Today's Message Index:
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1. 12:04 PM - Diode Reverse Voltage Leak or Invalid Measurement Setup? (Valin Thorn)
2. 01:21 PM - Re: Diode Reverse Voltage Leak or Invalid Measurement Setup? (Charlie England)
3. 04:51 PM - Re: Diode Reverse Voltage Leak or Invalid Measurement Setup? (Jeff)
Message 1
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| Subject: | 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)
Message 2
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| Subject: | Re: Diode Reverse Voltage Leak or Invalid Measurement |
Setup?
Not really relevant to your question, but the spec sheet has the wrong
symbol for the diodes (zener instead of Schottky).
In general, if you leave the ground side of a circuit floating and measure
from the floating ground to the actual supply ground, you will see some
voltage (in some cases, the full voltage of the supply). For your voltage
test of the diode pack, try inserting a 12V load (light, etc) in series
with the diode, connected from the diode anode (your red test lead point)
to the battery ground terminal. Measure voltage on the same points shown in
your pic. I'll bet that the load (light) won't illuminate, and you won't
see any voltage. All semiconductors have some leakage, but won't supply
significant current because the leakage is so low. Your spec sheet says
IsubR is 2 mA at 45 volts, so would be much lower at 12 volts.
My terminology probably isn't precise (I was a repair grunt; not an
engineer), but hopefully the idea is still there.
Charlie
On Sat, Jul 4, 2015 at 2:03 PM, Valin Thorn <valin@starflight.aero> wrote:
> Hello,
>
>
> We=99re getting set to do power on testing of our electrical system
and
> avionics. We=99re using the (Bob=99s) 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 th
e
> Ground Bus.
>
>
> The meter set up was with the VOM meter ground lead to the battery ground
> (not connected to the airplane=99s ground bus) and then the positiv
e prove to
> the various measurement locations.
>
>
> At first I thought we must have a pin out wrong in a connector somewhere
=93
> 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 1
2
> 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=99s p
ositive
> probe on the ground bus, it also shows 3.5 VDC. So not even sure this is
> a valid way to test that I=99m using
>
>
> Here=99s our Power Grid. Sorry Bob that it is in my pictogram form
at The
> anode side of Diode B is where I=99m seeing 3.5 VDC with 12VDC on t
he 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 singl
e
> 12 VDC battery and measure 4.9 VDC on the anode side of the diode with th
e
> cathode connected to the battery=99s positive terminal. See photo
below.
>
>
> My questions.
>
>
> 1. Is this even a valid voltage measurement setup? Is this real? I
f
> 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=99d sure appreciate some help with this Oh, and Happy I
ndependence Day!
>
>
> Thanks!
>
>
> Valin
> (Confused)
>
>
Message 3
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| Subject: | Re: Diode Reverse Voltage Leak or Invalid Measurement |
Setup?
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