Author Topic: Beginner's - removing diode  (Read 1605 times)

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Offline nForceTopic starter

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Beginner's - removing diode
« on: October 03, 2019, 11:58:01 am »
Hello, I have a simple question.

Why do I have to remove a diode from the circuit if, I want to test if it's OK?

Thanks.
 

Online xrunner

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Re: Beginner's - removing diode
« Reply #1 on: October 03, 2019, 12:06:32 pm »
It's possible that the stimulus applied to the diode in-circuit might be corrupted and flow around the diode because other components are connected to it. You can try to test it in-circuit but it might not give the correct reading.

At most you could only de-solder one lead also and test like that.
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Offline Zero999

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Re: Beginner's - removing diode
« Reply #2 on: October 03, 2019, 12:32:55 pm »
Hello, I have a simple question.

Why do I have to remove a diode from the circuit if, I want to test if it's OK?

Thanks.
It depends on the circuit. If the diode has other components in parallel with it, for example it's connected across the base-emitter junction of a BJT, it will read 0.6V in both directions, so will need to be removed from the circuit to be tested. If the diode is in series with the whole circuit, say for reverse polarity protection, it can be tested in circuit, as long as the connections to the power supply have been removed.
 
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Offline Kirill V.

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Re: Beginner's - removing diode
« Reply #3 on: October 03, 2019, 01:47:39 pm »
It depends if you want to test the diode or the rest of the circuit.
And if you are sure that the diode is not shunted, you can not remove it.
 
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Offline nForceTopic starter

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Re: Beginner's - removing diode
« Reply #4 on: October 05, 2019, 12:44:20 pm »
What about other components: resistors, capacitors, inductors? I think, here you can leave the components in the circuit. Only if we plug out the power supply, right?
 

Offline Zero999

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Re: Beginner's - removing diode
« Reply #5 on: October 05, 2019, 04:16:52 pm »
What about other components: resistors, capacitors, inductors? I think, here you can leave the components in the circuit. Only if we plug out the power supply, right?

No. Capacitors are seldomly used as a single. How are you doin to separate the capacitance/leakage from the one being tested and the others on the same bus?
The ESR of capacitors can be quite often tested in circuit, because the capacitor should form the lowest impedance path at AC.
 

Offline Zero999

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Re: Beginner's - removing diode
« Reply #6 on: October 06, 2019, 08:54:29 am »
The ESR of capacitors can be quite often tested in circuit, because the capacitor should form the lowest impedance path at AC.

Assuming you don't have an power-RF ground plane pair beneath it, and assuming the interconnection between caps are long and inductive.

Practically for a multilayer board, you need many MHz to see the difference, and I wonder not many people will spend money on a VNA just for measuring dead caps.

I don't think even 100kHz LCR meters are common, and they won't really address each one if you have 10 0402 MLCCs scattered around a small board.
That's true. I'm thinking more old school equipment such as linear power supplies, thermionic glassware and audio amplifiers.
 


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