This video is intended for experienced or supervised technicians. Microwave ovens can operate in excess of 4000 volts and can store 2000 volts even when unplugged.. Always take appropriate safety precautions when dealing with live circuits. For informational purposes only. Utilize the concepts in this video at your own risk. The Tech Circuit or Steve Morrison assumes no responsibility or liability for any errors or emissions in the content of this video. The information contained in this video is provided on an as is basis with no guarantees of completeness, accuracy, usefulness, or timeliness.
I have a 16 year old stand-alone SMPS that’s blowing its internal AC-side protection fuse instantaneously upon switch-on. In testing the diodes on the board (in-circuit), nearly every one is showing voltage in both directions (using Fluke 179 which reads 0L above 2.4V). Surely that many diodes (~10-15) didn’t all fail at the same time? I’m having trouble making any sense of it. The unit had been disconnected from power for several months when diodes were checked, so I would think (but am not certain) that the caps were all discharged enough to not be a factor. Any thoughts would be appreciated!
Damaged diodes usually short out and look like a wire. Leaky diodes usually read close to the same in both directions. if you are getting asymmetrical readings on each diode, they are probably good. Out of circuit is the only way to be certain.
Hello, thought I would ask if you have noticed that different DMM's measure diodes with different readings. I was measuring germanium diodes while troubleshooting and found some that had leakage in reverse bias. I checked with another DMM, and it measured open like it should. My Fluke 87-V failed showing my diode had leakage. My HP-34401A measured that it passed. I am thinking about a test circuit to check the different meters but wondered if you may know about this? Thanks Dan
The voltage used to bias the semiconductors is current-limited. If that current is very small, the voltage across the leakage (effective resistance), is smaller, and may more readily fall within the window of the meter's range. A larger test current may increase it beyond the meter's range. I'd say it thus depends on the amount of current used in the diode test function.
It sounds like you are picking up some parallel capacitance when measuring the reverse bias voltage of the diode. Just let it climb and I suspect it will exceed the maximum applied voltage of the meter and thus indicates the diode is likely good. Even if it does not climb above the 2 or 3 volt threshold of your meter, it is still likely picking up some in-circuit parallel continuity which isn't an indication that the diode is bad. Best to remove it and measure it if unsure.
@TheTechCircuit That would make sense. I just left the aviation field after 14 years and went into repairing VFDs, which is completely new to me. They do have capacitors that the busbars connect directly to the rectifiers. Thanks for clarifying 👍
This video is intended for experienced or supervised technicians. Microwave ovens can operate in excess of 4000 volts and can store 2000 volts even when unplugged.. Always take appropriate safety precautions when dealing with live circuits. For informational purposes only. Utilize the concepts in this video at your own risk. The Tech Circuit or Steve Morrison assumes no responsibility or liability for any errors or emissions in the content of this video. The information contained in this video is provided on an as is basis with no guarantees of completeness, accuracy, usefulness, or timeliness.
Awesome explanation. Thank you!
Straight to the point and well explained! Thank you!
Awesome video and in proper english too !! new sub indeed!
I have a 16 year old stand-alone SMPS that’s blowing its internal AC-side protection fuse instantaneously upon switch-on. In testing the diodes on the board (in-circuit), nearly every one is showing voltage in both directions (using Fluke 179 which reads 0L above 2.4V). Surely that many diodes (~10-15) didn’t all fail at the same time? I’m having trouble making any sense of it.
The unit had been disconnected from power for several months when diodes were checked, so I would think (but am not certain) that the caps were all discharged enough to not be a factor.
Any thoughts would be appreciated!
Damaged diodes usually short out and look like a wire. Leaky diodes usually read close to the same in both directions. if you are getting asymmetrical readings on each diode, they are probably good. Out of circuit is the only way to be certain.
@@TheTechCircuit thank you!
What voltage is the meter supplying to diode?
Do you have a video on how to test an LED with a diode?
Hello, thought I would ask if you have noticed that different DMM's measure diodes with different readings. I was measuring germanium diodes while troubleshooting and found some that had leakage in reverse bias. I checked with another DMM, and it measured open like it should. My Fluke 87-V failed showing my diode had leakage. My HP-34401A measured that it passed. I am thinking about a test circuit to check the different meters but wondered if you may know about this?
Thanks
Dan
The voltage used to bias the semiconductors is current-limited. If that current is very small, the voltage across the leakage (effective resistance), is smaller, and may more readily fall within the window of the meter's range. A larger test current may increase it beyond the meter's range. I'd say it thus depends on the amount of current used in the diode test function.
May i can ask something i get some diodes from old crt tvs circuit boards i checked with multimeter and shows 0.585 are good or faulty?
What about measuring rectifiers on VFD? The manual says reverse bias should be infinite, but when i measure it just keeps climbing. Is that infinite?
It sounds like you are picking up some parallel capacitance when measuring the reverse bias voltage of the diode. Just let it climb and I suspect it will exceed the maximum applied voltage of the meter and thus indicates the diode is likely good. Even if it does not climb above the 2 or 3 volt threshold of your meter, it is still likely picking up some in-circuit parallel continuity which isn't an indication that the diode is bad. Best to remove it and measure it if unsure.
@TheTechCircuit That would make sense. I just left the aviation field after 14 years and went into repairing VFDs, which is completely new to me. They do have capacitors that the busbars connect directly to the rectifiers. Thanks for clarifying 👍
Silicon.
Silicone is a whole 'nuther thing 😊