On 18 September 2017 at 23:28, george <g_ei...@hotmail.com> wrote: > The reason that DC is used commercially to measure resistance is simple, > if you use AC you may well get the reactive component as well as the > resistance coming into play. >
That may not be an an issue with a dual-phase lock-in amplifier, as the phase angle of the voltage can be resolved. In fact, it hints at something I have long thought about - using a lock-in amplifier as an LCR meter. I do however have a decent HP 4284A LCR meter, but it can't read very low impedances in the micro ohm range. I've put a couple of offers in on micro-ohm meters, but also bought a 150 W public-address (PA) audio amplifier for £25 (around $35). With that, and the lock-in amplifier, I should be able to make measurements, although I accept the uncertainty will be higher than a dedicated micro ohm meter. > > Such low resistance measurements commercially are normally only made on > high current power distribution networks as part of a periodic test regime > where you need to determine the quality/resistance of such things as bus > bar joints/connections and loop resistance. > > It is not a good idea to use copperslip around aluminium, there is an > aluminium based version that should be used, but, be warned, just like > copperslip it is an insulator, try putting your meter probes, set for > resistance, into a tub of both, I have. I do not know just why but the > aluminium version is just like sand, it gets everywhere when you use it. > So does Coperslip! It sure is messy, but I was given it free, and it stopped a leak. > > To check your joint I would use a four wire Kelvin set up using say 10 > Amps from my constant current bench supply and then use my Keithly 616 > digital electrometer to measure the voltage/s present across the joint, a > simple application of Ohms law will then give the resistance. > I don't have such an instrument, whereas the audio amplifier cost me very little, and I already have the lock-in amplifier. I did buy a lower powered (15 W, $5) amplifier from China, but thought by the time I box it up with a PSU, it would cost more than a PA amplifier with a built in mains supply. The PA amp, being in the UK, should also arrive a lot quicker than the units from China. > > 73 George G6HIG > Dave, G8WRB _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
