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

>> In fact a bi-stable relay that only needs to be powered for a few

> >> milliseconds for it to turn on or off, will use far less power than a

FET!

>> > > That's fine as long as the circuit can ensure that the switching doesn't

get

> reversed as a result of a short power dip or something else. > > latching relays typically need different inputs to switch each way. > (different polarity or on a different pin)

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

I'm not convinced that a latching relay would use less power than a > mosfet driven by a nor-nor cmos bistable.

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.

Reply to
Mr.T
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a

n't

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.

Reply to
kreed

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

Reply to
Terryc

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

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Reply to
Jasen Betts

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.

Reply to
Mr.T

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

beyond the point of it being able to

Best to monitor the battery to prevent total discharge in any case.

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?

the same thing,

Of course.

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

MrT.

Reply to
Mr.T

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.

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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Reply to
Jasen Betts

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