with modern ones.

author: Berni
author_rank: Super Contributor
timestamp: August 19, 2022, 06:18:58 pm
content: Quote from: ELS122 on August 19, 2022, 04:55:14 pmMore why you need such a resistor.You can make any resistor as crappy as you want by placing non linear semiconductor components in parallel or series with it, adjusting the values of the resistors around them to mix in the desired amount of non linearity and different arrangements can provide either positive or negative coefficients (both of those can be seen in resistors)