
Home  Forums  Rules  Articles  The diyAudio Store  Gallery  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc. 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
2nd March 2009, 12:31 PM  #1 
diyAudio Member
Join Date: Dec 2008

Your views on these resistor values
I recently bought some carbon resistors from ebay. They were advertised as 1 Watt 0.1 OHM and 2 Watt 0.47 ohm. Both were + or  5%.
I've just received them and measured them with my meter. The 0.1 Ohm resistors measure as 0.3 Ohm and the 0.47 Ohm measure as 0.7ohm. Even allowing for the + or 1 5% tolerance these measured values seem to be way out of line with what they should read. Are my observations correct or are those measured values what you would expect to see? Thanks 
2nd March 2009, 12:44 PM  #2 
diyAudio Member

If you short your measurement leads, does it read 0.2 ohms?
Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news?  W. S. Maugham Check out Linear Audio! 
2nd March 2009, 01:00 PM  #3 
diyAudio Member
Join Date: Dec 2008

Yes if I short the leads it reads 0.2 ohms, but I'm not shorting the leads when measuring the resistors.
At the same time as measuring the above I also measured a batch of 2.2K resistors and 2.7 ohm resistors. Both of these batches measured correctly (as have others) so it's not bad measuring skills. 
2nd March 2009, 01:59 PM  #4 
diyAudio Member
Join Date: Apr 2005
Location: Fredericia, DK

Sure Your not shorting the leads, but they are in series with the DUT. You have to add the resistance of the lead's wire/plugs/junctions when you measure such "small" resistors.
What if you had a 0.01r and tried measuring that the leads would still have ~0.2r. Best regards Ebbe 
2nd March 2009, 02:04 PM  #5 
diyAudio Member
Join Date: Dec 2005
Location: Buenos Aires

Your meter's leads have a resistance of about 0.2 ohms which are in series (they add up) with the resistor you're measuring. Thus 0.1 resistors appear to be 0.3 and 0.47 ohms show as 0.7 ohms.
You don't notice the difference while measuring 2.2K because the meter can't "see" the extra 0.2 ohms. Measuring very low or very high resistances with a standard DVM can show noticeable errors, that's why special instruments are sold. 
2nd March 2009, 02:28 PM  #6 
diyAudio Member
Join Date: Dec 2008

It is true that you learn something new every day
But now I feel silly Thanks for clearing this up. 
2nd March 2009, 02:59 PM  #7 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Hi,
string all your 0r1 resistors in series. Measure the resistance of the whole string. Subtract the 0r2 error that your leads have measured out at. Divide by the number of resistors. How close to nominal resistance are they now? Repeat for a string of 0r47 resistors. While they are in string apply a 100mA CCS source to them. Each 0r1 resistor will drop about 10.0mV read on a 200.0mV scale DMM. Compare all the voltage drops. now find matched pairs or quads. label them for future use when tight tolerance is called for. If you have a 100r 0.1% (four cheap 1% in series parallel would be better since 100mA will run very hot, or better still, 9 in series parallel) resistor you can add this to the string and measure the actual CCS current at the same time and from there calculate the resistance of every resistor to better than 1%. If you build a 190mA CCS you can do matching to better than 0.5% with care. Ingenuity and not much money can get you to 0.05%. Order up some lm317 regs and a decade range of 10r to 100k 0.1% precision resistors, included in your next purchase. Start buying your 1% metal film 600mW resistors in 100off packs. Rapid sell them for <£1.50/100. eventually you'll have the whole E24 range from 10r to 2M2 and E6 range extending out to 1r0 and 10M
__________________
regards Andrew T. 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Resistor values  Valvomaniac  Tubes / Valves  5  27th April 2007 04:50 AM 
Resistor Values For L Pad?  kelticwizard  MultiWay  8  16th February 2006 01:41 AM 
Resistor values  Stocker  Chip Amps  3  4th May 2004 03:10 PM 
Resistor values?  randytsuch  Chip Amps  2  11th July 2003 01:08 AM 
New To Site?  Need Help? 