Common diodes frequency? - Page 1
Forum section: electronics / projects

author: 001
author_rank: Super Contributor
timestamp: October 24, 2019, 01:42:26 pm
content: HiI`m looking for datasheet likehttps://www.vishay.com/docs/88755/uf4001.pdfBut no info what frequency diodes ratedIs it some wellknown thing what I did`nt know?

author: capt bullshot
author_rank: Super Contributor
timestamp: October 24, 2019, 01:52:27 pm
content: Yes, diodes aren't rated for frequency but for junction capacitance, turn-off charge and turn-off delay, repetitive peak current and the like.By looking at these parameters, one can bin diodes roughly to- Rectifiers (line frequency) - robust with large peak current but also large turn-off delay/charge (if specified at all) - not useful for frequencies above a few kHz- Fast rectifiers (SMPS) - low turn-off losses (due to low turn-off charge/delay), medium to low peak current in comparison to steady current, typical frequencies up to few 100kHz- General small signal - low capacitance, low steady forward current, usually working fine to MHz range- All kind of RF and other specials - read the datasheet to find out the intended application and frequency rangeThe UF400x is of the fast rectifier kind, not really suitable as an (power) line rectifier (that would be the 1N400x)

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 24, 2019, 01:53:03 pm
content: What exactly would you expect to see?The junction capacitance value would be a good start to model the frequency response in a given circuit.Note that with through-hole packages, the parasitic inductance may not be negligible and also something to take into account.A "simplified" model would be something like: a series inductor, a series resistor, and an ideal diode in parallel with a capacitor.

author: 001
author_rank: Super Contributor
timestamp: October 24, 2019, 01:57:27 pm
content: Thanx a lot!Sorry my question is wery mudyThe problem is about AC meterhttps://www.eevblog.com/forum/projects/shottky-diodes-instead-of-germanium-in-old-analog-meter/

author: 001
author_rank: Super Contributor
timestamp: October 24, 2019, 01:59:46 pm
content: Quote from: capt bullshot on October 24, 2019, 01:52:27 pmSome people "upgrade" 1N400x with UF400x and caps across it. Why?

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 24, 2019, 02:06:00 pm
content: One of the typical germanium diodes you could still find (but not everywhere) is the 1N34. Its specs are unimpressive (Vfd, reverse leakage current...), but it has a junction capacitance < 1pF, whereas the diode you posted has a typical 17pF.In an old analog meter in AC, which I kind of doubt was designed for high-frequency anyway, it's not likely to matter one bit IMO. (But we'd have to know more about this meter though.)

author: capt bullshot
author_rank: Super Contributor
timestamp: October 24, 2019, 02:11:42 pm
content: Quote from: 001 on October 24, 2019, 01:59:46 pmAs long as the peak current doesn't exceed the rating, one can use the UF400x as an line frequency rectifier. Upgrading and paralleling a cap reduces active and passive EMI in certain cases, there's some interesting reading about the purpose of the capacitors in parallel to the diodes (no matter if UF or 1N kind), especially interesting if you're dealing with HF in your lab or if you're a HAM radio operator. Can't provide a link, just remember having read about that, not where.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 24, 2019, 02:18:21 pm
content: Quote from: 001 on October 24, 2019, 01:59:46 pmDon't know. The UF series have a much lower typical reverse recovery time than their 1N counterparts. But paralleling a cap would kind of defeat that? You should tell us the context.A typical use of paralleling caps with diodes is in bridge rectifiers to reduce noise. The whole point would NOT be to improve the performance at high frequency (kind of the opposite).I don't really see a big difference between UF and 1N expect the recovery time, and I'm not sure it would really make a difference in the above case, so I don't really see the "upgrade" factor here.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2019, 02:57:31 pm
content: The junction capacitance and reverse recovery time are 2 properties not directly related. The capacitance depends on the size and doping levels. The reverse recovery time depends on the minority carrier lifetime - fast recovery usually comes with a higher leakage current, as a small life time also increases leakage.The cheap, slow rectifier diodes may not be tested for low leakage, but the typical leakage tends to be lower than with comparable size fast diodes. Fast diodes may have a smaller die and thus lower peak current.For the rectifier at a meter, it depends on the use: for protection (e.g. in parallel to the meter movement) the UF400x or 1N400x should be OK. For a rectifier to measure AC one would better use germanium or Schottky diodes because of the low forward voltage.

author: Cliff Matthews
author_rank: Unknown
timestamp: October 25, 2019, 01:20:34 pm
content: https://www.eevblog.com/forum/beginners/why-would-someone-use-10nf-capacitors-in-parallel-with-each-diode-of-a-fwr/