A properly functioning colloquially named zener diode will
have a forward conducting knee of 0.5-0.7 volts, and a reverse
conducting knee at the published zener voltage.
Any voltage below the conduction knee will result in little or
no current flow.
I think you are describing a bad zener, which could easily stop
your supply from bootstrapping.
-Chuck Harris
Mlynch001 wrote:
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I would put the odds that the Opto isolators are bad
at under 1%. They are very reliable, and pretty hard
to kill.
-Chuck Harris
Chuck,
You are 100% correct about that optocoupler. I pulled that component out of circuit and it tests good. Don't know why I even trust "in circuit" tests? Moving back to your "million dollar hint". . . .
I do have what the board shows as CR17 and this connects to PIN 1 of UC3844 (PCM Controller). Every diode on this board is labeled as CRxx. So the labels are no help in determining what the components actually are. That being said, CR17 looks like a classic glass package Zener Diode, so I lifted one end from circuit and then hooked it up to my TYPE 576 curve tracer to see what I had, expecting to see a typical ZENER curve. C17 tests as a Zener diode, but it acts in a VERY strange manner, unlike any Zener I have tested before. When I hook it up and increase the collector voltage control, the diode begins to conduct positive at the expected .6V and exhibits the other classic breakdown "knee" fairly quickly. For lack of a better explanation, it is acting like a "variable voltage" Zener. As I increase the collector sweep voltage, the breakdown voltage knee moves farther and farther left on the screen as the positive current increases and the positive current trace stays stationary at about .4V . A "known good" 5.1V Zener does not exhibit this strange characteristic. The known good ZENER knee appears and holds at the correct Zener voltage, regardless of the collector sweep voltage. I am fortunate enough to also have a 577 curve tracer as well, and it shows the same characteristic.
Is this some sort of diode that I have never heard of? Or could this be the troublesome component? Sorry if my explanation is not technically correct or as clear as it could be. I am dealing with something that I have not experienced before.
Any thoughts?
Sincerely,
