He's back xP
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Time to do some exercises
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xP
A baffling experience was recorded today :-X
Using a multimeter where you have to select the range the resistance of a forward biased diode was about 3.3 Mohm and infinite reverse biased
Then using the diode function the typical forward voltage drop was obtained without incident.
So what's the beef :grr well the auto ranging multimeter showed infinite resistance when forward biased and a few megohm when reverse biased :lmao: (and it has no diode test function)
:loco
I went back and rechecked all my results and even added a couple other multimeters to the mix
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Gonna have to think about this one for a while xP
Set the Ideal or the Centech to measure DC volts.
Check the polarity (and voltage) of the DC being put out by the Amprobe's when they are set to ohms or diode check functions.
I now see that this perceived "problem" is mere child's play for the Mighty G1 :dbtu:
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:tu:
The diode testing mode is showing 601 mV. It's a good diode with a reasonable forward drop.
These meters work by putting out some known, fixed current into the device and measuring the voltage. That current is going to be tiny! Remember that silicon diodces have a nominal forward drop of 0.7V; but the detailed model is that they start conducting at significantly below that, at small currents. That's why you can get readings below 600 mV for diodes.
Measuring ohms is a crapshoot. Multimeters measure resistance also by putting out a fixed current and measuring the voltage. The current used varies with the range: like millamp-level currents for the smallest ranges, and down into hundreds of nanonamperes for the biggest range.
Let's say that your multimeter is using a 100 nA test current for the multi-megohm range. The voltage drop would be 0.1V across a 1M resistor. The voltage across a diode conducting 100 nA could also be 0.1V, so that it reads as a 1M resistor. With a forward bias of 0.1V, it's possible for a diode to be conducting 100 nA.
Try this diode current calculator: https://www.everythingpe.com/calculators/diode-current-calculator
You can't pick a diode from a list here, needing to know the saturation current. This is like 2 to 4 nA for a 1N4148, so let's use, oh, 2.5 nA. Then we put in a voltage of 0.1 and hit calculate. The current is 1.1E-07A, which is 110 nA. So that checks out.
But actually, 3.3 MΩ was observed on saturated's auto-ranging meter. Can we use that to guess at approximately what current it is using for probing the component?
Yes! The meter is using around 20 nA. At that value, a 3.3M resistor drops a voltage of 0.06V. And 20 nA is in the ballpark of the forward current through a diode at 0.06V.
It's pretty uncommon to see reverse polarity on ohms function like that, but there is nothing official about how it has to be done.
I would check the polarity of the Capacitance function on that HD160 meter as well. In the instructions, it describes how to attach the probes one way, but then has a picture showing it the other way. You don't want to apply reverse polarity to a polarized capacitor.
Thanks for pointing that out WOW yeah that could be bad :grr
I'm gonna go ahead and say that's a big fault them having it that way :trouble
Also what if you guys were having me probe transistor leads and I kept reporting backassward results :grr xP
I can't say I'm over the moon with the auto range meter probably a dumb decision to try one :grr I don't like sitting there watching it think :grr
In other news I'm finding im gonna have to have a big come to Jesus about conventional current vs electron flow xP :'(
Conventional current is widely used for convenience and standardization, while electron flow reflects the true physical movement of electrons.
Both approaches are valid for circuit analysis, and understanding both enhances comprehension of electrical systems
Yes sir thanks I wondered why conventional current had not been eradicated but apparently it's not going anywhere.
Anyhow I'm gonna do some diy rectifier experiments
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they wanted me to have a 12.6 v transformer all I had was a 24 so I rigged this up
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Or maybe I can try the other primaries xP
Man I hate it when wierd stuff happens but I'm supposed to embrace it and welcome an opportunity to learn something
I tried the other primaries to 120V and got two different results from the red 208v primary
Nooooooo!?
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Went back and hooked it up again :tu:
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Scratches head :grr :'(
No changes at the variac for all 3 measurements?
If so, the 16V reading seems to be incorrect.
So it is like this?
Your transformer has a tapped primary side: red to white is full, black is half. Secondary side is yellow and blue.
If you're getting unexpected readings, take the voltage at the primary side to rule out that being flaky.
Disconnect the trafo and take winding resistance readings. If there is fluctuation, that points to an intermittent short in the windings.
Thanks guys sorry I should have mentioned when I tried the 208 and 240 NO VARIAC
MY APOLOGIES :duh
But anyway I appreciate the help as always and I reckon this was a loose connection or something I don't want to waste yalls help on it :grr so I'm moving on. Anyhow if it's really a thing maybe it will happen again while I'm making rectifier circuits and I'll see what the deal is :dbtu:
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Interesting. The 240V full coil doesn't have twice the resistance of the 120. This could be because of this: maybe the transformer primary wound from the orange side first: the upper half of the tap is closer to the core, and so it requires less length of wire to achieve its required number of windings. Then the black-to-white part of the coil is wound after the tap, requiring more wire to hit the same number of windings.
Yes sir thanks I believe you but that's kinda outa my pay grade xP
Anyhow I'm digging out another transformer because I need one with a center tap for the dual wave rectifier :tu:
Quote from: saturated on July 15, 2026, 01:15:21 PMThanks guys sorry I should have mentioned when I tried the 208 and 240 NO VARIAC
Then I expect the 16V secondary voltage you measured was the correct one when applying 120V to the 208V primary winding. The 3V reading will have been some kind of error.
You don´t know what wire gauge/thickness was used to wind that transformer.
THERE IS an optimum value achieved by calculation but winder uses what´s available, so sometimes not optimum.
Or wire simply can´t fit within EI window, same thing.