>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,
? 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.
>
? 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!
>
? 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.
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