26 Nov
2022
26 Nov
'22
11:11 a.m.
On 11/26/2022 12:47 PM CST Rob Jarratt via cctalk
<cctalk(a)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