If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #304
Joined: Sat Mar 11 2006, 02:19PM
Location:
Posts: 5
Hi, I am studying AS-Level physics, and for coursework I have to make and test a sensor -> I think I'm going to try and make one that detects the amount of charge at a terminal so you could, say, work out the voltage a battery will give out, etc. Note this is not a voltmeter - the key is measuring static charge without drawing a current.
Would MOSFETs be useable for this? I've read that they let a varying current through depending on the charge a their gate. Are there other ways of doing this? (It'd be better if you don't recommend one over the other though, as deciding that is part of my coursework!).
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Well there will always be a little leakage current just from like the plastic arround the leads and whatnot, but in most cases it can be ignored. When we are taking less than a hundred volts I can't say that will hold true.
As to other devices... I can't really think of anything...
Registered Member #32
Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549
To make this MOSFET thing work, you'd need to take all the charge that's on what you're measuring and put it on the MOSFET gate. You might as well put it onto a capacitor and measure the voltage on it, instead. Measuring a charge that's where you want it isn't a problem at all.
The hard thing is measuring a charge that's in an arbitrary environment. All you can measure remotely is the electric field from the charges, such as by measuring the voltage between two points. Capacitance is what defines how voltage and charge relate so without knowing the capacitance, measuring the voltage won't be much help.
Theoretically, you could measure the charge without affecting what you're measuring much. (Kind of like how a thermometer measures the temperature of something without affecting it much.) You could measure the voltage and take some charge off the thing (such as by sticking a tiny capacitor on it briefly). The capacitance of the thing should be the change in charge (the charge on the capacitor) divided by the change in the voltage (of the thing you are measuring). This will let you work out the original charge.
That's getting way too complicated for what you're doing, I'm sure. I wrote this so you might get a better idea of what you're looking for.
I'd say do something simpler, but still cool. Something like a speedtrap or maybe even a laser ranger.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
A common digital multimeter already works pretty much the way you describe, though it measures voltage, not charge. The circuit inside a DMM (like most chips nowadays) is made with MOSFET technology, so could happily function without drawing any current from the voltage under test.
In practice, the guys who make DMMs deliberately connect a resistor across the input, because it makes the meter more convenient to use and harder to destroy by static discharge. 11 Megohms is a common value.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Detecting the amount of charge: MOSFETs?
