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Subject: Re: Need idea for a remote lamp status indicator.
References: <3E1C60A9.email@example.com> <firstname.lastname@example.org>
Date: Thu, 09 Jan 2003 00:52:02 GMT
NNTP-Posting-Date: Wed, 08 Jan 2003 17:52:02 MST
"Roger Hamlett" wrote in
> "Spehro Pefhany" wrote in message
>> On Wed, 08 Jan 2003 12:32:25 -0500, the renowned Glenn Ashmore
>> >I am working up a design for a MOSFET based switch box to control
>> >the lights on my boat's mast from the cockpit. These lights cannot
>> >have more than a 3% voltage drop from the battery so wire runs have
>> >to be as short as possible and the run up the mast and back to the
>> >switch box is 130' long. The switch box will eliminate about 50' of
>> >wire from the mast to the cockpit but there is a drawback.
>> > From inside the cabin I can't see the switch position or the
>> > lights. I
>> >need a circuit design that will light an LED on the switch box when
>> >the light is on. Because of the possibility of a burned out bulb or
>> >broken wire the circuit has to know that current is actually flowing
>> >and not just that the MOSFET is on. The circuit must also have
>> >minimum effect on the power reaching the light.
>> >Specifics: The voltage is 12-13.8VDC and the load ranges from 2A to
>> >7A depending on the light. The MOSFETs are 30 amp IRFZ34 driven at
>> >close to max Vgs.
>> Why are you using such a wimpy MOSFET? You can get one with about
>> half the Rds(on) and double the Idm for a few cents more.
>> You could measure either the voltage across a shunt (which might be a
>> few feet of wire) or you could put a window comparator on the drain
>> voltage of the MOSFET. Too low and we assume the bulb is gone, too
>> high and the MOSFET is not working. You should probably also try to
>> detect the MOSFET failed ON. If we assume that 10 millivolts can be
>> reliably detected, you'd need a shunt of 5m ohms. That is about 5'
>> of AWG10 wire. Or an SMT resistor or two, if you prefer. You could
>> use an op-amp such as Microchip's MCP6022* for the comparator; it
>> has a very low Vos and uses little power.
>> Now, does it have to detect ONE of several parallel loads that have
>> failed, or is there just one lamp per MOSFET?
>> * Not confirmed, but I could have sworn I saw something like phase
>> reversal with this part when BOTH op-amps were out of the CM range,
>> although the data sheet claims that phase reversal doesn't occur
>> with this part.
> Or stick a Hall effect current transducer into the feed wire, and
> check for the current drawn falling below a specific level according
> to how many lights are on. One transducer in the power feed to the
> switch box, combine it's output with voltage feeds from the switches,
> so the unit knows how many lights are lit, and you can have a single
> LED, which signals that a bulb has failed. I must agree that given the
> cost, a lower Rds MOSFET would be a better choice.
> Best Wishes
See page 25 "Line monitor" ; http://members.shaw.ca/roma
it may be suitable with proper voltage divider
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