Resistor temperature coefficient  - Page 1
Forum section: electronics / projects

author: txescientist
author_rank: Regular Contributor
timestamp: June 20, 2013, 01:43:24 pm
content: I am using resistor voltage divider and have problems with temperature coefficients - ratio is drifting as much as 10% / 30*C or so. Ten 10M and 3 100k. It seems that 10M and 100k aren't have approximately the same temp. co. Is it better to use all carbon or all metal film? What would happen if you use 10M metal film and 100k carbon? Using this divider to sense high voltage from the output of HV PSU in negative feedback to regulate it. Available are only 1% metal film and 5% carbon film. Any help is very much appreciated !

author: Neilm
author_rank: Super Contributor
timestamp: June 20, 2013, 05:19:39 pm
content: The first thing that springs to mind it so check that you are not using the resistors at a higher voltage than they are rated for - you mention a HV PSU. The second thing is to double check that you are not over powering either of the resistors - that caught me out a few years ago. Many resistors give a headline power rating, but if you dig into the datasheet you will quickly find it forces a much lower maximum voltage than you thought.To answer your question, I have designed high voltage supplies and have successfully used metal film resistors that were suitably rated and had no problem at all (4000:1 divider ratio). It depends how much you want to pay for it.YoursNeil

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 05:24:07 pm
content: Carbon film have sloppy and high and irregular temp coeff's. Use metal film with metal film. Make sure the power handling of the 10meg resistor is small to reduce temperature rise. A 1% resistor is likely to be very stable and have a better temp coeff.Be aware of other leakage paths that might be affecting your 10meg, such as corona and creepage. It is sometimes better to use more than one resistor in series to get the least change in resistance due to applied voltage and at the same time reduce power dissipation to reduce value change with heat.

author: txescientist
author_rank: Regular Contributor
timestamp: June 20, 2013, 06:19:58 pm
content: It is a 500V stabilized PSU for Geiger counter. Resistors that I use are 300V rated so that is probably the issue. Will try new 0.6W metal film resistors tomorrow or the next day and report back. Thank you all for fast response!

author: asgard
author_rank: Regular Contributor
timestamp: June 20, 2013, 11:49:17 pm
content: A lot of precision power resistors are designed with a length of wire around the substrate in a partial-coil construction. I would then consider any potential parasitic inductive losses through the wire, or the joint between the pins and the resistive element. Then, if you identify significant loss due to RF emission then maybe some shielding could help you retain some of that energy.

author: txescientist
author_rank: Regular Contributor
timestamp: June 24, 2013, 01:04:40 pm
content: Used regular metal film resistors and the end results are great! I have put 5x10Meg and one 100k in series, voltage across 10.06V, voltage measured on 100k - 20.1mVWhen heated with a hair dryer, voltage changed to 20.2 and down to 20.1mV, so lets say 0.5-1% and this is acceptable error. Thank you all for suggestions.