what I used to replace the same is MUR 160 in my 422 had several that just went leaky. the 4148 was not fast enough
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On 11/25/20 12:19 AM, Tom Lee wrote:
Hi Jeff,
Please call me Tom. :)
Some critical diode parameters are rarely discussed in EE curricula, so you have every right to be flummoxed by datasheets. It is also not infrequently the case that manufacturers want to overwhelm you with a lot of data to hide the fact that they didn't tell you the one thing you really wanted to know. One of these is "reverse recovery time". It happens that PN junction diodes don't turn off right away. Until they do, the diode maintains a 0.7V anode-to-cathode voltage, even if that means that the current reverses direction. As you might imagine that can be a Very Bad Thing(tm) in certain circumstances. The time taken for a diode to return to the non-conducting state is the reverse recovery time. This value depends on both the forward and reverse current, but under typical conditions, the 1N4148 recovers in the several-nanosecond range. The venerable 1N400x is much slower, with 5-20us being typical. The UF400x is a fast recovery version of the 1N400x series, with typical recovery times in the range of 50ns-100ns.
On top of all that, you might care about capacitance, as in any circuit that has to support a high bandwidth. Frustratingly, manufacturers often leave out that data, making it difficult to figure out whether a given diode might be an acceptable replacement. In that case, you have to guess. A good guide to guessing is to assume that the junction capacitance is 10-20pF per ampere of rated forward current. This figure applies to junction diodes, and not to Schottky or other types. (But the same rule of thumb applies to the output capacitance of a bipolar transistor, as it happens.) The logic is that both capacitance and current-handling capacity are functions of device area, so the two should be correlated. Other factors affect the relationship as well, but current handling capacity is the simplest proxy for capacitance that is quantitatively useful.
In your circuit, you're looking for a diode with a breakdown voltage of 175V and a forward current rating of 100mA. The 1A rating of a 1N4004? or UF4004 means that it can handle those two fine, but will have 10 or so times the capacitance, and so will likely be unacceptable. A Schottky unfortunately has much higher capacitance per ampere, so it is also probably not a good choice here.
After all that, I often replace these types of diodes with series combinations of 1N4148, if there's room, because I am always impatient to fix the thing now, rather than wait for the right part to get shipped to me (if I can even find a vendor of these often-obsolete components). In this circuit, two in series should suffice to replace an FDH2161. Some fussy folks would advise using a resistor in parallel with each diode to guarantee equal voltage division, but they're unnecessary here, especially if the diodes come from the same batch.
Good luck with your continued debug. You are going to get this to work!
--Cheers,
Tom
