On Apr 8, 2006, at 4:51 AM, Peter Chadwick wrote:
> Rich said:
>
> >My guess is that a $1 MOV would have prevented this glitch.<
>
> Maybe. What I read was that during ground testing, the supply was
> unregulated, and went up to around 70 volts,
The Apollo's battery potential was 70v, NASA did this to reduce
current so they could save on conductor weight. Motor vehicles are
beginning to go to higher battery potentials to save on Cu expense and
on vehicle weight, especially diesel engined ones which require more
electrical energy for starting and for fuel injecting due to the
diesel's increased cylinder pressure Hybrids require even more
electrical energy. The hybrid car I drive has a 200v Ni-MHbattery with
individual 3-cell, 3.6v, blocks so that if one cell goes bad and
shorts, the entire battery does not need to be replaced.
> causing the thermostat to arc because the? voltage?it was trying to
> switch was too much - in terms of DV (Direct Voltage -? you really
> can't have a 'Direct Current Voltage', now, can you?).
Depends on the time period.
> The MOV would probably have just burnt out.
Not if the right one was selected for the job. For the quartz heater,
I chose an MOV that was rated at more recurrent peak current than the
heater was rated at. The MOV has not needed replacing.
> What is clear (and I think Rich is saying this, too) that underating
> switches, thermostats, relay contacts and solid state switches etc. is
> a poor idea.
No, I'm saying that there is no such thing as a non-inductive load and
there will always be more potential across opening contacts than across
closing contacts. For the aforementioned 24v relay. I know of no
toggle switch that is rated at the needed 400Vdc, so a better solution
is to put a diode across the coil to stop the reverse potential in its
tracks, and if one wants to speed up the release time of the relay, put
a resistor in series with the diode as per Fig. 7B on my Web site.
I just purchased a $117 switch that is rated at 600Vdc, 30A. It
will disconnect the 16. 30Vdc-no-load photovoltaic panels that will
power my 240v, 60Hz, 2500w Sunny Boy® inverter. The mechanism is a
handle-operated spring-toggled knife switch that stentorianly switches
in milliseconds. The box its in is about 12" tall, 4" deep and 6"
wide. My guess is that it would take c. 50kV to jump the open contacts
at sea-level. Needless to say, I'm planning to put a transient-V
suppressor across the PV array to absorb the reverse potential when
current stops suddenly as the switch opens.
>
> 20 odd years ago, VMOS devices were touted as ideal for relay drivers
> and the like, but it turned out that they needed VERY fast diode
> protection compared with bipolars....
How fast a diode?
cheers, Peter
>
>
> 73
>
> Peter G3RZP (I've been away all week, and haven't read all the items,
> so there's something I should have responded to, let me know)
Rich Measures, 805.386.3734, AG6K, www.somis.org
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