Teardown : Keithley 580 Micro-ohm meter - Page 1
Forum section: electronics / testgear

author: free_electron
author_rank: Super Contributor
timestamp: July 29, 2013, 02:40:37 am
content: And it's teardown time again. This time a Keithley 580 micro-ohm meter.Broken can't be fixed, rejected. So i picked it up for 50$A few hours of open case surgery and studying the service manual and the puppy was humming along fine.The piano-buttons were corroded heavily (bad contacts) and one of the BUZ71 switching mosfets was fried..The mechanical buttons select the measurement range and operate/standby. the soft buttons allow you to change the signal polarity, switch from pulse mode to continuous mode , do relative measurements and play with the triggering of the instrument (self-trigger or one-shot when you press it)Some operation background:Essentially this machine is a bipolar, pulsed, current source. (you can turn the pulsing of and go continous but that has drawbacks).By sending a known current through an unknown resistor and measuring the resulting voltage you can calculate the unknown resistor. the machine employs a kelvin-sensing system so the force and sense wires are individual. this allows the machine to compensate wiring resistance.The problem with sending current through resistors is that they heat up and , as a side-effect, will thus change their resistance. this meter solves this by using a pulsed current and a sample and hold system. it sends the measurement current (with the selected polarity) only for a very short time : about 75 milliseconds. after which it sleeps for a second. this duty cycle prevents the resistor under test from being heated substantially . This is what sets the keithley apart form other machines. those simply blast away current..since a current source is used there is another problem : when no resistance is connected the circuit is open. So there will be a high voltage standing between the testpoints ( well.. of course that's relative, in this machine it doesn't go much above 7 volts.. )The problem with this is that the high voltage can cause a false reading. Let's say we have a corroded contact in a relay and we want to measure the contact resistance. Initially the resistance may be bad so the meter will create a fairly large voltage (a few volts ) this voltage causes a flashthrough in the oxidation and all of a sudden the contact resistance drops. Subsequent readings now see this lower value . we essentially have falsified our reading becasue the initial measurement zapped the oxidation.... So , to counter this problem a dry circuit test is available on the lowest resistance settings.In this mode the current source are steered in such a way that there is never more than 20 millivolts generated across the sensing. this prevents the contamination or oxidation from being zapped and it allows you to find the bad contact. in essence they limit the open-loop voltage of the current source so that bad contacts can be found.This is what the output pulse looks like :..A peek inside the machine shows an old-skool buildup (this machine is fairly aged) . the measurmeent system is controlled by a motorola processor, eprom with firmware and a GPIO chip. the front panel LCD has an on-board LCd controller made by NEC.The pulse forming circuit is made with the little toroidal transformer next to the mains transformer. The current source employs precision resistors from tellabs ( one of which is a 4 wire resistor) and switched using mosfets (TO220 above the black tellabs resistors.A voltage dividing hybrid and a dual slope integrating ADC preceded by a sample and hold circuit form the remainder of the machine...

author: skipjackrc4
author_rank: Regular Contributor
timestamp: July 29, 2013, 03:28:44 am
content: Interesting, thank you.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 29, 2013, 04:07:46 am
content: Thanks not only for the pictures, but an explanation for how things work, what was broken and how you fixed it. I learn a lot from your posts.Definitely Bookmarked.Keep them coming. I appreciate amount of effort that goes into a post like the above.

author: ketil b
author_rank: Regular Contributor
timestamp: August 14, 2014, 11:27:29 pm
content: Hi sorry to pull this thread from the doldrums but I think this is better than staring a new thread.I have been studying the 580, with an eye to building a clone of it, and I have a question.What is the advantage of using the transformer in pulse generation circuit, over say just switching a reference voltage?thanksketil