K2 Resistor Question...
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Working on my K2 (page 48) Assembly, Part II and as I'm installing the
wad of 1/4 watt resistors, I have been measuring them after installation. R6 and R8 are both listed as 100 ohm resistors (BRN-BLK-BRN), but after installation, both show 61.6 ohms on three different DMM's..while R7 which is a 68 ohm resistor is showing 50.1 I recognize all of them read slightly lower than their "listed" values...but these seemed at such wide variance I didn't want to continue until I knew whether I should be replacing them or not. These three resistors all form a U around relay K12...so I didn't know if the relay would be sucking down my readings...but I'm placing the probe on the resistor wire on either side of the body of the resistor...so I can't see how something else could be causing that great of a difference.... Another identical 100 ohm resister still in the paper ladder to be installed later in the sequence is giving me a 97 ohm reading... Any advice would be appreciated. David King KE7EKA K2 #6048 ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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David,
I am not sure how best to put this... Imagine you put a resistor of infinite value in parallel with your 100 ohm resistor and measured it at the ends of the 100 ohm resistor; of course, you would see the 100 ohms; what if the two resistors were 100 ohms each? well, being in parallel, you would see 50 ohms; if you put a 0 ohm resistor in parallel with the 100 ohm, what would you see? so anything with some resistance to it, a resistor, a trace, a relay coil, will affect the resistance you measure. A nice mental exercise you can do with a handful of resistors is to tack solder them together into a cube, and start measuring across different points; maybe even make it simple, and do just a square first (4 resistors); you might start to develop some intuition about how this works. try it with same value resistors first... I think you will find your resistors are fine; when you get to a test sequence in the manual, you will see they tell you what to look for in resistance that is quite different and that this intuition gained will help you feel more comfortable. Alternatively, if you are more accustomed to math, you might just learn some of the associated applications of Ohm's law, or do both what the heck... [hidden email] wrote: > R6 and R8 are both listed as 100 ohm resistors (BRN-BLK-BRN), but > after installation, both show 61.6 ohms on three different > DMM's..while R7 which is a 68 ohm resistor is showing 50.1 > > I recognize all of them read slightly lower than their "listed" > values...but these seemed at such wide variance I didn't want to > continue until I knew whether I should be replacing them or not. > > These three resistors all form a U around relay K12...so I didn't know > if the relay would be sucking down my readings...but I'm placing the > probe on the resistor wire on either side of the body of the > resistor...so I can't see how something else could be causing that > great of a difference.... the rest of the circuit that is in parallel with the resistor is responsible for that variation. > Another identical 100 ohm resister still in the paper ladder to be > installed later in the sequence is giving me a 97 ohm reading... > And if you removed the ones you already soldered in, they would measure close to the same 100 ohms (or 97 in your case. > Any advice would be appreciated. > > David King > KE7EKA > K2 #6048 73 de Dave, W5SV _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Thanks to David, Don and Gary for making me feel better this morning
about my resistors. I have been testing each prior to installation but when came to that group, got lazy and decided to only check them AFTER installation...which then made me wish I hadn't gotten lazy!!! Great object lesson here...don't take shortcuts! It just didn't seem possible that measuring on the leads directly on either side of the component that there would be that much resistance draw after mounting...but the fact that it was so uniform on both of them, plus the third one (68 ohm) being down that much as well made me curious! I really appreciate the explanations...that's why I joined this reflector and am assembling the K2..to learn! I'm already dreading the empty feeling that will come about when it's done and I don't have it to retreat to in the evenings! But, I've got the SSB board setting in a box to assemble, and keep pouring over the options on the website, so as long as the XYL doesn't completely pull the purse strings shut, I'll be able to keep on building! Thanks. Quoting "David F. Reed" <[hidden email]>: > David, > > I am not sure how best to put this... > > Imagine you put a resistor of infinite value in parallel with your 100 > ohm resistor and measured it at the ends of the 100 ohm resistor; of > course, you would see the 100 ohms; what if the two resistors were 100 > ohms each? well, being in parallel, you would see 50 ohms; if you put a > 0 ohm resistor in parallel with the 100 ohm, what would you see? > > so anything with some resistance to it, a resistor, a trace, a relay > coil, will affect the resistance you measure. > > A nice mental exercise you can do with a handful of resistors is to > tack solder them together into a cube, and start measuring across > different points; maybe even make it simple, and do just a square first > (4 resistors); you might start to develop some intuition about how this > works. try it with same value resistors first... > > I think you will find your resistors are fine; when you get to a test > sequence in the manual, you will see they tell you what to look for in > resistance that is quite different and that this intuition gained will > help you feel more comfortable. > > Alternatively, if you are more accustomed to math, you might just learn > some of the associated applications of Ohm's law, or do both what the > heck... > > [hidden email] wrote: >> R6 and R8 are both listed as 100 ohm resistors (BRN-BLK-BRN), but >> after installation, both show 61.6 ohms on three different >> DMM's..while R7 which is a 68 ohm resistor is showing 50.1 >> >> I recognize all of them read slightly lower than their "listed" >> values...but these seemed at such wide variance I didn't want to >> continue until I knew whether I should be replacing them or not. >> >> These three resistors all form a U around relay K12...so I didn't >> know if the relay would be sucking down my readings...but I'm >> placing the probe on the resistor wire on either side of the body >> of the resistor...so I can't see how something else could be >> causing that great of a difference.... > > the rest of the circuit that is in parallel with the resistor is > responsible for that variation. > >> Another identical 100 ohm resister still in the paper ladder to be >> installed later in the sequence is giving me a 97 ohm reading... >> > > And if you removed the ones you already soldered in, they would measure > close to the same 100 ohms (or 97 in your case. > >> Any advice would be appreciated. >> >> David King >> KE7EKA >> K2 #6048 > > > 73 de Dave, W5SV ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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