Fluke choice for high impedance? - Page 1
Forum section: electronics / general

author: Glug
author_rank: Contributor
timestamp: January 19, 2012, 05:49:41 pm
content: I could use some help choosing a meter. I have the need to measure gigaohm resistors and high impedance circuits, and identify and match fairly small capacitors (mostly SMDs).The application is low level audio preamplifiers and buffers. I also need to match and bias fets, and match resistors - though analog methods still seem best for that.I understand the 80 series Flukes are my only real option for the high impedance. I would love to purchase a new 87V with warranty, but that strains my budget. Despite a lot of reading, I am hemming and hawing over which model, and whether an 83 or 85 would be just as good for my application. I don't see a compelling need for true RMS in my use cases.The big difference I see is the 87V's ability to measure capacitance above the 5uF limitation of all the rest. Even small power supply caps exceed that size. I have read some offhand remarks about the capacitance measurement of the 80's, and that other meters are better for that. Should I expect the capacitance measurement of the 80's to be adequate? I don't see myself spending a lot of money on a dedicated capacitance meter. I have not researched the gotchas of the $30 capacitance meters.So... Should a $60-$85 used 83, 85 or 87 meet my needs, in conjunction with a $30 capacitor meter? Is it worth spending more for an 87V just to get the increased capacitance measurement range? Am I missing anything?Thanks for the help!

author: saturation
author_rank: Super Contributor
timestamp: January 19, 2012, 07:42:06 pm
content: Gigaohm resistors? HH are not made for that; they max out <~ 1 gigaohm using conductance measurements, i.e., seimens. Gigaohms are the realm of insulation testers.

author: Unknown
author_rank: Unknown
timestamp: January 19, 2012, 11:09:56 pm
content: Quote from: saturation on January 19, 2012, 07:42:06 pmUsing insulation testers in pre-amp circuits sounds like a great idea. More common ways to measure high resistances in a lab are high resistance meters or electrometers, but these tend to be fairly specialized (how many labs stock GOhm resistors?) and expensive.For measurements in high impedance circuits, a high input impedance (>> 10 MOhm) might be useful. Many bench meters have > 10 GOhm input impedance at ranges up to 10 V DC or so. For even higher impedances, try electrometers, but expect to pay an order of magnitude more than a Fluke 87V for new ones. Keithley's low level measurement handbook (available from their website) has some good information about measuring high resistances or voltages from high-impedance sources.Why do you want to measure power supply caps? They are quite unstable (ever checked the tolerance?) and can vary wildly over temperature, so there's little point in measuring their capacitance with any accuracy. Measuring capacitance may tell you something about corrosion of the terminals or dried up electrolyte, but ESR tends to be a better measure for electrolytic cap condition.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 19, 2012, 11:22:09 pm
content: Quote from: alm on January 19, 2012, 11:09:56 pmIt is for audio. The laws of physics don't apply for audio stuff.

author: Unknown
author_rank: Unknown
timestamp: January 19, 2012, 11:25:49 pm
content: I missed that part. GOhm resistors in audio? Let me guess, the next question is what to do about the voltage noise and Johnson noise when combined with an opamp with 100MHz bandwith?

author: Glug
author_rank: Contributor
timestamp: January 20, 2012, 01:36:25 am
content: Quote from: alm on January 19, 2012, 11:09:56 pmComponent identification. The older Flukes have a 5uF limit.

author: amspire
author_rank: Super Contributor
timestamp: January 20, 2012, 02:28:28 am
content: Missing capacitance ranges on multimeters for electrolytics has never bothered me too much.I just use the ohms ranges and measure the charging time. A 1000uF cap will charge 1V in 1 second with a resistance range that uses a 1mA test current.For electrolytics, 10% or 20% accuracy is usually fine, so it is just a matter of learning the current and voltage levels of different ohms ranges. I have to admit, it was easier in the day of non-autoranging DMM's.It is a useful technique to have available. Otherwise, if your DMM measures up to 2000uF and you need to check a 10,000uF cap, how are you going to do it?You can buy a specialized LCR meter that may go up to 20,000uF, but what if you then need to test a 50,000uF cap?Richard

author: Glug
author_rank: Contributor
timestamp: January 20, 2012, 03:32:22 pm
content: Quote from: amspire on January 20, 2012, 02:28:28 amThat's a useful technique, especially with large caps, but I think we are off on a tangent there. The question is the limitation of 5uF. The only time I've ever had to work with large caps is when building phase converters. So that really is a corner case that I don't worry about.The idea is to optimize my next gear purchase to cover as many of my common use cases as reasonable. I don't want to spend a bunch more for a feature that ends up not meeting my needs, only to end up buying another dedicated meter.It sounds like I'll want to get a separate capacitance meter. How much I need to spend there is something I need to research. But, again, it is the need to accurately identify unknown caps, and match caps.I've poked around at bench meter specs a bit, but I haven't found anything reasonably priced that has a gigaohm measurement capability like the fluke 83, 85 and 87. Ultimately, I may need to build a rig for matching resistors, and use the fluke for coarse measurements.The conductance meters that test at 250, or a 1000 volts are not suitable for this application. They are intended to test insulation breakdown, and would fry my resistors.I did find this neat piece of gear:http://www.trifield.com/content/high-resistance-meter/