VAXmate PSU

[Eric Smith]

On Fri, Apr 10, 2020 at 10:14 AM Rob Jarratt via cctalk <
cctalk at classiccmp.org> wrote:

> D12 is an MBR3045PT. It tests correctly as a common cathode diode network.
> However, the forward voltage seems to be 0.19V. The datasheet (
> https://pdf1.alldatasheet.com/datasheet-pdf/view/53622/FAIRCHILD/MBR3045PT.html)
> would suggest it should be 0.76V at room temperature. I can see no physical
> damage to it though.
>

Even when they are good, the forward drop of a diode can be FAR lower than
the typical forward drop (around 0.7 for normal silicon diodes) if you're
putting significantly less that the rated current through it. 0.19V forward
drop is only slightly lower than typical characteristic at e.g. 10mA
current. See figure 3. I've seen more than enough variation of diodes from
typical curves for that alone to convince me that the diode is bad (though
obviously it may be).

The real test is how much it conducts in reverse. All diodes will pass a
small amount of reverse current. This one shouldn't pass more than 1 mA in
the reverse direction at room temperature, even with near the rated reverse
voltage (45V) applied.