Complicated issue indeed.
Reminds me of the famous phrase used by hardware designers for decades,
now...
"It's only software."
-WB2WIK/6
"Each success only buys an admission ticket to a more difficult problem." --
Henry Kissinger
> -----Original Message-----
> From: Richard [SMTP:2@mail.vcnet.com]
> Sent: Friday, March 15, 2002 4:07 AM
> To: Peter Chadwick; AMPS
> Subject: RE: [Amps] 10 Meters
>
>
>
> >Rich said:
> >
> >>a MOV across the contacts cures the problem better than a vacuum-
> >>relay since the reverse EMF still needs to be suppressed to prevent
> >>damage elsewhere.
> >
> >The problem is that because it's DC, even with no inductive load,
>
> EUR If wires are used to connect a (pretend) purely resistive load, the
> load always looks iinductive. And when the circuit opens, a reverse EMF
> at the opening contacts is guaranteed. Good engineering practice is to
> suppress reverse EMFs with a MOV or bi-lateral Si transient suppressor
> diode.
> - With an o'scope, I measured the transient V at the on/off contacts of
> the switch feeding a 24VDC garden-variety dpdt 15a relay. As the switch
> opened, there was 420v across the contacts of the 250v-rated switch.
> Murphy was right -- "Everything is more complicated than it looks".
>
> >and there will always be a reverse you need
> >a very long gap to ensure that the arc is broken. On AC of course, the
> zero
> >crossing helps the arc suppression - this is why switches have bigger AC
> >than DC ratings. Large arcs from things switching are presumably
> >undesirable in fighter aircraft, and they were talking of having 70 or
> >100kW of
> >power available. Kilovac developed vacuum contactors just for the job.
> >
> EUR One still needs MOVs. Boeing's lack of a $2 MOV in the main tank
> fuel
> -level detector's wiring reportedly (FAA) allowed an arc inside the tank
> that brought down a 747 off the East Coast.
>
> >A far cry from the RAF at the beginning of WW2, where many aircraft had
> > problems when the demands for electricity reached 500 watts!
> >
> EUR For a 4-engine WW2 bomber, using 110VDC instead of 24VDC would have
> saved many tons of scarce copper and allowed more payload. . Hindsight
> ...
> >
> cheers, Peter
>
> - R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
> www.vcnet.com/measures.
> end
>
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