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1. 06:59 AM - Re: Diode Reverse Voltage Leak or Invalid Measurement Setup? (Eric M. Jones)
2. 08:16 PM - Re: Diode Reverse Voltage Leak or Invalid Measurement Setup? (Valin Thorn)
Message 1
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| Subject: | Re: Diode Reverse Voltage Leak or Invalid Measurement |
Setup?
This is the Schottky diode that I sell with an appropriate heat sink, and it's
a great part. You could have contacted me, but this is a general interest anyway,
so an answer helps everyone.
All standard voltmeters have a built-in load. This is as it should be. If they
did not, then nothing would make much sense. For special purposes, voltmeters
can be built with arbitrarily high resistance (or a tiny load, called impedance
in meters, etc.), but they all still have some load.
All electronic parts, even relays and switches, have "leakage". Insulation can
still be measured in ohms, no matter how good, (millions and billions of ohms...)
So if you built a simple switch with nearly perfect insulation and applied a voltage
to it, a voltmeter with infinite ohms resistance would still measure a voltage
"through" the switch...whether or not the switch was on or off! That's
why voltages are not measured through switches, relays, and any other parts.
I note this in some of my literature, that a voltage measurement without a load
will give meaningless results. A Schottky diode has a very low Vf forward voltage
drop (lower than P-N diodes). This saves power, but the Schottky also has
a greater reverse current "leakage". The reverse current typically won't leak
enough power to light even the smallest LED, but some builders worry that this
is a defect.
It really isn't. It's just reality.
For those who are mystified by electrons. Check the attached.
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=444394#444394
Attachments:
http://forums.matronics.com//files/dabbling_with_electricity_103.pdf
Message 2
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| Subject: | Diode Reverse Voltage Leak or Invalid Measurement |
Setup?
Thanks Charlie and Eric! Very helpful.
Though after reading your article, Eric, I'm even more uncertain of how
electro-physics works. :)
Valin
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Valin
Thorn
Sent: Saturday, July 04, 2015 1:03 PM
Subject: AeroElectric-List: Diode Reverse Voltage Leak or Invalid
Measurement Setup?
Hello,
We're getting set to do power on testing of our electrical system and
avionics. We're using the (Bob's) Z-12 power grid at 28 VDC with a 12 VDC
bus and 12VDC hot battery bus.
We have schottkey diodes isolating the various buses.
With the positive battery terminal connected and the 12 VDC hot battery lead
connected, but NOT the battery ground to the single point ground, I checked
the voltage at various points and saw the 12+ VDC where it should be but was
surprised to measure about 3.5 VDC on the Main Bus and even the Ground Bus.
The meter set up was with the VOM meter ground lead to the battery ground
(not connected to the airplane's ground bus) and then the positive prove to
the various measurement locations.
At first I thought we must have a pin out wrong in a connector somewhere -
and maybe we do. After some trouble shooting, I was able to determine that
the voltage seems to be leaking in reverse past the diode from the 12 VDC
Hot Batt Bus to the 12VDC Bus and then through the 24 to 12 VDC converter to
the Main Bus. But somehow, when I put the meter's positive probe on the
ground bus, it also shows 3.5 VDC. So not even sure this is a valid way to
test that I'm using.
Here's our Power Grid. Sorry Bob that it is in my pictogram format. The
anode side of Diode B is where I'm seeing 3.5 VDC with 12VDC on the cathode
side, as expected, coming from the hot battery bus.
I did a test of a spare diode of the same model on the bench with a single
12 VDC battery and measure 4.9 VDC on the anode side of the diode with the
cathode connected to the battery's positive terminal. See photo below.
My questions.
1. Is this even a valid voltage measurement setup? Is this real? If
both the positive and negative side of the electrical system are showing 3.5
VDC relative to the unattached battery ground, then there is no electrical
potential difference/voltage between them.
2. Are these the wrong diodes for this application? (spec sheet
attached)
3. What should I do?
I'd sure appreciate some help with this. Oh, and Happy Independence Day!
Thanks!
Valin
(Confused)
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