Re: Circuit for 12V lights to come on each evening.

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FET!
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get

Easy enough for the circuit to check the state first in any case.


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You know of a mosfet with no losses at all, or you think the relay will
consume more in a few milliseconds than the mosfet will in a few hours?

MrT.



Re: Circuit for 12V lights to come on each evening.


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In this application, the Mosfet losses should be negligible, depending
on the load.

Also what happens with the latching relay if the battery discharges
beyond the point of it being able to
turn itself off ?  (especially as the battery ages). This could easily
happen as the LED's will be on overnight (due to the light sensor) and
even if the battery has sufficient capacity (and sufficient charging
current) to power them until dawn, (when switched off by the latching
relay) it would only need an overcast day to lower the battery's
charge rate to less than normal, and run the system out of juice
before dawn when it normally turns itself off.

Having the LED load locked "on" while charging may also prevent the
battery from ever reaching a decent charge (to unlatch the relay)  for
a lot longer than usual.  Every night that the charging stops before
it can get far enough, the LEDS will still be latched on killing
whatever charge may have been accumulated.

This would result in the load staying connected 24/7, probably
destroying the battery over time.

You can add battery level monitoring and auto-off, or a timer to do
the same thing, but these extra circuits being on all the time will
probably consume similar power than the FET would have in losses.









Re: Circuit for 12V lights to come on each evening.



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The idea was that they would only be one for a few hours each evening.
simple timer after the light sensor

Re: Circuit for 12V lights to come on each evening.



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Well "negligable" is purely subjective and the load is unknown, so that
statement is pointless.


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beyond the point of it being able to
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Best to monitor the battery to prevent total discharge in any case.


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Why on earth would you ever do that? If there is sufficient light for the
solar cells to charge the battery, wouldn't you have the circuit turn the
LED's OFF?


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the same thing,

Of course.

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Only if the design were bad, or the load current *extremely* small.

MrT.













Re: Circuit for 12V lights to come on each evening.


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hmm it seems they cost about 100nA (ballpark) to run.
that adds up quickly.



Re: Circuit for 12V lights to come on each evening.



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Since when is "100nA" a "power" loss? Seems you still fail to consider the
FET's on resistance and actual load current.
If it works for you, then I sure don't care  But your claim that "a latching
relay would use less *power* than a mosfet driven by a nor-nor cmos
bistable." is still a little shaky IMO though.

MrT.




Re: Circuit for 12V lights to come on each evening.


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have a look at some figures:

OMRON - G6BK-1114P-1-US-DC12      : 30 milliohms

FAIRCHILD SEMICONDUCTOR - FDU8580 : 9 milliohms

also, the relay costs twenty times the price of the mosfet, and can
only handle half the current.

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What lets the mosfet down is the gate leakage when on and the D-S leakage when
off.

I think there's more power to be lost in choosing an inefficient dark detector
and/or timer circuit.



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