> On 11/26/2022 12:47 PM CST Rob Jarratt via cctalk <cctalk@classiccmp.org>
> wrote:
>
>
> Just trying to decide what to replace the failed diode with, and looking at
> the UF400x series, as suggested by Mattis. It seems to me that as long as the
> PIV is 200V or higher it should be fine from that point of view, the
> switching speed is never higher than 70ns, while the original A114x (assuming
> it *is* an A114x) has a switching speed of 200ns (possibly even 200us from
> the datasheet).
>
> However, I am wondering about the forward voltage drop. The datasheets
> suggest that the A114x parts have a 1.3V forward voltage drop. I have a spare
> H7842 that was working (until I messed it up today, another story), so I
> tested the diode in that, its forward voltage appears to be 0.5V, using a
> little tester I have. The UF400x have ratings of either 1.0V or 1.7V.
>
> How sensitive is the circuit going to be to the forward voltage on the diode?
> Given that the forward voltage of the suggested replacement is higher, would
> it slow down the speed with which the transistor is switched off too much and
> cause it to be overloaded and fail?
>
> Thanks
>
> Rob
>
>
I'm not at all familiar with either this circuit or any of the mentioned
diodes. However, I would point out that a diode's forward voltage drop varies
with current through it. Usually, the datasheet will list the "max" forward
drop, at the rated current and typically at the lowest rated temp (the drop
decreases as temp rises.) So it is entirely possible that your tester is
putting a very small current through a high-current diode and getting that
0.5V. It might be useful to feed it something close to its rated current and
measure the drop for a more accurate estimate.
Will
