CYRUS PSU diode type PFR852 C923

[yestertech]

This is a fast recovery diode and seems quite expensive at £6 each
I have 3 that are short circuit. Oddly they seem to be used in parallel.
Would a standard 1N.... series suffice ?


Andy

[G6Tanuki]

The generic old-type 1N-type diodes, specially those for higher-voltage ratings, are nowhere near 'fast recovery' and can generate horrible levels of RF noise.

UF-series are what we should all be using. And if you need to deploy them in parallel, it's essential to use diodes from the same batch so their forward-resistance has a good chance of being well-matched.

Get ones from the same bandolier and they will _probably_ be matched well-enough.

But before blindly replacing the diodes, interrogate the downstream side of the load, to work out just _what_ caused the diodes to fail in the first place.

[Radio Wrangler]

Running diodes in parallel, even with control over getting the same batch is bad design. The current distribution is naturally unstable. If one diode takes even a little more current than than the others, it gets hotter than the others (no surprise there) and the raised temperature causes its 'Vbe' to reduce which causes it to take a larger proportion of the current and get hotter still.

The same effect can happen in a single large diode (or transistor) except that the designer of the diode will have taken care that there is just enough distributed resistance to overcome the current hogging effect of any subset of the area of the whole device.

Parallel diodes can be used with a little current sharing resistance in each path... but then that isn't really diodes in parallel any longer. The amount of resistance depends on the closeness of matching of the diodes.

Perhaps their failure and need for replacement may be seen as evidence of poor practice.

David

[Terry_VK5TM]

Originally Posted by yestertech

Would a standard 1N.... series suffice ?
Andy

UF5402 is a direct replacement, but as per others, you need to find out why they died first.