Need help locating this resistor

Thread Starter

born2fly

Joined Dec 5, 2020
30
Not sure what the ohms are for this resistor. I came up with 300G ohms but can't find one online. It reads .3ohms when tested. Can anyone help?

thanks,
rich
 

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Thread Starter

born2fly

Joined Dec 5, 2020
30
I suspect that the multiplier band is silver, not white, which would make it 3 ohms. 300G would be much larger and likely enclosed in glass.
I was hoping it was but I compared it to a similar size resistor with a silver band and it is definitely white. Checked it under the scope as well.
 

MisterBill2

Joined Jan 23, 2018
12,745
Use an ohm meter to find the value, since it does not appear that the item is damaged at all. And although the label calls it "R30" it might be an inductor, or even a fake.
 

sagor

Joined Mar 10, 2019
719
If the resistor has been running relatively hot, it is possible the color silver may have faded to white. Measuring the resistor, since it looks intact, should give a better idea. I don't think normal home-based multimeters will read 300G anyway...
I have some 50G resistors, and they are about 2 inches long (50mm).
Also, I doubt a 300G resistor at 1% tolerance would look anything like the one in the picture
 

Thread Starter

born2fly

Joined Dec 5, 2020
30
If the resistor has been running relatively hot, it is possible the color silver may have faded to white. Measuring the resistor, since it looks intact, should give a better idea. I don't think normal home-based multimeters will read 300G anyway...
I have some 50G resistors, and they are about 2 inches long (50mm).
Also, I doubt a 300G resistor at 1% tolerance would look anything like the one in the picture
Actually I used 3 different online calculators and that what they showed it should look like. It doesn't exhibit any signs of being hot or overheated and since the best I can get with the 2 devices I used to test it is .3 ohms, it seems to still be doing something and that's probably the best reading my equipment can give.
 

BobTPH

Joined Jun 5, 2013
5,510
That is a far cry from 300 GΩ. Given that the significant digits are correct, I would assume 0.3 is correct.
 

sagor

Joined Mar 10, 2019
719
When measuring low resistance, take a reading with the leads shorted, then take a reading with the device being tested. Difference is actual resistance. Many low cost home multimeters may have inaccurate resistance readings at the low end...
If it still reads 0.3 ohms, then something is wrong with those color bands on the resistor...
 

MisterBill2

Joined Jan 23, 2018
12,745
At this point, actually much earlier, common sense and a tiny bit of knowledge should have come into the picture and it would tell you that 300 gigohms is not a reasonable value for a resistor in any reasonable circuit. That resistance is probably more than the leakage resistance across the PCB between the device terminations! So immediately the conclusion is that the interpretation of the color code fails the "Mazelowski Ctrieria" of "is this number reasonable?" No, it is not reasonable, nor even just a few decades off from reasonable. And measuring the value shows even more that the reading of the bands is totally incorrect.
Next, examine the rest of the related circuit and it should reveal the purpose of the resistor in that segment of the circuit.

And I am rather amazed that there were no serious challenges about the believe-ability of such a resistor value.

In addition, this incident reminds us all that not everything presented on the internet is correct.
And one more question is why not show us a bit more of that circuit board? This could have been a test to see just what we might fall for.
 

Thread Starter

born2fly

Joined Dec 5, 2020
30
At this point, actually much earlier, common sense and a tiny bit of knowledge should have come into the picture and it would tell you that 300 gigohms is not a reasonable value for a resistor in any reasonable circuit. That resistance is probably more than the leakage resistance across the PCB between the device terminations! So immediately the conclusion is that the interpretation of the color code fails the "Mazelowski Ctrieria" of "is this number reasonable?" No, it is not reasonable, nor even just a few decades off from reasonable. And measuring the value shows even more that the reading of the bands is totally incorrect.
Next, examine the rest of the related circuit and it should reveal the purpose of the resistor in that segment of the circuit.

And I am rather amazed that there were no serious challenges about the believe-ability of such a resistor value.

In addition, this incident reminds us all that not everything presented on the internet is correct.
And one more question is why not show us a bit more of that circuit board? This could have been a test to see just what we might fall for.
So what value do you interpret it to be? Every chart I've looked at says the white ring is equal to "G". Highly unusual to see this but I have to say - I learn something new all the time and am grateful for it. This board is out of an air purifier that uses an ozone plate and other types of purification. The unit stopped producing power for the plate, still trying to hunt down the fault, looking like a transformer issue so far if the resistor is still good. Have not gotten to test voltages yet, just looking for anything obvious, only found a broken cap but that wasn't the fault.
 

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MisterBill2

Joined Jan 23, 2018
12,745
Resistors also have voltage ratings, and possibly stability ratings. asnd also temperature curves. An air cleaner is not a precision application, really, and so it could be a number of things. . It may be a power rating.
I see two high frequency high voltage transformers andzlso a very latge number of two position connectors. Some of those may be for safety interlock switches intended to prevent natural selection due to the high voltages. So the very fist step is to make sure that they are OK.
It does look a bit like a power resistor and imagine the voltage needed yo force current thru a 300G ohm resistor. At three ohms it may well be current limiting resistor for that nearby high voltage supply.
 

MrSalts

Joined Apr 2, 2020
2,111
If you measured 0.3Ω then that is what it is
Yes, that's what it is now. Who knows what it is supposed to be (or originally was). 0.3ojms seems like it could even be the resistance of tour test leads and your component could be a dead short low DCR inductor with milliohms of resistance. Can you measure inductance?
 

MisterBill2

Joined Jan 23, 2018
12,745
0.3 ohms is a reasonable value for a resistor in a transistor power circuit. And certainly that is some degree of power resistor based on it's size, relative to the others nearby.
It appears that the method of diagnostics being used here is to simply check every component and hope to find one out of spec. A vastly more efficient and more effective diagnostic approach is to understand what the circuit is supposed to be doing and then take some measurements to see what it is doing, and evaluate what may be causing the difference.
In this case the fault is a power oscillator that is not producing the required high voltage. The first check would be to determine if the oscillator power was present and at the correct voltage. Measuring the voltage across the filter capacitors would answer that question. Next would come a check of the highly stressed components, such as the power transistor driving the transformer primary. Checking the voltage across the emitter resistor can show if the transistor is flowing any current or not. If not, then checking the voltage at the connection between the transistor and the transformer will reveal that either there is the full supply voltage present, showing the transistor is not conducting, or almost no voltage, suggesting an open circuit in the direction toward the supply, or a shorter component downstream from the test point. (so far there are all voltage checks, relative to the circuit common point) If all seems reasonable so that the oscillator might be functional, then a measurement of AC voltage can reveal if that is the case. These checks would narrow the area of search to the power supply section or yje power transistor or the power transformer. Resistance checks of he transformer would probably verify continuity of the different windings, or not, leaving the transistor as a component to be checked.
 

Thread Starter

born2fly

Joined Dec 5, 2020
30
0.3 ohms is a reasonable value for a resistor in a transistor power circuit. And certainly that is some degree of power resistor based on it's size, relative to the others nearby.
It appears that the method of diagnostics being used here is to simply check every component and hope to find one out of spec. A vastly more efficient and more effective diagnostic approach is to understand what the circuit is supposed to be doing and then take some measurements to see what it is doing, and evaluate what may be causing the difference.
In this case the fault is a power oscillator that is not producing the required high voltage. The first check would be to determine if the oscillator power was present and at the correct voltage. Measuring the voltage across the filter capacitors would answer that question. Next would come a check of the highly stressed components, such as the power transistor driving the transformer primary. Checking the voltage across the emitter resistor can show if the transistor is flowing any current or not. If not, then checking the voltage at the connection between the transistor and the transformer will reveal that either there is the full supply voltage present, showing the transistor is not conducting, or almost no voltage, suggesting an open circuit in the direction toward the supply, or a shorter component downstream from the test point. (so far there are all voltage checks, relative to the circuit common point) If all seems reasonable so that the oscillator might be functional, then a measurement of AC voltage can reveal if that is the case. These checks would narrow the area of search to the power supply section or yje power transistor or the power transformer. Resistance checks of he transformer would probably verify continuity of the different windings, or not, leaving the transistor as a component to be checked.
Very informative info, thank you!
 

Thread Starter

born2fly

Joined Dec 5, 2020
30
Yes, that's what it is now. Who knows what it is supposed to be (or originally was). 0.3ojms seems like it could even be the resistance of tour test leads and your component could be a dead short low DCR inductor with milliohms of resistance. Can you measure inductance?
yes, but haven't tried yet.
 
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