Em 29/08/2013 05:39, RussellMc escreveu:

> What they all said, on average, and more, and less. Murphy says so.
>
> I would tend to use 1 x 1N400X per coil mounted at the relay - across coi=
l
> contacts if practical.
>
> It's quite likely that 1 only diode almost anywhere on the circuit would
> suffice and it's possible that you MAY get a useful improvement with
> esoteric diode types.
> If I was building these in production volumes I'd look at measuring what
> actually happens with various solutions. Those who like to do high wire
> without a safety net could probably get a good enough result with eg SPIC=
E.
> SPICE is usually an excellent way of modelling real world events.
> If "usually" is good enough for you then SPICE may be too. [FWIW about as
> good a SPICE as you can get is available for free. LTSPICE is free and th=
ey
> have recentyly upgraded it.]
>
> You said the relays were close together and separated from the driver by =
up
> to 20' of wire, and you specified the diuode to be near the relays rather
> than the driver.
> In the following "reasons" I also consider the more general case of relay=
s
> further apart on a 20' circuit and of a diode at the switch end. This is =
a
> superset of your problem but allows the solution to be more universal.
>
> My above 1 x 1N400X suggestion is based on:
>
> - Seems likely to work.
>
> - Has usually done OK in the past.
>
> - Cheap, available, robustish.
>
> - 1N400x will allow some voltage rise above what you'd see with eg a
> Schotkybut I'd expect it to be under a Volt extra and would be
> surprised if it was
> vastly more than a volt. Voltage rise is caused by the pre switch-off
> current flowing absolutely unimpeded into the new load. Given infinite
> impedance it will sigh and deliver infinite voltage. If only a perfect
> diode but with finite switching speed was present you'd get infinite
> voltage as it turned on. However, circuit capacitance and wire resistance
> alone will act as a reservoir with a time constant that the current will
> transfer into. The 20' of line will look (ATM) mainly like a small
> capacitor but to some extent like a transmission line with finite
> bandwidth. All told I'd expect real world situations to deliver and
> acceptably small 'blip' with a 1N400X.
>
> - When on the 1N400X sees coil voltage. A 1N4001 is 50 V (or so long ago
> memory says) so anything in 4001-4007 range will do at 12V. I usually buy
> 1N4007s as the cost difference is usually minimal and you then usually
> don't have to ask.
>
> - While 1 1N400x is rated at 1A continuous it has a surge current of
> amazing (perhaps about 20A? - see datasheet). Foor Irealy <=3D 1A it just
> works. Fir I_relay > 1A (and that would be an unusual relay) you can deci=
de
> if the dissipation is going to be OK and that will depend on switching du=
ty
> cycle but in 99.99% of cases a 1N400x will have adequate current and
> dissipation ratings.
>
> - As others have noted, if the diode is at the other end of a length of
> wire you will get radiation and also you will get increased voltage rise
> initially. Best (or worst) case Murphy will be able to use the radiation
> from your 20' slot antenna to crash your system or if used in a more majo=
r
> installation, take the whole plant down. Doing this is difficult. Murphy
> thrives on challenges.
>
> - Schottky would be better notionally in the forward direction but this
> does not seem of major advantage. In the reverse direction Schottky leaka=
ge
> rises magnificently with temperature - as long as you stick under 50 C th=
is
> is unlikely to matter. If you get to say 100C then SOME Schottkys have
> stunningly large reverse leakage. Many are poor but not so much to really
> notice in a relay driver. In some other applications Shottky reverse bias
> leakage can predominate.
>
> - If your 20 foot circuit is never broken and no relay ever gets unplugge=
d
> or disconnected then a far end diode will usually protect the electronic
> switch. However, if you have one only diode and it is at the far end then
> if the cct gets OC for ANY reason and your system turns a relay off with
> the diode missing you may need a new driver.
>
> 1N4007 is 1000 V rated and the lowest voltage versions usually encountere=
d
> are rate at 100V+. Schottky diodes start to get expensive above about 30V
> and may be cheaper again at 20V or 10V. A Schottky diode is extremely
> ESDsensitive and not overly bulletproof while 1N400x's can be used as
> bullets.
> Even a modest voltage pulse applied to a circuit with a say 30V Schottky =
in
> it could cause Schottky destruction. [[FWIW: That said, I have used many
> 1N5817s &  1N5819's (through hole) and do not recall ever having
> ESDdamaged one. ]]
>
> - Aren't those enough reasons? Why do you want more ? :-)
>
>
>     Russell McMahon
>     Applied Technology ltd
>     Auckland, New Zealand.


If I'm not mistaken, apart from the reverse stand-off voltage, the only
difference between the 1N400x parts is junction capacitance.
The parts from 1N4001 up to 1N4004 have the same higher capacitance and
the other devices have a smaller one.

Even the price is the same,  so I think it is better just to stock the
1N4007. That's what I do.


Isaac


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