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# Explain why DMM reads 00.0 on negative side of circuit

#### stspringer

May 10, 2019
116
Hello All,
Another question any help appreciated. Thanks in advance

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#### Martaine2005

May 12, 2015
4,963
That's actually really confusing.
Can you explain that again please?
All I can think of is the LED is acting as it should, a diode.
But as I couldn't decipher your question, or where you are probing, it's just a guess.

EDIT: LEDs don't have positive and negative legs.
EDIT2: Put the resistor before the LED and try your probing again. See if you get the opposite results on your DMM.
Martin

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#### stspringer

May 10, 2019
116
That's actually really confusing.
Can you explain that again please?
All I can think of is the LED is acting as it should, a diode.
But as I couldn't decipher your question, or where you are probing, it's just a guess.

EDIT: LEDs don't have positive and negative legs.
Martin

"LEDs don't have positive and negative legs."
Yes they do. Positive is the anode negative is the cathode. https://www.westfloridacomponents.c...w-to-tell-which-lead-is-positive-or-negative/

My picture shows the negative leg of the led. I put the black probe from my meter in the negative power rail. I put the red probe in the hole on the negative led leg. My meter reads .474 which is the resistance of the 470 ohm resistor. This is all done on the negative side of the circuit as shown in the picture.

Now with the black lead still on the negative power rail if I place the red probe on the positive side of the circuit on the other leg of the resistor my meter reads 00.0 which means there is continuity, but I was wondering why it would not read .474 ohms like the positive side did.

I hope you are clear on what I am saying about the more positive side or more negative side of the circuit, which I point out in the picture.

"EDIT2: Put the resistor before the LED and try your probing again. See if you get the opposite results on your DMM.
Martin"

The resistor is bridging the negative and positive sides of the circuit on the breadboard see the picture.

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#### BobK

Jan 5, 2010
7,682
Are you measuring Ohms on a powered circuit? That is a no-no.

Bob

#### Martaine2005

May 12, 2015
4,963
I put the black probe from my meter in the negative power rail. I put the red probe in the hole on the negative led leg
That will measure the resistor only.
Now with the black lead still on the negative power rail if I place the red probe on the positive side of the circuit on the other leg of the resistor my meter reads 00.0 which means there is continuity, but I was wondering why it would not read .474 ohms like the positive side did.

Sorry you lost me. Please edit the circuit picture with connections from your DMM.
Also, LEDS as you already stated have anodes and cathodes. Which tells you which leg to connect to the positive and negative of a power source.
Martin

#### stspringer

May 10, 2019
116
Are you measuring Ohms on a powered circuit? That is a no-no.

Bob

No, it is not Powered Bob I learned that from you and others

#### Martaine2005

May 12, 2015
4,963
Are you measuring Ohms on a powered circuit? That is a no-no.

Bob
That never even crossed my mind. Well spotted.

#### stspringer

May 10, 2019
116
That never even crossed my mind. Well spotted.

No, it is not Powered

#### Martaine2005

May 12, 2015
4,963
Can you clarify post 5 please.

Martin

#### stspringer

May 10, 2019
116
The circuit is not powered. I will try to explain it better.

My circuit, the side with the short red jumper wire, going to the anode leg of the led, I am calling this the more positive side.
My circuit, the side with the small black jumper wire feeding the other leg of the resistor to complete the circuit, I am calling the more negative side.

The resistor is the bridging the breadboard center channel to complete the circuit.

If I keep my black meter probe on any negative power rail, and then I place my red meter probe on the more "positive side" on the "negative cathode side" of the LED "the side with the short red jumper wire" my meter reads .474 ohms.

If I keep my black meter probe on any negative power rail, and I then place my red meter probe on the more "negative side" the side with the short black jumper wire" on the other resistor leg my meter reads 00.0 to 00.3

I will upload more pics

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#### Martaine2005

May 12, 2015
4,963
Is this what you are doing? Excuse the terrible pic

Martin

#### stspringer

May 10, 2019
116
View attachment 45659Is this what you are doing? Excuse the terrible pic

Martin

Pretty much please see my new pics. The resistor is hard to see in my new pics but it is in line with the negative leg of the led across the breadboard gap and fed by the small black jumper wire.

#### Martaine2005

May 12, 2015
4,963
Ok we got there in the end.
What you are reading is perfectly normal. You are measuring the resistor. Then you are just probing a wire between the resistor and power source, it's a dead short for the DMM.

Martin

#### stspringer

May 10, 2019
116
Ok we got there in the end.
What you are reading is perfectly normal. You are measuring the resistor. Then you are just probing a wire between the resistor and power source, it's a dead short for the DMM.

Martin
Why do you say it is a dead short? Isn't 00.0 on the ohm setting meaning continuity? Also there is no power going to this breadboard.

#### Martaine2005

May 12, 2015
4,963
Yes, continuity means a loop without a break.
What you are doing *is* just putting the meter probes together.

Martin

#### stspringer

May 10, 2019
116
Yes, continuity means a loop without a break.
What you are doing *is* just putting the meter probes together.

Martin
Ok so here is where I am confused. Why am I getting a good resistor reading .474 ohms only on one side of the resistor, if the resistor is the bridge to complete the circuit?

#### Martaine2005

May 12, 2015
4,963

This Is continuity.

Martin

#### stspringer

May 10, 2019
116

This Is continuity.

Martin
Yes but I don't see "understand" where I have a break. Why is the resistor braking my path?

#### Martaine2005

May 12, 2015
4,963
You don't have a break. You have very low Ohms reading. A short Or a link or electrically connected.

Martin

#### stspringer

May 10, 2019
116
You don't have a break. You have very low Ohms reading. A short Or a link or electrically connected.

Martin
So in my case it would be electrically connected, right? I mean the circuit works when I apply power

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