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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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 Posted: Tue Sep 09, 2008 6:47 am Post subject: Electronics 101...Advanced. |
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Occasionally I get an inquiry from a customer who is checking out his electrical system and finds what they view as an error--They test solid state components that are supposed to be OFF, or diodes that are supposed to not conduct, and they measure a voltage on the output. This they think must indicate a failure.
Usually, I can't figure this out from the description until some emails or calls have been exchanged, but it always boils down to the following--
The existence of an "output voltage", even a very substantial voltage, does not mean much. If there was a load connected from the output, there would be no measured voltage (on a standard voltmeter).
Voltmeters always possess a load. Digital types are usually 10 MegOhms, which is small enough to interfere very little with the circuit being measured, but for many applications can lead to incorrect or puzzling results--for example measuring dry cell batteries, or the output of solid state switches or diodes--In these tasks, output load is important. (It is trivial to make a voltmeter with GigaOhms or greater resistance but they find little use except in the rare experiment.)
"The problem with the world is that only the intelligent people want to be
smarter, and only the good people want to improve."
- Eolake Stobblehouse
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Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
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trigo(at)mail.telepac.pt
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| Posted: Tue Sep 09, 2008 1:36 pm Post subject: Electronics 101...Advanced. |
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Eric
Now that you initiated a pedagogic thread, you should go to the bottom of it.
For us, non-EE and electronically-challenged (should I say ignorant?) guys, that is a real situation! For me, it’s a “Been-there-done-that” case … J
I did measure, with my digital voltmeter, one used AA battery and one just off the blister, and both readings were 1.5V
Can you enlighten us?
Carlos
[quote] --
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MTBehnke
Joined: 13 Sep 2007
Posts: 3
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| Posted: Tue Sep 09, 2008 4:19 pm Post subject: Re: Electronics 101...Advanced. |
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| Quote: | Occasionally I get an inquiry from a customer who is checking out his electrical system and finds what they view as an error--They test solid state components that are supposed to be OFF, or diodes that are supposed to not conduct, and they measure a voltage on the output. This they think must indicate a failure.
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So a real life example might be someone checking to see if their essential bus diode is working by measuring voltage using their multimeter. One might find 12-14V at the main bus when the e-bus alternate feed is on and the battery contactor is off and therefore assume the diode isn't working. Further tests might reveal that while you might measure a non-zero voltage at the main bus, there is no current available if one was to try to power a device, such as a light.
Anyone ever have this happen?
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Mike Behnke
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Wed Sep 10, 2008 6:32 am Post subject: Re: Electronics 101...Advanced. |
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Mike,
I am certain there are, at least among my customers. With reference to the negative ground, an extremely high impedance voltmeter will measure voltages on everything, everybody, everywhere that is not firmly connected to ground. Even the disconnected output terminal of a mechanical relay or switch, or disconnected wires or metal will display a voltage.
Carlos,
A battery is only useful if it can deliver current (or power). Battery test meters differ from voltmeters because they impose a small load on the battery being measured. For 1.5V AA cells a load of 100 mA will do, so a load R of 1.5V/0.100A. So a 15 Ohm load will do while measuring the voltage across the cell.
"Democracy is the theory that the common people know what they want and deserve to get it good and hard." -- H. L. Mencken
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Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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