Trying to determine if a diode UF5** (UF5 is all I can see, anything else is not visible ) is operable using only a Volt/Ohm meter. Is there a minimum voltage necessary to get a valid Ohm reading. This looks like a fairly robust diode and I get very high resistance in either direction with my ohm meter. Is it possible that I need a minimum voltage which my ohm meter is not providing for a valid test, or is this diode "dead"?

 

UF 54001/7 are Ultrafast diodes, put Dvm on Diode test, Black lead to Cathode ( stripe)and measure approx 560 to 700mV one way and open circuit the other.

 

I am doing this over the phone with my son and he has only an expensive volt/ohm meter which does not have the Diode test. Can we rely on the Ohm test only? And can you tell me what is the feature of "ultrafast". Does that just mean it responds to reversals much quicker? Thanks for your reply.

 

You can only test diodes on Diode test, Ohms won't give a correct reading, Ultrafast diodes are used in switchmode psus usually, for high speed rectified AC.

 

I just want to find out if the diode has failed. At this point I am not interested in whether the diode meets specification. Could I at least expect to see a significantly less resistance in the forward direction. I might guess that the Diode test is not done above maybe a 9 volt battery voltage and just measures the voltage drop. Thanks again for your reply.

 

Yes you can put the diode in series with a battery and measure the battery voltage, it will be 0.7V less than the battery voltage in one direction and Zero volts in the other.