Cheap soviet precision voltage divider (under 1$ per pcs) - Page 1
Forum section: electronics / general

author: shodan@micron
author_rank: Contributor
timestamp: February 27, 2021, 08:43:56 pm
content: void

author: Anders Petersson
author_rank: Regular Contributor
timestamp: March 03, 2021, 08:11:40 am
content: What's happening at 25 C with sample #2?

author: picburner
author_rank: Frequent Contributor
timestamp: March 05, 2021, 08:12:24 am
content: Can I ask you where did you find these devices?I guess not on ePay

author: salbayeng
author_rank: Frequent Contributor
timestamp: March 05, 2021, 08:33:25 am
content: Wow , great find , I could have used a pair of these a while back making a differential amplifier (measuring 100mV FSD on a 100v common mode)I just used 1% metal oxide with a trimpot for balancing but it drifted all over the place (thermal shielding wouldn't have helped, I think all the TCR's were different) Too cheap to buy the analog devices $20 diff amp with the laser trimmed resistors.

author: salbayeng
author_rank: Frequent Contributor
timestamp: March 05, 2021, 08:58:47 am
content: Quote from: shodan@micron on March 05, 2021, 08:29:35 amHaving a high VCR is not helpful for making differential amplifiers, I'm not sure how someone could measure VCR without a known good resistor for comparison. Sure with a pair of dividers and a variable amplitude 0-100v AC signal you could determine the difference in VCR between two dividers. (Using AC cancels any thermal EMF).There is also the problem of 1/F noise that can be voltage dependant (well known in carbon composition resistors, and to a lesser extent in thick film resistors) this sometimes appears as "castle noise" as the voltage steps up and down like the battlements on a castle. Some related reading here:https://www.effectrode.com/wp-content/uploads/2018/12/Measurement_of_1f_Noise_in_Carbon_Composition_and_Thick_Film_Resistors.pdf. On a microscopic scale you get quantum mechanical tunneling between grains of the resistor so as the voltage gets higher the electric field increases , and the current takes shortcuts between grains. apparently resistor elements with tight bends are worse as there are shortcuts in the inside of the bends, wire wound resistors don't have bends, Metal foil resistors also have low 1/F noise (even with bends).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 08, 2021, 05:45:27 pm
content: I've been burned by both noise from thick film resistors and by HV non-linearity in metal oxide resistors. Gellerlabs is gone, but the JCan noise measurement project worked really well for noise measurements. Some of it is still accessible via the wayback machine.https://web.archive.org/web/20181109003553/http://www.gellerlabs.com/JCan%20References.htmAlso the original article here-http://physicsopenlab.org/wp-content/uploads/2016/10/JCan-NV-article.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2021, 06:10:21 pm
content: Measuring just the thermal / Johnson noise is kind of pointless, as it is the same for all resistor types (no matter if carbon, metal oxide, TaN, wire wound or thich film). The relevant part would be the excess noise that can cone up with an applied voltage / current. With a high resistane this can get a bit more tricky. The usual configuraion is a kind of bridge circuit with 4 of the DUT resistors (could get away with 2 samples here when using 4 and 6 M from each divider). The point is than how much the noise goes up with an applied votlage.For smaller resistors, like in the 10 K range this is now relatively simple, as one could use moder AZ OPs and measure at a relatively low frequency (e.g. 0.01-0.1 Hz range). For 4x10 K one could get away with something like an LTC2057 as a x 100 amplifer and than directly to a good DMM. For the bridge drive a simple 9-12 V battery should be OK, espeically if the resistors are well matched.Asuming the same technology for the smaller resistor parts, one could use just the lower resistance part of the dividers.For the M Ohms range this becomes tricky as there are few OPs for this range. Classical non AZ amplifier with JFETs have some 1/f noise on there own which make the measurement a bit tricky, though it may work if the resistor noise is high. It would be possible to use AC excitation of the bridge, though this adds some complications.I would consider this quite some effort to measure the excess noise for 10 M resistors.