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strange DVM jacks

I have a Fluke 8845A benchtop DVM. I've been occasionally measuring the slow self-discharge of a supercap. I've been just poking the leads of the cap into the banana jack sockets in the front of the DVM now and then. The cap voltage has crept down over some weeks to about 1.7 so far. But yesterday, I poked the leads into the DVM and it read 2.5!

Turns out that the banana jacks for IN HI and IN LO are actually split, to sort of make a 4-wire connection to an ordinary banana plug, and if you hit the ones on one side it reads 1.7, but hitting the other side reads 2.5. Plugging in banana probes or test leads into the DVM (or using another DVM) reads 1.7.

OK, I'm mildly disabled from a gigantic rum+coke at lunch, but I can't envision how this would read 2.5, or even why these jacks are split. There's a separate pair of jacks for 4W ohms measurements.

My handheld Fluke has split terminals too. Maybe that's for some sort of alarm function. That one beeps if you connect it wrong, but the

8845 doesn't. 
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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin
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It's for a 4 wire Kelvin bridge connection. 2 conductors in each lead. In order to get the 6.5 digit accuracy out of the 8845A, you need the $80 leads: or at least a banana jack which connects the source output and measure input connections in the jack together.

Look out the window at the big tower on the hill. If the super cap is connected to the source output, the measure input is picking up its voltage from leakage across the jack and will read low. If connected to the measure input, measure input is picking up RF from your friendly neighborhood RF noise source. If you have the super cap is connected to measure input on one jack, and source output on the other, I have no idea what might happen.

I think (not sure) that those are just the split connections brought out to individual banana jacks.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

It's set to a voltage range, not ohms. The 2.5 reading is steady. I doubt that it's rectifying RF.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

Kinda looks like a Kelvin bridge to me: Perform 4-wire measurement with only two leads Patented split terminal jacks for the 2 x 4 ohms function allow you to perform 4-wire measurements using only two leads instead of four. A special test lead accessory is available to enable you to establish the connection. You get excellent resolution and accuracy plus the convenience and ease of using a single pair of leads.

You're probably right about it NOT being RF that's causing a 0.8v increase in reading, but I couldn't think of anything else. (I tend to blame the things with which I'm most familiar). If you don't want to move the setup, try putting a "space blanket" (aluminized mylar) over the instrument and see if the reading decreases.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

Is it doing some kind of MAX HOLD on those readings? No telling with all that hitting and banging you're doing to the leads and meter.

Reply to
bloggs.fredbloggs.fred

split banana sockets are necessary for auto ranging multimeters. A manual ranging probably has the more solid type.

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Using Opera's mail client: http://www.opera.com/mail/
Reply to
David Eather

It's pretty weird, doing 4-terminal measurements with just two probes. You may as well use two cheap probes and zero out the lead resistance.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

It doesn't matter, since we're all going to roast to death soon anyhow.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

It's mostly useful for measuring resistances, where there's current going through the leads. A 4 wire Kelvin bridge arrangement takes the lead resistance out of the measurement so that you don't have to short the leads and zero the resistance every time you measure something. Zeroing the resistance might be useful for low precision resistance measurements, such as commodity ESR meters, but would be a PITA if you were trying to utilize the entire 6.5 digit resolution of the Fluke

8846A. Just move the leads or connectors, and you have to recalibrate again. 2x4 wire resistance simplifies precision measurements. 2x4-wire ohms makes it easier to make accurate resistance measurements, without having to change cable configurations and without having to work with a bench full of silicone spaghetti. 
--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

Here it feels like pot-roasting (not roasting) over the past few days :) Media is claiming it is the longest such heatwave since 76, when the Minister for Drought (seriously) suggested bathing with a friend(!) and was in that office for only two weeks.

Reply to
Tom Gardner

Sounds like a fun guy, as Ministers go.

I just managed to sprint outside and back to fetch the newspaper, without freezing to death. It's 57F. Once safely back indoors, I opened the Chronicle and the headline on the front page shrieks HEAT WAVE EXPECTED TO CONTINUE.

Maybe that's why I'm skeptical about extinction from global warming. It's always cold here.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

From that always reliable source, wackypedia

formatting link

"In 1976, during Britain's driest summer in over 200 years, he was made Minister for Drought (but nicknamed 'Minister for Rain'). Howell was charged by the Prime Minister with the task of persuading the nation to use less water ? and was even ordered by No. 10 to do a rain dance on behalf of the nation.

"The appointment provoked much public mirth, but in true Brummie style, the Lozells-born MP responded by inviting reporters to his home in Moseley where he revealed he was doing his bit to help water rationing by sharing baths with his wife, Brenda. Days later, heavy rainfall caused widespread flooding, and he was made Minister of Floods. Additionally, during the harsh winter of 1978?1979 he was appointed Minister for Snow."

The temp has dropped 10C overnight, phew.

In contrast to SF, in the UK we are /very/ well aware of the difference between "weather" and "climate". We /know/ we expect climate but get weather. That's why the weather is the default conversation here!

I remember dipping my toe in Half-moon Bay, and being surprised at how cold the water was. I didn't know about the "California Current" then.

Reply to
Tom Gardner

Speak up, speak up! It's hard to hear, with the central heat running. Seriously.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

The problem with Heat waves, is most aren't. As I used to understand it a heat wave is when temperatures are above 90 for an extended period, including night and day. The last so called heat wave we had recently it was below 90 at night (70ish).

Cheers

Reply to
Martin Riddle

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