Need help determining Ohm value of resistor.

[CAS]

I have found over the years that when using the 'diode' mode on any meter that I only use it as an indication if the component is faulty or not. As you have seen for yourselves, that actually reading you get varies depending on the meter.

[green]

I was intrigued to know what the result would be on my own meter, so I took a 1N4148 diode and a 33Ω resistor and put them in parallel. The displayed result was .032V.

ToddB66 - Can you confirm the reading on your meter was 027, or was it actually .027 (which would make more sense)?

Wondered what my meters would read. Used a 1N4148 diode and two 1 percent resistors, 30 and 500 ohms. One meter has an extra digit resolution. Diode (.549v, .5948v)
30 ohm resistor (30.2 ohms, .022v, 30.24 ohms,.0222v) 500 ohm resistor (497 ohms, .321v, 497.5 ohms, .3122v). Based on your readings I was thinking the meter displayed the voltage with 1 ma current. (E=IR) It appears my meters use less than 1 ma and it's not constant. The diode reading changes with temperature.

[Qiaozhi]

The 33 ohm resistor is i parallel with the diode. If you measure a 33 resistor with the diode range thats the figure I would expect. A lot of SMPS seem to have resistors in parallel with diodes and fets etc which make it a pain when trying to fault find.

I was intrigued to know what the result would be on my own meter, so I took a 1N4148 diode and a 33Ω resistor and put them in parallel. The displayed result was .032V.

ToddB66 - Can you confirm the reading on your meter was 027, or was it actually .027 (which would make more sense)?

[CAS]

The 33 ohm resistor is i parallel with the diode. If you measure a 33 resistor with the diode range thats the figure I would expect. A lot of SMPS seem to have resistors in parallel with diodes and fets etc which make it a pain when trying to fault find.

[Qiaozhi]

On the subject of ALT CODES, I found this useful reference for entering Greek characters).
https://rotkel.de/Infothek/Tastaturk...-Alphabet.html

It's in German, but the table is self-explanatory, and lists the correct codes to use for Windows, Mac, Linux and Latex.
That's how I inserted the omega character (for Ohms) in my previous post.

[Qiaozhi]

The only puzzling thing here is the 027 reading, and appears to be a resistance value (close to 33Ω). However, in the diode test mode, I would have expected to see a voltage reading on the meter of around 0.7V. Perhaps this a feature of the meter. i.e. if it cannot detect a diode, it shows the resistance value.

[ToddB66]

Question about testing diode (Continued)

CAS,

(This supplements my post #19)

One more thought; since the rectifier-type diode and resister are on the PCB solely as requirements for the LED and the LED is working fine, I'm just going to assume that the rectifier-diode and resistor are soldered correctly in the circuit in a manner that's functionally correct for the LED. Therefore, I've decided to assemble the air freshener unit back together and forget about using any of the PCB components for experimental testing. I'll just buy a mixed bag of components for experimental purposes, as was suggested to me earlier in this thread.

At least my interest in the PCB components has been time well spent for me, as I have studied my electronics books again, conversed with you and Qiaozhi and got a better understanding of how diodes and resistors work together.

Again, thanks for helps on this.

ToddB66

[ToddB66]

CAS,

As near as I can determine, it appears your circuit schematic agrees with the PCB circuit at my end.

So I guess you are indicating that when I'm testing the diode with the meter set in the diode position and the meter leads touching the diode leads positive to anode and then switching the positive test lead to cathode, that the 027 meter reading is a false reading....right ?

If you agree with the above statement, then do you also agree that I would have to clip one of the diode leads and re-test to get a true diode test, i.e. current flow blocked in one direction, but not in the other direction ?

Bottom line, for my "testing diodes project", me thinks I'll just go buy an assortment of diodes to play with, rather than clip a diode lead on my PCB. As mentioned previously, the LED is working, so I'm going to assume that the rectifier diode is good.

I really appreciate the time you are spending to help me unravel this.

Looking forward to your reply to my two questions above and any other comments you decide to make.

Best regards,

ToddB66

[CAS]

This is the circuit as I see it.



So the resistor and diode are in parallel. You are just measuring the resistor.

[ToddB66]

Questio about testing diode.

CAS....Thanks for your last reply !

Here's the top-side view of the PCB, showing the LED, resistor and diode.

Does this help you make a determination of what could be causing the 027 meter reading to be the same, regardless of whether the meter test leads are touching the diode leads the correct way and then reversing the test leads ?

In reference to the picture of the underside of the PCB in my previous post #15, I assume you can see the circuit tracks plainly enough when you double-click the attachment.

Thanks,

ToddB66

[CAS]

By the look from the picture, it looks like the led is in parallel with the diode or resistor. This would give you same reading both ways. Bit hard to say exactly without seeing the other side of the board and knowing how its all connected.

[ToddB66]

Question about testing a diode.

Qiaozhi,

I got a reading of 027 both ways, i.e. with the positive test lead on the anode lead of the diode and with the test leads reversed. There were no decimal points showing on the meter screen. To confirm, I did have the meter set in the diode position and the screen was showing the mv symbol.

Can the 027 reading be converted "close to 0.7V" mathematically ?

I prefer NOT to clip a lead on the diode if possible.

Thanks,
ToddB66

Qiaozhi,

Attached are two pictures showing the inside of my True Air model 04531GM air freshener and the top & bottom of the PCB for the LED, including the 33 Ohm, plus/minus 10 percent resistor and black-colored rectifier diode.
If I uploaded both pictures correctly, you should see the top side and bottom side of the PCB......if not, I'll leave the one shot and try to upload the correct second view. BTW, I plugged the unit into a wall outlet and the LED works, despite our suspicion that the diode might be faulty (since I got the same 027 reading with the meter leads touching the diode leads both ways.

[CAS]

I got a reading of 027 both ways, i.e. with the positive test lead on the anode lead of the diode and with the test leads reversed. There were no decimal points showing on the meter screen. To confirm, I did have the meter set in the diode position and the screen was showing the mv symbol.

Can the 027 reading be converted "close to 0.7V" mathematically ?

I prefer NOT to clip a lead on the diode if possible.

Thanks,
ToddB66

If you got a reading the same both ways, it sounds like either there is a resister in parallel with the diode or the LED you mentioned on the board is in parallel but reversed. You need to look at the tracks on the board and determine what else is connected to the diode.
Some meters do not give an accurate reading in the 'diode' mode. In reality, the diode mode will only give you an indication of whether the diode/transistor is ok. I have had components that read ok but will break down under load so dont take the reading as gospel. The only real test is to read voltages when it is running to determine if they are working as they should.
If you want to practice on components, go buy a mixed bag of resistors, diodes etc and test those. The solder them into basic circuits so you can understand what readings you get when components are not in isolation. If you were in AUS I would post you a collection to play with.

[ToddB66]

As you can see from Post #4, the Geotech website does accept ALT-CODES. Which indicates that you either have a Windows 8 problem, or something specific to your computer.

Qiaozhi,

I think your analysis is probably correct. I'm hoping my Son can find the problem and make the necessary changes/corrections to my computer. Anyway, as mentioned before, I can temporarily use Word Pad to type posts, then Copy/Paste into Geotech posting windows.

Thanks again !

ToddB66

[ToddB66]

Question about testing a diode.

Set your meter to the diode test position, and put the leads across the diode (positive lead on anode, and negative lead on cathode). The reading should be close to 0.7V. Then try the leads the other way round. The reading will be something different (most likely there will be no reading). If in doubt, snip off one lead and retest.

Qiaozhi,

I got a reading of 027 both ways, i.e. with the positive test lead on the anode lead of the diode and with the test leads reversed. There were no decimal points showing on the meter screen. To confirm, I did have the meter set in the diode position and the screen was showing the mv symbol.

Can the 027 reading be converted "close to 0.7V" mathematically ?

I prefer NOT to clip a lead on the diode if possible.

Thanks,
ToddB66